290 Pages

# MATLAB Programming

Course Number: MA 340, Fall 2009

College/University: Capitol College

Word Count: 57955

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Contents 1 Engineering Problem Solving 1.1 1.2 1.3 1.4 Problem-Solving Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Problem Solving Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Computing Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 4 8 Computing Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....

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Engineering Contents 1 Problem Solving 1.1 1.2 1.3 1.4 Problem-Solving Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Problem Solving Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Computing Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 4 8 Computing Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 14 2 Matlab Technical Computing Environment 2.1 2.2 2.3 2.4 2.5 2.6 Workspace, Windows, and Help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Scalar Mathematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Basic Mathematical Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Computational Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Display Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Accuracy and Precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 37 3 Files and File Management 3.1 3.2 3.3 3.4 3.5 File Management Denitions and Commands . . . . . . . . . . . . . . . . . . . . . . 37 Saving and Restoring Matlab Information . . . . . . . . . . . . . . . . . . . . . . . . 39 Script M-Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Errors and Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Matlab Search Path, Path Management, and Startup . . . . . . . . . . . . . . . . . . 49 i 4 Trigonometry and Complex Numbers 4.1 4.2 4.3 51 Trigonometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Complex Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Two-Dimensional Plotting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 81 5 Arrays and Array Operations 5.1 5.2 5.3 Vector Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Matrix Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Array Plotting Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 101 6 Mathematical Functions and Applications 6.1 6.2 6.3 6.4 6.5 6.6 Signal Representation, Processing, and Plotting . . . . . . . . . . . . . . . . . . . . . 101 Polynomials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Partial Fraction Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Functions of Two Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 User-Dened Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Plotting Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 135 7 Data Analysis 7.1 7.2 7.3 7.4 Maximum and Minimum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Sums and Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Statistical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Random Number Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 155 8 Selection Programming 8.1 8.2 8.3 8.4 8.5 Relational and Logical Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Selection Statements in User-Dened Functions . . . . . . . . . . . . . . . . . . . . . 169 Update Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 ii 8.6 Applied Problem Solving: Speech Signal Analysis . . . . . . . . . . . . . . . . . . . . 175 180 9 Vectors, Matrices and Linear Algebra 9.1 9.2 9.3 9.4 Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Solutions to Systems of Linear Equations . . . . . . . . . . . . . . . . . . . . . . . . 196 Applied Problem Solving: Robot Motion . . . . . . . . . . . . . . . . . . . . . . . . . 202 207 10 Curve Fitting and Interpolation 10.1 Minimum Mean-Square Error Curve Fitting . . . . . . . . . . . . . . . . . . . . . . . 207 10.2 Applied Problem Solving: Hydraulic Engineering . . . . . . . . . . . . . . . . . . . . 213 10.3 Interpolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 10.4 Applied Problem Solving: Human Hearing . . . . . . . . . . . . . . . . . . . . . . . . 219 11 Integration and Dierentiation 223 11.1 Numerical Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 11.2 Numerical Dierentiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 12 Strings, Time, Base Conversion and Bit Operations 239 12.1 Character Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 12.2 Time Computations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 12.3 Base Conversions and Bit Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 13 Symbolic Processing 250 13.1 Symbolic Expressions and Algebra . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 13.2 Manipulating Trigonometric Expressions . . . . . . . . . . . . . . . . . . . . . . . . . 257 13.3 Evaluating and Plotting Symbolic Expressions . . . . . . . . . . . . . . . . . . . . . 258 13.4 Solving Algebraic and Transcendental Equations . . . . . . . . . . . . . . . . . . . . 259 13.5 Calculus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 13.6 Linear Algebra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 iii Section 1 Engineering Problem Solving Engineering often involves applying a consistent, structured approach to the solving of problems. A general problem-solving approach and method can be dened, although variations will be required for specic problems. Problems must be approached methodically, applying an algorithm, or step-by-step procedure by which one arrives at a solution. 1.1 Problem-Solving Process The problem-solving process for a computational problem can be outlined as follows: 1. Dene the problem. 2. Create a mathematical model. 3. Develop a computational method for solving the problem. 4. Implement the computational method. 5. Test and assess the solution. The boundaries between these steps can be blurred and for specic problems one or two of the steps may be more important than others. Nonetheless, having this approach and strategy in mind will help to focus our eorts as we solve problems 1. Problem Denition: The rst steps in problem solving include: Recognize and dene the problem precisely by exploring it thoroughly (may be the most dicult step). 1 Determine what question is to be answered and what output or results are to be produced. Determine what theoretical and experimental knowledge can be applied. Determine what input information or data is available Many academic problems that you will be asked to solve have this step completed by the instructor. For example, if your instructor ask you to solve a quadratic algebraic equation and provides you with all of the coecients, the problem has been completely dened before it is given to you and little doubt remains about what the problem is. If the problem is not well dened, considerable eort must be expended at the beginning in studying the problem, eliminating the things that are unimportant, and focusing on the root problem. Eort at this step pays great dividends by eliminating or reducing false trials, thereby shortening the time taken to complete later steps. After dening the problem: Collect all data and information about the problem. Verify the accuracy of this data and information. Determine what information you must nd: intermediate results or data may need to be found before the required answer or results can be found. 2. Mathematical Model: To create a mathematical model of the problem to be solved: Determine what fundamental principles are applicable. Draw sketches or block diagrams to better understand the problem. Dene necessary variables and assign notation. Reduce the problem as originally stated into one expressed in purely mathematical terms. Apply mathematical expertise to extract the essentials from the underlying physical description of the problem. Simplify the problem only enough to allow the required information and results to be obtained. Identify and justify the assumptions and constraints inherent in this model. 3. Computational Method: A computational method for solving the problem is to be developed, based on the mathematical model. Derive a set of equations that allow the calculation of the desired parameters and variables. 2 Develop an algorithm, or step-by-step method of evaluating the equations involved in the solution. Describe the algorithm in mathematical terms and then implement as a computer program. Carefully review the proposed solution, with thought given to alternative approaches. 4. Implementation of Computational Method: Once a computational method has been identied, the next step is to carry out the method with a computer, whether human or silicon. Some things to consider in this implementation: Assess the computational power needed, as an acceptable implementation may be hand calculation with a pocket calculator. If a computer program is required, a variety of programming languages, each with dierent properties, are available. A variety of computers, ranging from the most basic home computers to the fastest parallel supercomputers, are available. The ability to choose the proper combination of programming language and computer, and use them to create and execute a correct and ecient implementation of the method, requires both knowledge and experience. In your engineering degree program, you will be exposed to several programming languages and computers, providing you with some exposure to this issue. The mathematical algorithm developed in the previous step must be translated into a computational algorithm and then implemented as a computer program. The steps in the algorithm should rst be outlined and then decomposed into smaller steps that can be translated into programming commands. One of the strengths of Matlab is that its commands match very closely to the steps that are used to solve engineering problems; thus the process of determining the steps to solve the problem also determines the Matlab commands. Furthermore, Matlab includes an extensive toolbox of numerical analysis algorithms, so the programming eort often involves implementing the mathematical model, characterizing the input data, and applying the available numerical algorithms. 5. Test and Assess the Solution: The nal step is to test and assess the solution. In many aspects, assessment is the most open-ended and dicult of the ve steps involved in solving computational problems. The numerical solution must be checked carefully: A simple version of the problem should be hand checked. 3 The program should be executed on obtained or computed test data for which the answer or solution is either known or which can be obtained by independent means, such as hand or calculator computation. Intermediate values should be compared with expected results and estimated variations. When values deviate from expected results more than was estimated, the source of the deviation should be determined and the program modied as needed. A reality check should be performed on the solution to determine if it makes sense. The assumptions made in creating the mathematical model of the problem should be checked against the solution. 1.2 Problem Solving Example As an example of the problem solving process, consider the following problem: A small object is launched into ight from the ground at a speed of 50 miles/hour at 30 degrees above the horizontal over level ground. Determine the time of ight and the distance traveled when the ball returns to the ground. 1. Problem Denition: As stated above, this problem is well dened. The following items could be considered in further dening the problem. Additional information needed: Properties of the object and the ight medium could aect the ight trajectory. For example, if the object is light in weight, has a relatively large surface area, and travels in air, the air resistance would aect its ight. If the air were moving (by wind, for example), the ight of the object would also be aected. If this information is not available, it will be necessary to assume that the medium has no aect on the trajectory of ight. Acceleration of gravity also aects the ight. If no further information is available, it would be reasonable to assume that the gravitational acceleration at the surface of the Earth would apply. The accuracy of the initial speed and angle of the object is needed to determine the necessary accuracy of the quantities to be computed. The problem is one for which we have some physical insight, as it could represent the throwing of a baseball, although it is thrown from the ground instead of shoulder level. From this interpretation, the initial speed and angle seem reasonable. Unit conversions needed: 1 mile = 5280 feet 1 hour = 60 minutes = 3600 seconds 360 degrees = 2 radians 4 Output or results to be produced: It is clear from the problem statement that the results to be produced are the time of ight and the distance traveled. The units for these quantities havent been specied, but from our knowledge of throwing a baseball, computing time in seconds and distance in feet would be reasonable. Theoretical and experimental knowledge to be applied: The theory to be applied is that of ballistic motion in two dimensions. Input information or data: This includes the object initial velocity of 50 miles per hour at an angle 30 degree above horizontal. 2. Mathematical Model: To pose this problem in terms of a mathematical model, we rst need to dene the notation: Time: t (s), with t = 0 when the object is launched. Initial velocity magnitude: v = 50 miles/hour. Initial angle: = 30 . Horizontal position of ball: x(t) (ft). Vertical position of ball: y(t) (ft). Acceleration of gravity: g = 32.2 ft/s2 , directed in negative y direction. The key step in developing a mathematical model is to divide the trajectory into its horizontal and vertical components. The initial velocity can be divided in this way, as shown in Figure 1.1. From basic trigonometry, we know that vh = v cos vv = v sin Figure 1.1: Initial velocity (v) divided into horizonal (vh ) and vertical (vv ) components. Given the horizontal and vertical components of the initial velocity, the horizontal and vertical positions can be determined as functions of time. Since there is no external force acting to retard the horizontal motion, the object will move at a constant speed of vh in the horizontal direction x(t) = vt cos In the vertical direction, the object motion is retarded by gravity and its position is 1 y(t) = vt sin gt2 2 5 3. Computational Method: Using the model developed above, expressions for the desired results can be obtained. The object will hit the ground when its vertical position is zero 1 y(t) = vt sin gt2 = 0 2 which can be solved to yield two values of time t = 0, 2v sin g The second of the two solutions indicates that the object will return to the ground at the time tg = 2v sin g The horizontal position (distance of travel) at this time is x(tg ) = vtg cos 4. Computational Implementation: The equations dened in the computational method can be readily implemented using Matlab. The commands used in the following solution will be discussed in detail later in the course, but observe that the Matlab steps match closely to the solution steps from the computational method. % Flight trajectory computation % % Initial values g = 32.2; v = 50 * 5280/3600; theta = 30 * pi/180; % gravity, ft/s^2 % launch velocity, ft/s % launch angle, radians % Compute and display results disp(time of flight (s):) % label for time of flight tg = 2 * v * sin(theta)/g % time to return to ground, s disp(distance traveled (ft):) % label for distance xg = v * cos(theta) * tg % distance traveled % Compute and plot flight trajectory t = linspace(0,tg,256); x = v * cos(theta) * t; y = v * sin(theta) * t - g/2 * t.^2; plot(x,y), axis equal, axis([ 0 150 0 30 ]), grid, ... xlabel(Distance (ft)), ylabel(Height (ft)), title(Flight Trajectory) Comments: 6 Words following percent signs (%) are comments to help in reading the Matlab statements. A word on the left of an equals sign is known as a variable name and it will be assigned to the value or values on the right of the equals sign. Commands having this form are known as assignment statements. If a Matlab command assigns or computes a value, it will display the value on the screen if the statement does not end with a semicolon (;). Thus, the values of v, g, and theta will not be displayed. The values of tg and xg will be computed and displayed, because the statements that computes these values does not end with a semicolon. The three dots (...) at the end of a line mean that the statement continues on the next line. More than one statement can be entered on the same line if the statements are separated by commas. The statement t = linspace(0,tg,256); creates a vector of length 256. The expression t.^2 squares each element in t, making another vector. In addition to the required results, the ight trajectory has also been computed and plotted. This will be used below in assessing the solution. The plot statement generates the plot of height against distance, complete with a title and labels on the x and y axes. 5. Testing and Assessing the Solution: This problem is suciently simple that the results can be computed by hand with the use of a calculator. There is no need to generate test data to check the results. One could even question the need to use the power of Matlab to solve this problem, since it is readily solved using a calculator. Of course, our objective here is to demonstrate the application of Matlab to a problem with which you are familiar. Executing the statements above provides the following displayed results: time of flight (s): tg = 2.2774 distance traveled (ft): xg = 144.6364 Computing these quantities with a calculator can be shown to produce the same results. The ight trajectory plot that is generated is shown in Figure 1.2. This demonstrates another strength of Matlab, as it has taken care of the details of the plot generation, including axis scaling, label placement, etc. This result can be used to assess the solution. Our physical intuition tell us that this result appears to be correct, as we know that we could throw a baseball moderately hard and have it travel a distance of nearly 150 feet and that maximum height of about 20 feet also seems reasonable. The shape of the trajectory also ts our observations. 7 Flight Trajectory 30 Height (ft) 20 10 0 0 50 Distance (ft) 100 150 Figure 1.2: Matlab generated plot of ight trajectory 1.2.1 Programming Style Programs that are not documented internally, while they may do what is request, can be dicult to understand when read a few months later, in order to correct or update them. Thus, it is extremely important to develop the art of writing programs that are well structured, with all of the logic clearly described. This is known as programming style. Elements of good programming style include: Use comments liberally, both at the beginning of a script, to describe briey what it does and any special methods that may have been used, and also throughout the script to introduce dierent logical sections. Describe each variable briey in a comment when it is initialized. Separate sections of code with blank lines. Indent multiple line structures to make them stand out as a logical unit. Use spaces in expressions to make them more readable (for example, on either side of operators and equal signs). 1.3 Computing Software Before discussing Matlab in more detail, a brief discussion on computing software is useful. Computer software contains the instructions or commands that the computer is to execute. Categories include: System software Languages Tools System Software Computer system software provides: 8 An interface between the user and the hardware An environment for developing, selecting, and executing software. The operating system is the most important part of the systems software, managing the computer resources, including: The individual user sessions on multiuser computer systems. The division of CPU time across various tasks. The random access memory, or RAM, where both instructions and data are stored. The secondary storage systems, such as disk drives, CD-ROM drives, tape drives, or any other device in which information is stored in binary form on some medium. Information in secondary storage is organized into units called les. When a computer is turned on, it loads the operating system into RAM (usually from secondary storage) before a user can execute a program. The system software may also include one or more command shells: Programs that direct the interaction with users. In most cases, when a you type a command, you are interfacing with the shell. Early shells provided a text only interface, but modern computers (particularly personal computers) have graphical user interfaces (GUIs) that allow users to express what they want to do by using a pointing device and selecting among choices displayed on the screen. System software also includes: Language compilers Text editors Utilities for le management Utilities for printing Computer Languages Computer languages are used to develop programs to be executed. The can be described in terms of levels. Machine language: Lowest-level language Binary-encoded instructions that are directly executed by the hardware. 9 Language is closely tied to the hardware design. Machine specic: machine language for one computer is dierent from that of a computer having a dierent hardware design. Assembly language: Also unique to a specic computer design. Commands or instructions are written in text statements instead of numbers. Assembly language programming requires good knowledge of the specic computer hardware. This is a tedious process, but it results in programs that are very fast, because they take advantage of the specic computer hardware. System software called an assembler translates the assembly language program to a machine language program for execution on the hardware. High-level languages: Have English-like command syntax. Include languages such as Basic, Fortran, COBOL, Pascal, Ada, C, C++, and Java. Supported on many computer designs and knowledge of the specic hardware is not as critical in writing a program. System software called a compiler translates the program into machine language. Compilation Process: Original program is called the source program Translated machine language program is called the object program. Errors detected by the compiler (called compile errors or syntax errors) must be corrected and compilation performed again. The process of compiling, correcting errors (or debugging) must often be repeated several times before the program compiles without compile errors. Execution Process: To prepare the object program for execution, system software is applied to link other machine language statements to the object program and then load the program into memory. The program is executed and new errors, called execution errors, run-time errors or logic errors (also called bugs) may be identied. 10 These program errors are caused when the programmer errs in determining the correct steps to be taken in the problem solution, such as an attempt to divide by zero. These errors must be corrected in the source program, which must again be compiled and link-loaded for execution. The process of program compilation, linking, and execution is shown in Figure 1.3. Figure 1.3: High-level language program development Even when a program executes without an error message, the results must be checked carefully to be sure that they are correct. The computer performs the steps precisely as specied in the source; if the wrong steps are specied, the computer will execute these wrong (but syntactically correct) steps and produce a result that is incorrect. Matlab Mathematical Computation Tool Matlab and other mathematical computation tools are computer programs that combine computation and visualization power that make them particularly useful tools for engineers. Matlab is both a computer programming language and a software environment for using that language eectively. The name Matlab stands for Matrix laboratory, because the system was designed to make matrix computations particularly easy. Dont worry if dont know what a matrix is, as this will be explained later. The Matlab environment allows the user to manage variables, import and export data, perform calculations, generate plots, and develop and manage les for use with Matlab. The Matlab environment is an interactive environment: Single-line commands can be entered and executed, the results displayed and observed, and then a second command can be executed that interacts with results from the rst command that remain in memory. This means that you can type commands at the Matlab prompt and get answers immediately, which is very useful for simple problems. Matlab is a executable program, developed in a high-level language, which interprets user commands. Portions of the Matlab program execute in response to the user input, results are displayed, and the program waits for additional user input. 11 When a command is entered that doesnt meet the command rules, an error message is displayed. The corrected command can then be entered. Use of this environment doesnt require the compile-link/load-execution process described above for high-level languages. While this interactive, line-by-line execution of Matlab commands is convenient for simple computational tasks, a process of preparation and execution of programs called scripts is employed for more complicated computational tasks: A script is list of Matlab commands, prepared with a text editor. Matlab executes a script by reading a command from the script le, executing it, and then repeating the process on the next command in the script le. Errors in the syntax of a command are detected when Matlab attempts to execute the command. A syntax error message is displayed and execution of the script is halted. When syntax errors are encountered, the user must edit the script le to correct the error and then direct Matlab to execute the script again. The script may execute without syntax errors, but produce incorrect results when a logic error has been made in writing the script, which also requires that the script be editted and execution re-initiated. Script preparation and debugging is thus similar to the compile-link/load-execution process required for in the development of programs in a high-level language. 1.4 Computing Terminology Denitions of computing terms: Command: A user-written statement in a computer language that provides instructions to the computer. Variable: The name given to a quantity that can assume a value,. Default: The action taken or value chosen if none has been specied. Toggle: To change the value of a variable that can have one of two states or values. For example, if a variable may be on or o and the current value is on, to toggle would change the value to o. Arguments: The values provided as inputs to a command. Returns: The results provided by the computer in response to a command. Execute: To run a program or carry out the instructions specied in a command. Display: Provide a listing of text information on the computer monitor or screen. 12 Echo: To display commands or other input typed by the user. Print: To output information on a computer printer (often confused with display in the textbook). 13 Section 2 Matlab Technical Computing Environment Matlab provides a technical computing environment designed to support the implementation of computational tasks. Briey, Matlab is an interactive computing environment that enables numerical computation and data visualization. Matlab has hundreds of built-in functions and can be used to solve problems ranging from the very simple to the sophisticated and complex. Whether you want to do some simple numerical or statistical calculations, some complex statistics, solve simultaneous equations, make a graph, or run and entire simulation program, Matlab can be an eective tool. Matlab has proven to be extraordinarily versatile and capable in its ability to help solve problems in applied math, physics, chemistry, engineering, nance almost any application area that deals with complex numerical calculations. 2.1 Workspace, Windows, and Help Running Matlab Unix: From a terminal window, type matlab, followed by the Enter key Win95: double-click on the Matlab icon or select Matlab from Start/Programs Display Windows Command window Enter commands and data, display results Prompt >> or EDU>> 14 Graphics (Figure) window Display plots and graphs Created in response to graphics commands M-le editor/debugger window Create and edit scripts of commands called M-les When you begin Matlab, the command window will be the active window. As commands are executed, appropriate windows will automatically appear; you can activate a window by clicking the mouse in it. Getting Help help On-line help, display text at command line help, by itself, lists all help topics help topic provides help for the specied topic help command provides help for the specied command help help provides information on use of the help command helpwin On-line help, separate window for navigation. helpdesk Comprehensive hypertext documentation and troubleshooting demo Run demonstrations intro Interactive introduction to Matlab Interrupting and Terminating Matlab Ctrl-C (pressing the Ctrl and c keys simultaneously): Interrupts (aborts) processing, but does not terminate Matlab. You may want to interrupt Matlab if you mistakenly command it to display thousands of results and you wish to stop the time-consuming display. quit: Terminates Matlab exit: Terminates Matlab Select Exit under File menu: Terminates Matlab (MS Windows) 2.2 Scalar Mathematics Scalar mathematics involves operations on single-valued variables. Matlab provides support for scalar mathematics similar to that provided by a calculator. For more information, type help ops. The most basic Matlab command is the mathematical expression, which has the following properties: 15 Mathematical construct that has a value or set of values. Constructed from numbers, operators, and variables. Value of an expression found by typing the expression and pressing Enter 2.2.1 Numbers Matlab represents numbers in two form, xed point and oating point. Fixed point: Decimal form, with an optional decimal point. For example: 2.6349 -381 0.00023 Floating point: Scientic notation, representing m 10e For example: 2.6349 105 is represented as 2.6349e5 It is called oating point because the decimal point is allowed to move. The number has two parts: mantissa m: xed point number (signed or unsigned), with an optional decimal point (2.6349 in the example above) exponent e: an integer exponent (signed or unsigned) (5 in the example). Mantissa and exponent must be separated by the letter e (or E). Scientic notation is used when the numbers are very small or very large. For example, it is easier to represent 0.000000001 as 1e-9. 2.2.2 Operators The evaluation of expressions is achieved with arithmetic operators, shown in the table below. Operators operate on operands (a and b in the table). Operation Addition Subtraction Multiplication Right division Left division Exponentiation Algebraic form a+b ab ab ab ba ab Matlab a+b a-b a*b a/b a\b a^b Example 5+3 23-12 3.14*0.85 56/8 8\56 5^2 Examples of expressions constructed from numbers and operators, processed by Matlab: >> 3 + 4 16 ans = 7 >> 3 - 3 ans = 0 >> 4/5 ans = 0.8000 Prompt >> is supplied by Matlab, indicates beginning of the command. ans = following the completion of the command with the Enter key marks the beginning of the answer. Operations may be chained together. For example: >> 3 + 5 + 2 ans = 10 >> 4*22 + 6*48 + 2*82 ans = 540 >> 4 * 4 * 4 ans = 64 Instead of repeating the multiplication, the Matlab exponentiation or power operator can be used to write the last expression above as >> 4^3 ans = 64 Left division may seem strange: divide the right operand by the left operand. For scalar operands the expressions 56/8 and 8\56 produce the same numerical result. >> 56/8 ans = 7 >> 8\56 ans = 7 We will later learn that matrix left division has an entirely dierent meaning. Syntax 17 Matlab cannot make sense out of just any command; commands must be written using the correct syntax (rules for forming commands). Compare the interaction above with >> 4 + 6 + ??? 4 + 6 + | Missing operator, comma, or semi-colon. Here, Matlab is indicating that we have made a syntax error, which is comparable to a misspelled word or a grammatical mistake in English. Instead of answering our question, Matlab tells us that weve made a mistake and tries its best to tell us what the error is. Precedence of operations (order of evaluation) Since several operations can be combined in one expression, there are rules about the order in which these operations are performed: 1. Parentheses, innermost rst 2. Exponentiation (^), left to right 3. Multiplication (*) and division (/ or \) with equal precedence, left to right 4. Addition (+) and subtraction () with equal precedence, left to right When operators in an expression have the same precedence the operations are carried out from left to right. Thus 3 / 4 * 5 is evaluated as ( 3 / 4 ) * 5 and not as 3 / ( 4 * 5 ) . 2.2.3 Variables and Assignment Statements Variable names can be assigned to represent numerical values in Matlab. The rules for these variable names are: Must start with a letter May consist only of the letters a-z, digits 0-9, and the underscore character (_) May be as long as you would like, but Matlab only recognizes the rst 31 characters Is case sensitive: items, Items, itEms, and ITEMS are all dierent variable names. Assignment statement: Matlab command of the form: variable = number variable = expression 18 When a command of this form is executed, the expression is evaluated, producing a number that is assigned to the variable. The variable name and its value are displayed. If a variable name is not specied, Matlab will assign the result to the default variable, ans, as shown in previous examples. Example 2.1 Expressions with variables >> screws = 32 screws = 32 >> bolts = 18 bolts = 18 >> rivets = 40; >> items = screws + bolts + rivets items = 90 >> cost = screws * 0.12 + bolts * 0.18 + rivets * 0.08 cost = 10.2800 Variables: screws, bolts, rivets, items, cost Results displayed and stored by variable name Semicolon at the end of a line (as in the line >> rivets=40;) tells Matlab to evaluate the line but not to display the results Matlab workspace: Variables created in the Command window are said to reside in the Matlab workspace or Base workspace. The workspace retains the values of these variables, allowing them to be used in subsequent expressions. Thus, for the example above, we are able to compute the average cost per item by the following: >> average_cost = cost/items average_cost = 0.1142 Because average cost is two words and Matlab variable names must be one word, an underscore was used to create the single Matlab variable average_cost. If an expression contains variables, the value of the expression can be computed only if the values of the variables have been previously computed and still reside in the workspace. To compute the value of y = x + 5, you must rst supply a value of x: 19 >> x=2 x= 2 >> y=x+5 y= 7 If you have not supplied a value for x, Matlab will return a syntax error: >> y=x+5 ??? Undefined function or variable x. Precedence of operations involving variables Consider the computation of the area of a trapezoid whose parallel sides are of length b1 and b2 and whose height is h: A= (b1 + b2 )h 2 Matlab statement: area = 0.5*(base_1 + base_2)*height; Deleting the parentheses: area = 0.5*base_1 + base_2*height; This would compute A= b1 + b2 h 2 The incorrect result would be computed, but there would be no error messages, as the command has been written in correct Matlab syntax. Style: writing arithmetic expressions Use parentheses often Consider the equation to convert from temperature in Fahrenheit (TF ) to temperature in Celsius (TC ): 5 TC = (TF 32) 9 This is computed correctly by the Matlab command: TC = 5/9*(TF-32) 20 Matlab would rst compute TF-32, replacing it with a value. Then 5/9 would be computed and this value would multiply the previously computed value of TF-32. Easier to understand: TC = (5/9)*(TF-32) Use multiple statements Consider the equation: H(s) = s2 + 4s + 13 s3 2s2 + 4s + 5 Matlab commands: numerator = s^2 + 4*s + 13; denominator = s^3 - 2*s^2 + 4*s + 5; H = numerator/denominator; Special variables: ans: default variable name pi: ratio of circle circumference to its diameter, = 3.1415926... eps: smallest amount by which two numbers can dier inf or Inf : innity, e.g. 1/0 nan or NaN : not-a-number, e.g. 0/0 date: current date in a character string format, such as 19-Mar-1998. flops: count of oating-point operations. Commands involving variables: who: lists the names of dened variables whos: lists the names and sizes of dened variables clear: clears all variables, resets default values of special variables clear var: clears variable var clc: clears the command window, homes the cursor (moves the prompt to the top line), but does not aect variables. clf: clears the current gure and thus clears the graph window. more on: enables paging of the output in the command window. more off: disables paging of the output in the command window. When more is enabled and output is being paged, advance to the next line of output by pressing Enter ; get the next page of output by pressing the spacebar. Press q to exit out of displaying the current item. Example 2.2 Listing and clearing variables >> who Your variables are: 21 average_cost bolts >> whos Name average_cost bolts cost items rivets screws cost items Size 1x1 1x1 1x1 1x1 1x1 1x1 rivets screws Bytes 8 8 8 8 8 8 Class double double double double double double array array array array array array Grand total is 6 elements using 48 bytes Each variable occupies 8 bytes of storage. These variables each have a size 1x1 because they are scalars, as opposed to vectors or matrices. Redening variables A variable may be redened simply by executing a new assignment statement involving the variable. Note that previously issued commands involving the redened variable wont be automatically reevaluated. Example 2.3 Variable redenition >> screws = 32; >> bolts = 18; >> rivets = 40; >> items = screws + bolts + rivets items = 90 >> screws = 36 screws = 36 >> items items = 90 After computing items, screws was changed to 36, overwriting its previous value of 32. Note that the value of items has not changed. Unlike a spreadsheet, Matlab does not recalculate the number of items based on the new value of screws. When Matlab performs a calculation, it does so using the values it knows at the time the requested command is evaluated. In this example, to recalculate items, the items assignment statement must be re-issued: 22 >> items = screws + bolts + rivets items = 94 Command reuse and editing Commands can be reused and editted using the following operations: Press the up arrow cursor key () to scrolls backward through previous commands. Press Enter to execute the selected command. The down arrow cursor key () scrolls forward through commands The left () and right arrow () cursor keys move within a command at the Matlab prompt, allowing the command to be edited. The mouse can also be used to reposition the command cursor, by positioning the mouse cursor and pressing the left mouse button. Other standard editing keys, such as delete Del , backspace BkSp , home Home , and end End , perform their commonly assigned tasks. Once a scrolled or edited command is acceptable, pressing Enter with the cursor anywhere in the command tells Matlab to process it. Escape key Esc erases the current command at the prompt. Windows copy and paste operations: Copy: Highlight a command to be copied by placing the mouse cursor at the beginning of the text to be highlighted, press the left mouse button, and drag the mouse cursor through the text, releasing the mouse button when you reach the end of the text to be copied. The selected, or highlighted, text will appear as white text on black background instead of the reverse. In Unix, the highlighted text is automatically copied (stored internally). In MS Windows, the highlighted text is copied by pulling down the Edit menu and selecting Copy (or typing Ctrl+C). Paste: To paste the copied text in Unix, move the mouse cursor to the desired location, press the middle mouse button, and the text will be pasted into the window. In MS Windows, reposition the cursor, pull down the Edit menu and select Paste (or type Ctrl+V). Punctuation and Comments Semicolon (;) at the end of a command suppresses the display of the result 23 Commas and semicolons can be used to place multiple commands on one line, with commas producing display of results, semicolons supressing Percent sign (%) begins a comment, with all text up to the next line ignored by Matlab Three periods (. . .) at the end of a command indicates that the command continues on the next line. A continuation cannot be given in the middle of a variable name. Example 2.4 Use of punctuation >> screws = 36 screws = 36 >> items items = 90 >> items = screws + bolts + rivets items = 94 >> screws=32, bolts=18; rivets=40; % multiple commands screws = 32 >> items = screws + bolts + rivets; >> cost = screws*0.12 + bolts*0.18 + rivets*0.08; >> average_cost = cost/... % command continuation items average_cost = 0.1142 2.3 Basic Mathematical Functions Matlab supports many mathematical functions, most of which are used in the same way you write them mathematically. Elementary math functions (enter help elfun for a more complete list): abs(x) sign(x) exp(x) log(x) log10(x) sqrt(x) rem(x,y) Absolute value |x| Sign, returns 1 if x < 0, 0 if x = 0, 1 if x > 0 Exponential ex Natural logarithm ln x Common (base 10) logarithm log10 x Square root x Remainder of x/y. For example, rem(100,21) is 16. Also called the modulus function. 24 Information about these functions is displayed by the command help function. For example: >> help sqrt SQRT Square root. SQRT(X) is the square root of the elements of X. Complex results are produced if X is not positive. See also SQRTM. Example 2.5 Use of math functions x= 2 2 1 2 y = ex /2 2 z = 20 log10 y >> x = sqrt(2)/2 x= 0.7071 >> y = exp(-(x^2)/2)/sqrt(2*pi) y= 0.3107 >> z = 20*log10(y) z= -10.1533 Example 2.6 Solving for quadratic roots Problem: Solve for s: 2s2 + 10s + 12 = 0 Analysis: Derive and apply the quadratic equation by rst expressing the quadratic polynomial in parametric form as2 + bs + c = 0 Assuming a = 0, rewrite the equation as c b s2 + s + = 0 a a 25 To solve, complete the square in s s+ b 2a 2 b 2a 2 + c =0 a Rewrite to leave only the term involving s on the left hand side s+ b 2a 2 = 1 2a 2 b2 4ac Take the square root of each side to nd the solutions s1,2 = 1 b 2a 2a b2 4ac There are two solutions, so it is necessary to rewrite the equation in a dierent form to develop the Matlab commands. b x= 2a b2 4ac y= 2a s1 = x + y s2 = x y Matlab session: >> >> >> >> >> >> s1 a=2; b=10; c=12; x = -b/(2*a); y = sqrt(b^2-4*a*c)/(2*a); s1 = x+y = -2 >> s2 = x-y s2 = -3 2.4 Computational Limitations To understand the computational limitations of Matlab and other computer software, it is necessary to consider the methods for representing numbers in digital computers. 26 Fixed-Point Numbers All computer instructions and data are represented and stored in binary (base 2) form. In the representation of xed-point numbers, the value of each digit in the number depends on its position relative to the xed decimal point. For example the integer value 53 in base 10 is represented by the binary or base 2 value 00110101, denoted by 5310 = 001101012 In base 10, this means 5310 = 5 101 + 3 100 Similarly, 001101012 = 0 27 + 0 26 + 1 25 + 1 24 + 0 23 + 1 22 + 0 21 + 1 20 = 3210 + 1610 + 410 + 110 Thus, each bit position represents an increasing power of 2, beginning with 20 on the right. Floating-Point Numbers To represent non-integer numbers, a number representation known as oating-point must be dened. Floating-point numbers are used on computers to approximate a subset of the real numbers. While there are many possible oating-point number systems, all such systems consist of zero, a set of positive numbers, and the corresponding set of negative numbers. The set of base 10 oating-point numbers consists of every number that can be written in the form m 10e , where m (the mantissa) is in the range 0 m < 10 and contains p digits (p is called the precision). e (the exponent) is an integer that lies between emin (the minimum exponent) and emax (the maximum exponent). Our denition is for base 10 oating-point numbers, since you either are, or will become, accustomed to base 10. However, computers use base 2 numbers of the form m 2e . In Matlab, when you enter a number in base 10, it is converted to base 2. Similarly, a base 2 result will converted to base 10 before being displayed. The essential properties of base 10 and base 2 number systems are the same, so our discussion of oating-point numbers will focus on base 10 represntations. To represent a real number x with a oating-point number, we rst round x to the closest real number y that has a p-digit mantissa. There are then two possibilities: If y is a member of the set of oating-point numbers, we say that x is represented by y in the oating-point number system. The absolute dierence |x y| is called the roundo error. 27 If y is not a member of the set of oating-point numbers, we say that x is not representable in the oating-point number system. This can happen if x is too large, which is called overow error; or if the absolute value of x is too small, which is called an underow error. In Matlab, numbers are typically represented in a oating-point representation conforming to a standard established by the Institute of Electrical and Electronics Engineers (IEEE) in 1985. In the IEEE double-precision standard used by Matlab, there are 53 bits in the mantissa and 11 bits in the exponent, for a total of 64 bits to represent a scalar number. This provides a range of values extending from 10308 to 10308 . A single-precision representation is usually considered to be a 32-bit representation, but this is not supported by Matlab. Two Matlab functions, realmax and realmin, display the largest and the smallest numbers, respectively, that can be represented: >> realmax ans = 1.7977e+308 >> realmin ans = 2.2251e-308 If two numbers dier by one in the least-signicant of the p digits of the mantissa, this dierence is given by the value of the special variable eps: >> eps ans = 2.2204e-016 Note that e means exponent, not the base of the natural logarithms, so that ans above represents 2.2204 1016 . For an example of exponent overow, consider >> x = 2.5e200; >> y = 1.0e200; >> z = x*y z= Inf The result for z should have been 2.5e400, but since this overowed the exponent, the result was displayed as innity, represented by the special variable Inf. For an example of exponent underow, consider >> x = 2.5e-200; >> y = 1.0e-200; 28 >> z = x*y z= 0 Here z should have been 2.5e-400, but due to exponent underow, the result is displayed as 0. Due to the nite accuracy of the representation of numbers in a computer, errors can be made in computations. For example, we know that 1 5 0.2 = 0; however, Matlab produces the following: >> 1 -0.2 -0.2 -0.2 -0.2 -0.2 ans = 5.5511e-017 The result is a very small number, but it is not exactly zero. The reason is that the binary number corresponding to 0.2 is 0.001100110011001100 . . . This representation requires an innite number of digits. The consequence is that the computer works with an approximate value of 0.2. Subtracting the approximate value of 0.2 from 1 ve times does not yield exactly 0. 2.5 Display Options There are several ways to display the value of a variable. The simplest way is to enter the name of the variable. The name of the variable will be repeated, and the value of the variable will be printed starting with the next line. There are several commands that can be used to display variables with more control over the form of the display. Number Display Options Matlab follows several rules in displaying numerical results. Integers: Displayed as integers, as long as they contain 9 digits or less. For 10 digits or more, they are displayed in scientic notation, described below. For example: >> x = 5 x= 5 Short xed point: The default is to display as a decimal number with four digits to the right of the decimal point, and up to three digits to the left of the decimal point, if this is possible. This is known as the short format. For example: 29 >> 610./12. ans = 50.8333 Short oating point: When the result has more than three digits to the left of the decimal point, it is displayed in scientic notation (called the short e format). Scientic notation expresses a value as a number between 1 and 10 multiplied by a power of 10. An example of the short e format, which shows four digits to the right of the decimal point: >> 61000./12. ans = 5.0833e+003 In this format, 5.08333 is called the mantissa, e means exponent, which has the value +003. Written in the more common scientic notation, the result is 5.0833 103 . The default behavior can be changed by specifying a dierent numerical format within the Preferences menu item in the File menu, or by typing a FORMAT command at the command prompt. The table below indicates the aect of changing the display format for the variable ratio, computed as: >> ratio = 610./12 ratio = 50.8333 Matlab Command format short format long format short e format long e format short g ratio 50.8333 50.83333333333334 5.0833e+001 5.083333333333334e+001 50.8333 Comments 4 decimal digits 14 decimal digits 4 decimal digits plus exponent 14 decimal digits plus exponent better of format short or format short e (default), switching for ans > 1000 better of format long or format long e 2 decimal digits positive, negative, or zero format long g format bank format + 5.083333333333334e+001 50.83 + The bank format is a xed format for dollars and cents. The + format displays the symbols +, , and blank for positive, negative, and zero results, respectively. Note: The display formats do not change the internal representation of a number; only the display changes. The internal representation is rounded when the display is shortened. 30 For example: >> format long >> 5.78/57.13 ans = 0.10117276387187 >> format short >> ans ans = 0.1012 Thus, the short result is displayed as 0.1012 instead of 0.1011, which would have been the result if the long display had been truncated to four decimal digits. format compact suppresses many of the line feeds that appear between displays and allows more lines of information to be seen together on the screen. For example: >> temp = 78 temp = 78 >> format compact >> temp=78 temp = 78 In the examples shown in these notes, it is be asseumed that this command has been executed. The command format loose can be used to return to the less compact display mode. Displaying Values and Text There are three ways to display values and text in Matlab, to be described in this section: 1. By entering the variable name at the Matlab prompt, without a semicolon. 2. By use of the command disp. 3. By use of the command fprintf. From the prompt: As demonstrated in previous examples, by entering a variable name, an assignment statement, or an expression at the Matlab prompt, without a semicolon, the result will be displayed, proceeded by the variable name (or by ans if only an expression was entered). For example: 31 >> temp = 78 temp = 78 disp: There are two general forms of the command disp that are useful in displaying results and annotating them with units or other information: 1. disp(variable): Displays value of variable without displaying the variable name. 2. disp(string): Displays string by stripping o the single quotes and echoing the characters between the quotes. String: A group of keyboard characters enclosed in single quote marks (). The quote marks indicate that the enclosed characters are to represent ASCII text. >> temp=78; >> disp(temp); disp(degrees F) 78 degrees F Note that the two disp commands were entered on the same line so that they would be executed together. fprintf One of the weaknesses of Matlab is its lack of good facilities for formatting output for display or printing. A function providing some of the needed capability is fprintf. This function is similar to the function of the same name in the ANSI C language, so if you are familiar with C, you will be familiar with this command. The fprintf function provides more control over the display than is provided with disp. In addition to providing the display of variable values and text, it can be used to control the format to be used in the display, including the specication to skip to a new line. The general form of this command is: fprintf(format string, list of variables) The format string contains the text to be displayed (in the form of a character string enclosed in single quotes) and it may also contain format speciers to control how the variables listed are embedded in the format string. The format speciers include: w.d%f Display as xed point or decimal notation (defaults to short), with a width of w characters (including the decimal point and possible minus sign, with d decimal places. Spaces are lled in from the left if necessary. Set d to 0 if you dont want any decimal places, for example %5.0f. Include leading zeros if you want leading zeroes in the display, for example %06.0f. Display using scientic notation (defaults to short e), with a width of w characters (including the decimal point, a possible minus sign, and ve for the exponent), with d digits in the mantissa after the decimal point. The mantissa is always adjusted to be less than 1. Display using the shorter of tt short or short e format, with width w and d decimal places. Newline (skip to beginning of next line) 32 w.d%e w.d%g \n The w.d width speciers are optional. If they are left out, default values are used. Examples: >> fprintf(The temperature is %f degrees F \n, temp) The temperature is 78.000000 degrees F >> fprintf(The temperature is %4.1f degrees F \n, temp) The temperature is 78.0 degrees F It should be noted that fprintf only displays the real part of a complex number, which is an important data type to be discussed later in the course. 2.6 Accuracy and Precision Physical measurements cannot be assumed to be exact. Errors are likely to be present regardless of the precautions used when making the measurement. Quantities determined by analytical means are not always exact either. Often assumptions are made to arrive at an analytical expression that is then used to calculate a numerical value. The use of signicant digits provides a method of expressing results and measurements that conveys how good these numbers are. Signicant Digits A signicant digit is dened as: Any digit used in writing a number, except: Zeros used only for location of the decimal point Zeros that do not have any nonzero digit on their left. Numbers larger than 10 that are not written in scientic notation can cause diculties in interpretation when zeros are present. For example, 2000 could contain one, two, three, or four signicant digits. If the number is written in scientic notation as 2.000 103 , then clearly four signicant digits are intended. Table 2.1 gives several examples. When reading instruments, such as a measuring tape, thermometer, or fuel gauge, the last digit will normally be an estimate. That is, the instrument is read by estimating between the smallest graduations on the scale to get the nal digit. It is standard practice to count the doubtful digit as signicant. In performing arithmetic operations, it is important to not lose the signicance of the measurements, or, conversely, to imply precision that does not exist. The following are rules for determining the number of signicant digits that should be reported following computations. Multiplication and division: The product or quotient should contain the same number of signicant digits as are contained in the number with the fewest signicant digits. 33 Quantity 4782 600 6.0 102 6.00 102 31.72 30.02 46.0 0.02 0.020 600.00 Signcant Digits 4 1, 2, or 3 2 3 4 4 3 1 2 5 Table 2.1: Signicant digits Examples: (2.43)(17.675) = 42.95025 = 43.0 (2.279h)(60 min/h) = 148.74 min = 148.7 min The conversion factor is exact (a denition). (4.00 102 kg)(2.2046lbm/kg) = 881.84lbm = 882.lbm The conversion factor is not exact, but it should not dictate the precision of the result if this can be avoided. A conversion factor should be used that has one or two more signicant gures than will be reported in the result. 589.62/1.246 = 473.21027 = 473.2 Addition and subtraction: The result should show signicant digits only as far to the right as is seen in the least precise number in the calculation. Examples: 1725.463 + 189.2 + 16.73 = 1931.393 = 1931.4 897.0 0.0922 = 896.9078 = 896.9 Combined operations: While it would be good practice to convert the result of each operation to the proper number of signcant digits before applying another operation, it is normal practice to perform the entire calculation and then report a reasonable number of signicant gures. Rounding: In rounding a value to the proper number of signicant gures, increase the last digit retained by 1 if the rst gure dropped is 5 or greater. For example, consider the rounding performed by Matlab in displaying results in short format: >> 2/9, 2.222222222222/4, 2/3, -2/3 34 ans = 0.2222 ans = 0.5556 ans = 0.6667 ans = -0.6667 Accuracy, Precision, and Errors Accuracy is a measure of the nearness of a value to the correct or true value. Precision refers to the repeatability of a measurement, this how close successive measurements are to each other. Errors: All measurements will have some degree of error. Identiable and correctable errors are called systematic. Accidental or other nonidentiable errors are called random. Systematic Errors: Some errors will always have the same sign (+ or ) and are said to be systematic. Consider measure the distance between two points with a 25m steel tape. First, if the tape is not exactly 25.000m long, compared with the standard at the U.S. Bureau of Standards, there will be a systematic error. However, this error could be removed by applying a correction. A second source of error can be due to the temperature at the time of use and at the time when the tape was compared with the standard. A mathematical correction can be applied if this temperature is measured and the coecient of thermal expansion of the tape is known. Another source of systematic is due to the dierence in the tension applied to the tape while in use and the tension employed during standardization. Knowing the weight of the tape, the tension applied, and the length of the suspended tape, a correction can be calculated. Random Errors: Even if all of the systematic errors have been eliminated, a measurement will still not be exact, due to random errors. For example, in reading a thermometer, the reading must be estimated when the indicator falls between graduations. Furthermore, variations in the ambient air temperature could cause the measurement to uctuate. These errors can thus ssproduce measurements that are either too large or too small. Repeated measurements of the same quantity will vary due to random error. However, it is impossible to predict the magnitude and sign of the accidental error present in any one measurement. Repeating measurements and averaging the results will reduce random error in the average. However, repeating measurements will not reduce the systematic error in the average result. Roundo Errors: Rounding o a number to n decimal places, following the rule described in the preceding section, produces an error whose absolute value is not larger than 0.5 10n . For example, assuming that the Matlab value of is correct, the error associated with rounding to 4 decimal places is: >> E = pi - 3.1416 E= 35 -7.3464e-006 This error is indeed smaller than the bound shown above. We have used the denition: error = true value - calculated value This is also called the absolute error, although the qualier absolute may be used to indicate the absolute value of the error dened above. The error can be compare to the true value by calculating the relative error relative error = error/(true value) For our example: >> E/pi ans = -2.3384e-006 This relative error can also be expressed in per cent: >> 100*E/pi ans = -2.3384e-004 36 Section 3 Files and File Management In using Matlab, you will read and write several types of les, or collections of computer data. In this section, you will learn File management denitions and commands Saving and restoring information Script M-les Errors and debugging Search path, path management, and startup 3.1 File Management Denitions and Commands To describe le management, several concepts and denitions are introduced, then a few Unix commands are concisely described, followed by a description of Matlab commands in greater detail. File: A collection of computer data, either ASCII text or binary, that is stored on an external memory device, such as a disk or tape drive. The format of the le, or the order, size, and location of the data items stored in the le, is determined by the program that writes the le on the external memory device. File System: The combination of hardware and software that allows les to be stored (written), retrieved (read), and managed. Structure of le system: The le system in Unix and MS Windows has a tree structure, with the base called the root, dividing sequentially into branches, called directories or folders. Files can be thought of as leaves along the branches. File types: For our purposes, there are two types of les: binary and text (also called ASCII text or plain text). Binary les contain both the machine instructions of executable programs and 37 data stored in machine-readable form. Text les contain keyboard characters represented by one byte (8 bits) per character. Text, but not binary, les can be created and modied with text editors and printed on printers. Binary les can only be read an operated upon by programs designed to handle the internal formats of these les. File management: The process, implemented with a set of user commands, to manage the les in a le system. This involves dening the tree structure by creating or deleting directories and managing les by creating, moving, renaming, or removing them and listing their names and attributes. In a Unix terminal window, a MS-DOS window, or a Matlab Command window, le management is by typed commands (called command-line driven). In MS-Windows, le management is handled by a graphical user interface (GUI). Directory: Branch of a le tree, containing les and subdirectories; also called a folder. Root directory: Base of le system, denoted by / in Unix and C:\ in MS-DOS and MS Windows. Present working directory: The branch of the le system in which you are currently located. Also called current directory. You have direct access to the les in this directory. Using change directory commands, you can move to another directory. Home directory: When you log into your Unix account, your present working directory, is called your home directory. User les: The user is given permission to manage all les in directories (branches) said to be below the home directory. Path: Sequence of directories (branches) leading to a specic directory or le. Path separator: Character used to separate the directories (folders) in the path / in Unix, \ in MS-DOS and MS Windows. Absolute path: A path beginning at the le system root, such as /home/ford/teach/e6/ also called the full path. Relative path: A path beginning at the present working directory. For example, if the present working directory is /home/ford/, then teach/e6 is the relative path to the directory e6. Unix File Management The following are the basic Unix commands for le management. For more detailed information, type man command. Most of these operations can be done from Matlab, so it wont be necessary to learn them in great detail. pwd: Print working directory displays the full path of the present working directory. cd path: Change to directory given by path, which can be either a relative or an absolute path. ls: Display a list of the names of the directories and les in the present working directory. 38 ls -l: Display a list of names and associated access permissions, owner, size, and modication date of the directories and les in the present working directory. mkdir dir: Create the directory named dir in the present working directory. rm le: Delete le from current directory. more le: Display the contents of le (text le only), one screen at a time (press spacebar to display the next screen, q to quit). cp le1 le2: Make of copy of le1 named le2. mv le1 le2: Change the name of le1 to le2. lp le: Prints le (which must be a text le) on the users default printer. Matlab File Management Matlab provides a group of commands to manage user les that are similar to those of Unix. For more information, type help iofun. pwd: Print working directory displays the full path of the present working directory. cd path: Change to directory (folder) given by path, which can be either a relative or absolute path. dir or ls: Display the names of the directories (folders) and les in the present working directory. what: Display the names of the M-les and MAT-les in the current directory. delete le: Delete le from current directory type le: Display contents of le (text le only, such as an M-le). 3.2 Saving and Restoring Matlab Information It is good engineering practice to keep records of calculations. These records can be used for several purposes, including: To revise the calculations at a later time. To prepare a report on the project. Matlab provides several methods for saving information from a workspace session. Saving the session output with the diary command and saving and loading the variables with the save and load command are described in this section. 39 3.2.1 Diary Command The diary commands allows you to record all of the input and displayed output from a Matlab interactive workspace session. The commands include: diary le: Saves all text from the Matlab session, except for the prompts (>>), as text in le, written to the present working directory. If le is not specied, the information is written to the le named diary. diary off: Suspends diary operation. diary on: Turns diary operation back on. diary: Toggles diary state Example 3.1 Use of diary command Consider again the example involving roots of the quadratic equation. Problem: solve for s: s2 + 5s + 6 = 0 s= b b2 4ac 2a Matlab session: >> >> >> >> >> >> >> s1 diary roots a=1; b=5; c=6; x = -b/(2*a); y = sqrt(b^2-4*a*c)/(2*a); s1 = x+y = -2 >> s2 = x-y s2 = -3 The le roots is written in your current working directory. It can be displayed by the Matlab command type roots. It can also be displayed in a Unix terminal window by the command more roots or printed with the command lp roots. a=1; b=5; 40 c=6; x = -b/(2*a); y = sqrt(b^2-4*a*c)/(2*a); s1 = x+y s1 = -2 s2 = x-y s2 = -3 diary off Note that this is nearly the same as the display in the command window, with the exception that the Matlab prompt (>>) is not included. 3.2.2 Saving and Retrieving Matlab Variables There will be occasions when you want to save your Matlab variables so that you can later retrieve them to continue your work. In this case, you must save the information in the Matlab binary format, so that the full precision of the variables is retained. Files in this Matlab binary format are known as MAT-les and they have an extension of mat. Storing and Loading Workspace Values save save data save data 1 x y load data 1 Stores workspace values (variable names, sizes, and values), in the binary le matlab.mat in the present working directory Stores all workspace values in the le data.mat Stores only the variables x and y in the le data_1.mat Loads the values of the workspace values previously stored in the le data_1.mat Exporting and Importing Data There are also situations in which you wish to export Matlab data to be operated upon with other programs, or to import data created by other programs. This must be done with text les written with save or read with load. To write a text le data1.dat in the current working directory containing values of Matlab variables in long e format: save data1.dat -ascii Note that the individual variables will not be identied with labels or separated in any way. Thus, if you have dened variables a and b in the Matlab workspace, the command above will output rst the values in a, then the values in b, with nothing separating them. Thus, it is often desirable to write text les containing the values of only a single variable, such as: 41 save data2.dat a -ascii This command causes each row of the array a (more on arrays later) to be written to a separate line in the data le. Array elements on a line are separated by spaces. The separating character can be made a tab with the command: save data2.dat a -ascii -tab The .mat extension is not added to an ASCII text le. However, it is recommended that ASCII text le names include the extension .dat so that it is easy to distinguish them from MAT-les and M-les (to be described in the next section). For an example of importing text data, suppose that a text le named data3.dat contains a set of values that represent the time and corresponding distance of a runner from the starting line in a race. Each time and its corresponding distance value are on a separate line of the data le. The values on a line must be separated by one or more spaces. Consider a data le named data3.dat containing the following: 0.0 0.1 0.2 0.0 3.5 6.8 The load command followed by the lename will read the information into an array with the same name as the base name of the data le (extension removed). For example, the command load data3.dat reads the data in the le data3.dat and stores it in the Matlab array variable named data3, having two columns and three rows. Matlab can also import data from and export data to two common formats that are used in spreadsheet and database program formats. The more common of these le formats is the comma separated values or .csv format. This is an ASCII text le, with values on each line separated by commas, as the name implies. The second format is the spreadsheet .wk1 format, often used as an interchange format for dierent spreadsheet programs. The Matlab import and export commands for these formats are: A = csvread(file) csvwrite(file,A) A = wk1read(file) wk1write(file,A) Reads a comma separated value formatted le file. The result is returned in A. The le can only contain numeric values. Writes matrix A into file as comma separated values. Reads all the data from a WK1 spreadsheet le named file into matrix A. Writes matrix A into a WK1 spreadsheet le with the name file. .wk1 is appended to the lename if no extension is given. Use the help facility for information on options for these commands. 42 3.3 Script M-Files For simple problems, entering commands at the Matlab prompt in the Command window is simple and ecient. However, when the number of commands increases, or you want to change the value of one or more variables, reevaluate a number of commands, typing at the Matlab becomes tedious. You will nd that for most uses of Matlab, you will want to prepare a script, which is a sequence of commands written to a le. Then, by simply typing the script le name at a Matlab prompt, each command in the script le is executed as if it were entered at the prompt. For more information, type help script. Script File: Group of Matlab commands placed in a text le with a text editor. Matlab can open and execute the commands exactly as if they were entered at the Matlab prompt. The term script indicates that Matlab reads from the script found in the le. Also called M-les, as the lenames must end with the extension .m, e.g. example1.m. M-les are text les and may be created and modied with any text editor. You need to know how to open, edit, and save a le with the text editor you are using. On a PC or Macintosh, an M-le editor may be brought up by choosing New from the File menu in the Matlab Command window and selecting M-le. The script M-le is executed by choosing Run Script... from the File menu on a PC or Macintosh, or simply typing the name of the script le at the prompt in the Matlab command window. Example 3.2 Quadratic root nding script Create the le named qroots.m in your present working directory using a text editor: % qroots: Quadratic root finding script format compact; a=1 b=5 c=6 x = -b/(2*a); y = sqrt(b^2-4*a*c)/(2*a); s1 = x+y s2 = x-y To execute the script M-le, simply type the name of the script le qroots at the Matlab prompt: >> qroots a= 1 b= 5 c= 43 6 s1 = -2 s2 = -3 Comments: % qroots: % allows a comment to be added format compact: Suppresses the extra lines in the output display a=1, b=5, c=6: Sets values of a, b, c, will display result x & y: Semicolon at end of command means these intermediate values wont be displayed s1, s2: Compute and display roots Search rules for qroots command: 1. Display current Matlab variable qroots if dened 2. Execute built-in Matlab command qroots if it exists 3. Execute qroots.m if it can be found in the Matlab search path (described below) Commands within the M-le have access to all variables in the Matlab workspace and all variables created by the M-le become part of the workspace. Commands themselves are not normally displayed as they are evaluated. Placing the echo on command in the M-le will cause commands to be displayed. The command echo off (the default) turns o command display and echo by itself toggles the command echo state. You could repeatedly edit qroots.m, change the values of a, b, and c, save the le, then have Matlab execute the revised script to compute a new pair of roots. Matlab functions useful in M-les: Command disp(ans) echo [on|off] input(prompt) keyboard pause pause(n) waitforbuttonpress Description Display results without identifying variable names Control Command window echoing of script commands Prompt user with text in quotes, accept input until Enter is typed Give control to keyboard temporarily. Type Return to return control to the executing script M-le. Pause until user presses any keyboard key Pause for n seconds Pause until user presses mouse button or keyboard key 44 The input command is used for user input of data in a script and it is used in the form of an assignment statement. For example: a = input(Enter quadratic coefficient a: ); When this command is executed, the text string Enter quadratic coefficient a: is displayed as a user prompt in the command window. The user then types in data value, which is assigned to the variable a. Because this input command ends with a semicolon, the entered value of a is not displayed when the command is completed. Example 3.3 Revised quadratic roots script Change script for computing quadratic roots by: 1. Prompting for input of coecients a, b, and c; 2. Format the display of the computed roots s1 and s2; Script rqroots.m: % rqroots: Revised quadratic root finding script format compact; % prompt for coefficient input a = input(Enter quadratic coefficient a: ); b = input(Enter quadratic coefficient b: ); c = input(Enter quadratic coefficient c: ); disp() % compute intermediate values x & y x = -b/(2*a); y = sqrt(b^2-4*a*c)/(2*a); % compute and display roots s1 = x+y; disp(Value of first quadratic root: ),disp(s1); s2 = x-y; disp(Value of second quadratic root: ),disp(s2); Two examples of the results from this script: 45 >> rqroots Enter quadratic coefficient a: 1 Enter quadratic coefficient b: 5 Enter quadratic coefficient c: 6 Value of first quadratic root: -2 Value of second quadratic root: -3 >> rqroots Enter quadratic coefficient a: 1 Enter quadratic coefficient b: 4 Enter quadratic coefficient c: 8 Value of first quadratic root: -2.0000+ 2.0000i Value of second quadratic root: -2.0000- 2.0000i M-le Commands Command edit test Description Open test.m for editing using the built-in Matlab text editor, same as Open... in File menu Eective Use of Script Files The following are some suggestions on the eective use of Matlab scripts: 1. The name of a script le must follow the Matlab convention for naming variables; that is, the name must begin with a letter and may include digits and the underscore character. 2. Do not give a script le the same name as a variable it computes, because Matlab will not be able to execute that script le more than once unless the variable is cleared. Recall that typing a variable name at the command prompt causes Matlab to display the value of that variable. If there is no variable by that name, then Matlab searches for a script le having that name. For example, if the variable rqroot was created in a script le having the name rqroot.m, then after the script is executed the rst time, the variable rqroot exists in the Matlab workspace. If the script le is modied and an attempt is made to run it a second time, Matlab will display the value of rqroot and will not execute the script le. 3. Do not give a script le the same name as a Matlab command or function. You can check to see whether a function already exists by using the the which command. For example, to see whether rqroot already exists, type which rqroot. If it doesnt exist, Matlab will display rqroot not found. If it does exist, Matlab will display the full path to the function. For more details as to the existence of a variable, script, or function having the name rqroot, type exist(rqroot). This command returns one of the following values: 46 0 1 2 3 4 5 6 7 if if if if if if if if rqroot rqroot rqroot rqroot rqroot rqroot rqroot rqroot does not exist is a variable in the workspace is an M-le or a le of unknown type in the Matlab search path is a MEX-le in the Matlab search path is a MDL-le in the Matlab search path is a built-in Matlab function is a P-le in the Matlab search path is a directory 4. As in interactive mode, all variables created by a script le are dened as variables in the workspace. After script execution, you can type who or whos to display information about the names, data types and sizes of these variables. 5. You can use the type command to display an M-le without opening it with a text editor. For example, to view the le rqroot.m, the command is type rqroot. 3.4 Errors and Debugging You will nd that you will seldom get your scripts to run correctly the rst time. However, you shouldnt despair, as this is also the case for experienced programmers. You will need to learn to identify and correct the errors, known in computer jargon as bugs. Syntax Errors The most common type of error is the syntax error, a typing error in a Matlab command (for example, srqt instead of sqrt. These errors are said to be fatal, as they cause Matlab to stop execution and display an error message. Unfortunately, since the Matlab command interpreter can easily be confused by these errors, the displayed error message may not be very helpful. It is your job as a programmer to make sense of the message and correct your script, a process known as debugging. Some examples of syntax errors and associated messages include: Missing parenthesis >> 4*(2+5 ??? 4*(2+5 | A closing right parenthesis is missing. Check for a missing ")" or a missing operator. This message is helpful, as it even points out the location of the error. Missing operator >> 4(2+5) 47 ??? 4( | Missing operator, comma, or semi-colon. In this error, an operator such as * was left out after 4. Misspelled variable name >> 2x = 4*(2+5) ??? 2 | Missing operator, comma, or semi-colon. Here, if you intended the variable name to be x2 instead of 2x, the error message is misleading. You are at least shown the location of the error, allowing you to identify the error. There are a large number of possible syntax errors, many of which you will discover in writing scripts for this course. With experience you will gradually become more adept at understanding and correcting your mistakes. Run-time and Logic Errors After correcting syntax errors, your script will execute, but it still may not produce the results you desire. Run-time Errors: These errors occur when your script is run on a particular set of data. They typically occur when the result of some operation leads to NaN (not a number), Inf (innity), or an empty array (to be covered later in the course). Logic Errors: These are errors in your programming logic, when your script executes properly, but does not produce the intended result. When this occurs, you need to re-think the development and implementation of your problem-solving algorithm. The following are suggestions to help with the debugging of Matlab scripts or M-les: Execute the script on a simple version of the problem, using test data for which the results are known, to determine which displayed results are incorrect. Remove semicolons from selected commands in the script so that intermediate results are displayed. Add statements that display variables of interest within the script. Place the keyboard command at selected places in the script to give temporary control to the keyboard. By doing so, the workspace can be interrogated and values changed as necessary. Resume script execution by issuing a return command at the keyboard prompt. Later in the course, the Matlab debugging functions will be introduced to provide additional tools for correcting these errors. 48 Programming Style As discussed previously, good programming style requires that comments be included liberally in a script le. Furthermore, the rst comment line (known as the H1 line) before any executable statement is the line that is searched by the lookfor command. This command aids the user in nding a suitable Matlab command: lookfor xyz Looks for the string xyz in the rst comment line (the H1 line) in all M-les found on matlabpath. For all les in which a match occurs, lookfor displays the H1 line. Searches the entire rst comment block of each M-le. lookfor xyz -all For example: >> lookfor roots rqroots.m: % rqroots: Revised quadratic root finding script POLY Convert roots to polynomial. ROOTS Find polynomial roots. Thus, the rst line of a Matlab script should be a comment containing the script name and key words describing its function. The next several lines (to be searched with the -all option of lookfor) should contain comments giving the authors name, the date the script was created, and a detailed description of the script. 3.5 Matlab Search Path, Path Management, and Startup Matlab search path: Ordered list of directories that Matlab searches to nd script and function M-les stored on disk. Commands to manage this search path: matlabpath: Display search path. addpath dir: Add directory dir to beginning of matlabpath. If you create a directory to store your script and function M-les, you will want to add this directory to the search path. rmpath dir: Remove directory dir from the matlabpath. path(p1,p2): Changes the path to the concatenation of the two path strings p1 and p2. Thus path(path,p) appends a new directory to the current path and path(p,path) prepends a new path. If p1 or p2 are already on the path, they are not added. For example, the following statements add another directory to Matlabs search path: Unix: path(path,/home/ford/matlabm) DOS: path(path,TOOLS\GOODSTUFF) editpath: Edit matlabpath using Matlab editor, same as Set Path in File menu. 49 which test: Display the directory path to test.m. Used to tell you which M-le script will be executed when you enter the command test. Matlab at Startup Two les are executed at startup: matlabrc.m and startup.m matlabrc.m: Comes with Matlab, shouldnt be modied. Sets default Figure window size and placement, as well as a number of other default features. startup.m: An optional M-le to be created by the user, typically containing commands that add personal default features. It is common to put addpath or path commands in startup.m to append additional directories to the Matlab search path. Since startup.m is a standard script M-le, then there are no restrictions as to what commands can be placed in it. For example, you might want to include the command format compact in startup.m so that the compact results display format becomes the default. 50 Section 4 Trigonometry and Complex Numbers In this section, we will consider in greater detail two scalar mathematics tools that are important to engineers: trigonometry and complex numbers. We will nd that these two topics are closely related. 4.1 Trigonometry Denitions In quadrant I: sin = y , r x , r y , x = arcsin y r x r y x = sin1 y r x r y x cos = = arccos = cos1 tan = = arctan = tan1 r= x2 + y 2 51 sin(alpha) cos(alpha) tan(alpha) asin(z) acos(z) atan(z) atan2(y,x) Sine of alpha Cosine of alpha Tangent of alpha Arcsine or inverse sine of z, where z must be between 1 and 1. Returns an angle between /2 and /2 (quadrants I and IV). Arccosine or inverse cosine of z, where z must be between 1 and 1. Returns an angle between 0 and (quadrants I and II). Arctangent or inverse tangent of z. Returns an angle between /2 and /2 (quadrants I and IV). Four quadrant arctangent or inverse tangent, where x and y are the coordinates in the plane shown in the gure above. Returns an angle between and (all quadrants), depending on the signs of x and y. Example: >> z = sqrt(2)/2 z= 0.7071 >> alpha = asin(z) alpha = 0.7854 >> alpha_deg = alpha*180/pi alpha_deg = 45.0000 Where sqrt() is square root and asin() is inverse sine or arcsine. Note that Matlab only works in radians, so the result alpha returned by asin is in radians. Using the property that 2 radians equals 360 degrees, a statement has been written to compute alpha_deg, the angle in degrees. Note that care must be taken in computing an angle from an inverse trigonometric function. The functions asin and atan will yield angles only in quadrants I and IV: I for a positive argument and IV for a negative argument. The function acos yields angles only in quadrants I and II: I for a positive argument and II for a negative argument. The function atan2 is best used to compute angles, as the lengths of both the x and y sides of the triangle are used in the calculation. Consider a case for quadrant II, with x = 1, y = 1: >> x=-1; >> y=1; >> r=sqrt(x^2+y^2) r= 1.4142 >> theta_1=(180/pi)*asin(y/r) theta_1 = 45 >> theta_2=(180/pi)*atan(y/x) theta_2 = -45 52 >> theta_3=(180/pi)*atan2(y,x) theta_3 = 135 Observe that only theta_3 is the proper result, in quadrant II. Example 4.1 Estimating the height of a building Problem: Consider the problem of estimating the height of a building, as illustrated in Fig. 4.1. If the observer is a distance D from the building, the angle from the observer to the top of the building is , and the height of the observer is h, what is the building height? Figure 4.1: Estimating Building Height Solution: Draw a simple diagram as shown in Fig. 4.1. The building height B is B =h+H where H is the length of the triangle side opposite the observer, which can be found from the triangle relationship tan = H D Therefore, the building height is B = h + D tan If h= 2 meters, D= 50 meters, and is 60 degrees, the Matlab solution is >> h=2; >> theta=60; >> D=50; >> B = h+D*tan(theta*pi/180) B= 88.6025 53 Solution of Triangles Law of sines: b c a = = sin sin sin Law of cosines: a2 = b2 + c2 2bc cos Law of tangents: tan 1 ( ) ab 2 = a+b tan 1 ( + ) 2 Included angles: + + = radians = 180 Example 4.2 Use of triangle relations In Figure 4.2, measurements of distances and angles around a lake have been indicated. The problem is to use these measurements to calculate the distance DE across the lake. Figure 4.2: Distance and angle measurements near a lake The solution is to sequentially apply the various triangle relations to determine the distances CF , CD, and EF , which can then be used to determine the specied distance DE. First, determine the length of the sides of triangle ACF, beginning with the right triangle relationship on the bottom right: sin 30 = 245 AC 54 Solving this equation for AC: AC = 245 sin 30 From the included angles in triangle ACD: D = 180 30 45 Applying the law of sines to triangle ACD: AC CD = sin 30 sin D Solving for CD: CD = AC sin 30 sin D From the construction of angle A in triangle ACF: A = 180 30 45 Triangles ACF and ACD are similar (have the same angles), which means that the lengths of corresponding sides are proportional: AC CD = CF AC Solving for CF : CF = (AC)2 CD From the similar triangle, angle F must be the same as angle CAD, so F = 30 . Applying the Law of sines to triangle ACF: AF AC = sin 45 sin F Solving for AF : AF = AC sin 45 sin F 55 From the included angles in triangle AEF: E = 180 15 F Applying the Law of sines to triangle AEF: AF EF = sin 15 sin E Solving for EF : EF = AF sin 15 sin E Finally, the distance DE across the lake is: DE = CF CD EF A Matlab script to compute DE, with angles in degrees and distances in feet: % Lake distance problem % AC = 245/sin(30*pi/180) D = 180-30-45 CD = AC*sin(30*pi/180)/sin(D*pi/180) CF = AC^2/CD F = 30 AF = AC*sin(45*pi/180)/sin(F*pi/180) E = 180-15-F EF = AF*sin(15*pi/180)/sin(E*pi/180) DE = CF - CD - EF The displayed results from Matlab: AC = 490.0000 D= 105 CD = 253.6427 CF = 946.6073 F= 30 56 AF = 692.9646 E= 135 EF = 253.6427 DE = 439.3220 The nal step in the solution to this problem is to carefully consider the results to determine if they make sense. This is done visually by comparing the computed lengths with those of Fig. 4.2. 4.1.1 Hyperbolic Functions The hyperbolic functions are functions of the natural exponential function ex , where e is the base of the natural logarithms, which is approximately e = 2.71828182845904. The inverse hyperbolic functions are functions of the natural logarithm function, ln x. The curve y = cosh x is called a catenary (from the Latin word meaning chain). A chain or rope, suspended from its ends, forms a curve that is part of a catenary. Matlab includes several hyperbolic functions, as described briey in the table below. sinh(x) cosh(x) tanh(x) asinh(x) acosh(x) atanh(x) Hyperbolic sine of x; 1 (ex ex ) 2 Hyperbolic cosine of x; 1 (ex + ex ) 2 sinh(x) Hyperbolic tangent of x; cosh(x) Inverse hyperbolic sine of x; ln(x + x2 + 1) Inverse hyperbolic cosine of x; ln(x + x2 1) for x 1 Inverse hyperbolic tangent of x; 1 ln 1+x for |x| 1. 2 1x 4.2 Complex Numbers Complex numbers nd widespread applications in many elds. They are used throughout mathematics, applied science, and engineering to represent the harmonic nature of vibrating systems and oscillating elds. A powerful feature of Matlab is that it does not require any special handling for complex numbers. In this section, we develop the algebra and geometry of complex numbers and describe how they are represented and handled by Matlab. 57 4.2.1 Denitions and Geometry Imaginary number: The most fundamental new concept in the study of complex numbers is the imaginary number j. This imaginary number is dened to be the square root of 1: j= 1 j 2 = 1 You may be more familiar with the imaginary number being denoted by i, which is the common notation in mathematics. However, in engineering, an electrical current is denoted by i, so j is used for the imaginary number. Rectangular Representation: A complex number z consists of the real part x and the imaginary part y and is expressed as z = x + jy where x = Re[z]; y = Im[z] This form of representation for complex numbers is called the rectangular or cartesian form since z can be represented in rectangular coordinates by the point (x, y) in a plane having a horizontal axis being the real axis and the vertical axis being the imaginary axis, as shown in Figure 4.3. This plane is called the complex plane. Figure 4.3: The complex number z in the complex plane In Matlab, i and j are variable names that default to the imaginary number. You have to be careful with their use, however, as they can be overridden and used as general variables. You can insure that j is the imaginary number by explicitly computing it as the square root of 1: 58 >> j = sqrt(-1) j= 0+ 1.0000i The result is displayed in rectangular form, with i used as the imaginary number. A general complex number can be formed in three ways: >> z = 1 + j*2 z= 1.0000+ 2.0000i >> z = 1 + 2j z= 1.0000+ 2.0000i >> z = complex(1,2) z= 1.0000 + 2.0000i In the rst method above, the imaginary number j explicitly multiplies the real number (imaginary part) 2. In the second method, the imaginary number j is used as notation to produce an imaginary part of 2. Note however, that j cannot precede the imaginary part: >> z = 1 + j2 ??? Undefined function or variable j2. The error message indicates that Matlab interprets j2 as a variable that has not been dened and thus does not have a value. In Matlab, the function real(z) returns the real part and imag(z) returns the imaginary part: >> z = 3 + 4j z= 3.0000+ 4.0000i >> x = real(z) x= 3 >> y = imag(z) y= 4 We call z = x + jy the Cartesian or rectangular representation of z, with real component x and imaginary component y. We say that the Cartesian pair (x, y) codes the complex number z. Polar Representation: Dening the radius r and the angle of the complex number z shown in Figure 4.3, z can be represented in polar form and written as z = r cos + jr sin 59 or, in shortened notation z=r where Math denition Magnitude: Argument or angle: For example: >> z = 3 + 4j; >> r = abs(z) r= 5 >> theta = angle(z) theta = 0.9273 Recall that angles in Matlab are given in radians. To compute the angle in degrees, the result in radians must be multiplied by (360/2) or (180/): >> theta = (180/pi)*angle(z) theta = 53.1301 Principal value of the complex argument: The angle is dened only for nonzero complex numbers and is determined only up to integer multiples of 2, since adding 2 radians rotates the complex number one revolution around the axis and leaves it in the same location. The value of that lies in the interval < is called the principal value of the argument of z, and is the value computed by angle(z) in Matlab. Example 4.3 Principal value of complex argument For the complex number z = 1 + j 3 r= 12 + ( 3)2 = 2 and = z = tan1 3 = + 2n 3 |z| = r = = tan1 x2 + y 2 y y = sin1 x r = cos1 x r Matlab abs(z) angle(z) The principal value of is /3 and therefore z = 2(cos /3 + j sin /3) Conrming with Matlab: 60 >> z = 1 + j*sqrt(3) z= 1.0000+ 1.7321i >> theta = angle(z) theta = 1.0472 >> pi/3 ans = 1.0472 >> r = abs(z) r= 2.0000 >> z1 = r*(cos(theta) + j*sin(theta)) z1 = 1.0000+ 1.7321i >> theta2 = theta +2*pi theta2 = 7.3304 >> z2 = r*(cos(theta2) + j*sin(theta2)) z2 = 1.0000 + 1.7321i Polar to rectangular conversion: To obtain the rectangular representation from the polar representation, apply the trigonometric relationships between the radius and angle and the real and imaginary parts: x = r cos y = r sin For example: >> r = 5; theta = 0.9273; >> z = r*cos(theta) + j*r*sin(theta) z= 3.0000+ 4.0000i The complex number z can be written as z = x + jy = r cos + jr sin = r(cos + j sin ) 61 Exponential Representation: The base of the natural logarithms, e = 2.71828182845904, is used to develop the exponential representation for complex numbers, through the Euler (sounds like oiler) formula ej = cos + j sin From this identity, two additional formulas can be derived: cos = 1 j e + ej 2 1 ej ej 2j sin = These formulas are derived and discussed in greater detail in a calculus course. Our purpose here is to use them to represent the complex number z in the exponential form z = rej = r cos + jr sin = r(cos + j sin ) Note that this has the same functional form as the polar representation for z. While it appears to dier from the polar representation only in notation at this point, we will continue to expand on the properties of the exponential representation to show that the dierences are more than symbolic. The graphical representation shown in Figure 4.3 still applies. Magnitude of z: |z| = r Angle or phase of z: = z Exponential representation of z: z = |z|ej z = rej Conrming with Matlab: >> z = 3 + 4j z= 3.0000+ 4.0000i >> r = abs(z) r= 5 >> theta = angle(z) theta = 0.9273 >> z = r * exp(j*theta) z= 3.0000+ 4.0000i 62 Consider the special case for which the magnitude |z| = r = 1 z = ej For this case, z lies on on a circle of radius 1 centered on the origin of the complex plane, at angle , as shown in Figure 4.4. Figure 4.4: Complex number z = ej in the complex plane There are several values of for which you should know the value of ej , as shown in Figure 4.4. = 0: = /2: = : = 2: = /2: ej0 = cos 0 + j sin 0 = 1 + j0 = 1 ej/2 = cos /2 + j sin /2 = 0 + j1 = j ej = cos + j sin = 1 + j0 = 1 ej2 = cos 2 + j sin 2 = 1 + j0 = 1 ej(/2) = cos(/2) + j sin(/2) = 0 j1 = j Using the function exp(x), these values can be conrmed in Matlab: >> z = exp(j*0) z= 1 >> z = exp(j*pi/2) z= 0.0000+ 1.0000i >> z = exp(j*pi) z= -1.0000+ 0.0000i >> z = exp(j*2*pi) z= 1.0000- 0.0000i >> z = exp(-j*pi/2) z= 0.0000- 1.0000i Summary: Complex Number Representations 63 We can summarize the representations of the complex number z as follows: z = x + jy = r = rej = |z|ej z 4.2.2 Algebra of Complex Numbers The algebraic operations of addition, subtraction, multiplication, and division can be dened for complex numbers in much the same way as they are dened for real numbers. Also, additional algebraic operations can be dened for complex numbers that have no meaning for real numbers. The complex operations have simple geometric interpretations. Addition and Subtraction: The complex numbers z1 and z2 are added (or subtracted) by separately adding (or subtracting) the real and imaginary parts: z1 + z2 = (x1 + jy1 ) + (x2 + jy2 ) = (x1 + x2 ) + j(y1 + y2 ) z1 z2 = (x1 + jy1 ) (x2 + jy2 ) = (x1 x2 ) + j(y1 y2 ) As shown in Figure 4.5, the geometric interpretation of complex addition is the parallelogram rule, where z1 + z2 lies on the node of a parallelogram formed from z1 and z2 . For complex subtraction, z2 is represented in the complex plane by reversing its direction and then adding to z1 using the parallelogram rule, as shown in Figure 4.5. If z is given in polar or complex exponential Figure 4.5: Addition and subtraction of complex numbers form, it must be converted to rectangular form to perform the addition. In Matlab, complex addition is performed in the same way as it is performed for real numbers: >> z1 = 1 + 2j 64 z1 = 1.0000+ 2.0000i >> z2 = 4 + 3j z2 = 4.0000+ 3.0000i >> z3 = z1 + z2 z3 = 5.0000+ 5.0000i Multiplication: The product of z1 and z2 is found using the rst-outer-inner-last (FOIL) method from polynomial multiplication, applying the identity j 2 = 1, and writing the result in rectangular form: z1 z2 = (x1 + jy1 )(x2 + jy2 ) = x1 x2 + jx1 y2 + jx2 y1 + j 2 y1 y2 = (x1 x2 y1 y2 ) + j(x1 y2 + x2 y1 ) Multiplication is better understood if the complex exponential representations are used: z1 z2 = r1 ej1 r2 ej2 = r1 r2 ej(1 +2 ) Here, we have used the mathematical property that exponents j1 and j2 of common base e add. We say from the above that the magnitudes multiply and the angles add. |z1 z2 | = r1 r2 (z1 z2 ) = 1 + 2 In Matlab, complex multiplication is performed in the same way as it is performed for real numbers. In the example below, magnitudes and angles have been computed to allow you to conrm that magnitudes multiply and angles add: >> z1 = 1 +0.5j; z2 = 2 + 1.5j; >> z4 = z1 * z2 z4 = 1.2500+ 2.5000i >> magz1 = abs(z1) magz1 = 1.1180 >> magz2 = abs(z2) magz2 = 2.5000 65 >> magz4 = abs(z4) magz4 = 2.7951 >> mag_test = magz1 * magz2 mag_test = 2.7951 >> angz1 = angle(z1) angz1 = 0.4636 >> angz2 = angle(z2) angz2 = 0.6435 >> angz4 = angle(z4) angz4 = 1.1071 >> ang_test = angz1 + angz2 ang_test = 1.1071 Rotation: There is a special case of complex multiplication that is important to understand. When z1 = r1 ej1 and z2 = ej2 (i.e., the magnitude of z2 is 1), then the product of z1 and z2 is z1 z2 = r1 ej1 ej2 = r1 ej(1 +2 ) As shown in Figure 4.6, z1 z2 is just a rotation of z1 through the angle 2 . A particular case of Figure 4.6: Rotation of complex numbers rotation results from the multiplication by j. Recalling that ej/2 = j, the product jz1 becomes: jz1 = r1 ej(1 +/2) Thus, multiplying by j results in a rotation by /2 or 90 , producing a result that is perpendicular to z1 in the complex plane. For example: 66 >> z1 = 3 + 4j; >> z2 = z1 * exp(j*pi/2) z2 = -4.0000+ 3.0000i >> z3 = j * z1 z3 = -4.0000+ 3.0000i >> theta1 = (180/pi) * angle(z1) theta1 = 53.1301 >> theta2 = (180/pi) * angle(z2) theta2 = 143.1301 >> theta_diff = theta2 - theta1 theta_diff = 90 Complex Conjugate. For every complex number z = x+jy = rej there is the complex conjugate z = x jy Plotting z in the complex plane, by replacing y with y , as shown in Figure 4.7, we see that the angle or phase of z is . Thus, Figure 4.7: Complex conjugate z = x jy = rej The mathematical procedure for nding a complex conjugate is to replace j with j , changing the sign of the imaginary part of the complex number. For example: 67 >> z = 3 + 4j z= 3.0000+ 4.0000i >> zconj = conj(z) zconj = 3.0000- 4.0000i >> theta = angle(z) theta = 0.9273 >> r = abs(z) r= 5 >> zconj1 = r * exp(-j*theta) zconj1 = 3.0000- 4.0000i Magnitude Squared: The product of z and its complex conjugate is z z = (x jy)(x + jy) = x2 + jxy jxy j 2 y 2 = x2 + y 2 = r2 = |z|2 or, |z|2 = z z and we see that the magnitude squared |z|2 is the product of z and its complex conjugate z . For example: >> z = 3 + 4j; >> zz = conj(z) * z zz = 25 >> zmsq = abs(z)^2 zmsq = 25 Division: This operation is dened as the inverse operation of multiplication. The quotient z1 /z2 is obtained in rectangular form by multiplying both the numerator and denominator of the quotient by the conjugate of z2 : z1 z2 = = = x1 + jy1 x2 + jy2 (x1 + jy1 )(x2 jy2 ) (x2 + jy2 )(x2 jy2 ) (x1 + jy1 )(x2 jy2 ) 2 x2 + y2 2 68 = x1 x2 + y1 y2 x2 y1 x1 y2 +j 2 2 x2 + y2 x2 + y2 2 2 Division is more easily performed in exponential form z1 z2 r1 ej1 r2 ej2 r1 ej1 ej2 r2 ej2 ej2 r1 j(1 2 ) e r2 = = = Thus, the magnitude of the quotient is the quotient of the magnitudes and the angle of the quotient is the dierence of the angle of the numerator and the angle of the denominator: r1 z1 = z2 r2 z1 z2 For example: >> z2 = 2 + 1.5j z2 = 2.0000+ 1.5000i >> z4 = 1.25 + 2.5j z4 = 1.2500+ 2.5000i >> z1 = z4/z2 z1 = 1.0000+ 0.5000i >> magz2 = abs(z2) magz2 = 2.5000 >> magz4 = abs(z4) magz4 = 2.7951 >> magz1 = abs(z1) magz1 = 1.1180 >> magtest = magz4/magz2 magtest = 1.1180 >> argz2 = angle(z2) argz2 = 69 = 1 2 0.6435 >> argz4 = angle(z4) argz4 = 1.1071 >> argz1 = angle(z1) argz1 = 0.4636 >> ang_test = argz4 - argz2 ang_test = 0.4636 Algebraic Rules The algebraic rules you have learned for real variables and numbers apply to complex variables and numbers. Commutativity: Addition and multiplication can be done in either order without changing the result. z1 + z2 = z2 + z1 z1 z2 = z2 z1 Associativity: Sums and products of three or more variables can be performed in sequential groups of two without changing the result. (z1 + z2 ) + z3 = z1 + (z2 + z3 ) (z1 z2 )z3 = z1 (z2 z3 ) Distributivity: Multiplication can be distributed across a sum without changing the result. z1 (z2 + z3 ) = z1 z2 + z1 z3 Matlab Functions for Complex Numbers To summarize, the following are the Matlab functions for complex numbers: abs(z) angle(z) conj(z) imag(z) real(z) Complex magnitude |z| (absolute value for real z) Phase angle or argument of z Complex conjugate z Complex imaginary part Im(z) Complex real part Re(z) 70 4.2.3 Roots of a Quadratic Polynomial In a previous example, we found that the roots of the quadratic polynomial as2 + bs + c = 0 are given by: s1,2 = 1 b 2a 2a b2 4ac For the values of the coecients considered in that example, the resulting roots were real. However, for other values of the coecients, the roots can be complex. Having now reviewed complex numbers, we can investigate the problem of quadratic roots in more detail. There are three dierent cases for the solution, dependent on the value of the discriminant of the quadratic equation: d = b2 4ac Overdamped (d > 0): Both roots are real and are given by s1,2 = 1 b 2a 2a b2 4ac The roots are located symmetrically about the point b/2a. When b = 0, they are located symmetrically about 0 at the points (1/2a) 4ac (in this case, 4ac > 0). Critically Damped (d = 0): The two roots are real and equal (we say they are repeated): s1,2 = b 2a Underdamped (d < 0): The square root in the quadratic equation produces an imaginary number, so the roots are complex s1,2 = 1 b (4ac b2 ) 2a 2a 1 b 4ac b2 = j 2a 2a Note that in this case, the roots s1 and s2 are complex conjugates: s2 = s 1 The roots are purely imaginary when b = 0 s1,2 = j c a In Matlab, it is not necessary to determine which of the three cases applies to a given problem, as the square root function will appropriately return real or imaginary values as needed. 71 Example: >> >> >> >> s1 a=1; b=4; c=8; x = -b/(2*a); y = sqrt(b^2-4*a*c)/(2*a); s1 = x + y = -2.0000+ 2.0000i >> s2 = x - y s2 = -2.0000- 2.0000i 4.3 Two-Dimensional Plotting One of the major advantages of Matlab are its capabilities for displaying its results in the form of two-dimensional and three-dimensional graphics, providing what has come to be known as scientic visualization. In this section, Matlab capabilities are explained for producing two-dimensional plots in the context of the display of complex variables in the complex plane. In later sections, these two-dimensional graphics will be extended to other variables, in the context of the presentation of those variable types. For more information, type help graph2d. Example: Plotting Complex Variables The plot command can be used to plot complex variables in the complex plane. For example: >> z = 1 + 0.5j; >> plot(z,.) This creates a graphics window, called a Figure window, named by default Figure No. 1, which becomes the active (top) window. The plot produced is shown in Figure 4.8, with z plotted as a point (due to the command .) in the complex plane, with the real value (1.0) on the horizontal (x) axis and the imaginary value (0.5) on the vertical (y) axis. The axes have been scaled automatically, with a range of 0.0 to 2.0 on the horizontal axis and a range of -0.5 to 1.5 on the vertical axis. Numerical scales and tick marks have been added automatically. This plot was exceedingly easy to produce, but it has many deciencies that we can improve upon. Here are some improvements to consider: Control the scaling of the axes Produce a plot that is square instead of rectangular, having the same scale on both axes Include several complex variables on a single plot 72 1.5 1 0.5 0 0.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Figure 4.8: Simple plot of a complex number Label the axes Title the plot Label the plotted complex variables The commands to implement each of these improvements will be introduced and discussed. The resulting plot produced by each command wont be given, so you might try each of them in Matlab as you read this. Finally, when all of the commands have been discussed, a Matlab example session using them will be given and the results will be displayed. 4.3.1 2D Plotting Commands For more information, type help graph2d. Colors and Markers Color and markers can be specied by giving plot an additional argument following the complex variable name. This optional additional argument is a character string (enclosed in single quotes) consisting of characters from the following table: 73 Symbol y m c r g b w k Color yellow magenta cyan red green blue white black Symbol . o x + * s d v ^ < > p h Marker + hexagram Examples: plot(z1,b.) plot(z2,go) plot(z3,r*) % plot variable z1 as a blue point % plot variable z2 as a green circle % plot variable z3 as a red asterisk Customizing Plot Axes The axis command provides control over the scaling and appearance of both the horizontal and vertical axes of a plot. This command has many features, so only the most useful will be discussed here. For more complete information, refer to on-line help. The primary features are given in the following table Command axis([xmin xmax ymin ymax]) axis square axis equal axis normal axis(auto) axis off Description Dene minimum and maximum values of the axes Produce a square plot instead of rectangular Equal scaling factors for both axes Turn o axis square, equal Return the axis to automatic defaults Turn o axis background, labeling, grid, box, and tick marks. Leave the title and any labels placed by the text and gtext commands Turn on axis background, labeling, tick marks, and, if they are enabled, box and grid axis on Adding New Curves Using the hold command to add lines to an existing plot: 74 Command hold on hold off ihold Description Retain existing axes, add new curves to current axes when new plot commands are issued. If the new data does not t within the current axes limits, the axes are rescaled (for automatic scaling only) Releases the current gure window for new plots Logical command that returns 1 (True) if hold is on and 0 (False) if hold is o Plot Grids, Axes Box, and Labels There are several commands to control the appearance of the plot. These include: Command grid on grid off grid box on box off box title(text) xlabel(text) ylabel(text) text(x,y,text) gtext(text) Description Adds dashed grid lines at the tick marks Removes grid lines (default) Toggles grid status (o to on, or on to o) Adds axes box, consisting of boundary lines and tick marks on top and right of plot Removes axes box (default) Toggles box status Labels top of plot with text in quotes Labels horizontal (x) axis with text in quotes Labels vertical (y) axis with text in quotes Adds text in quotes to location (x,y) on the current axes, where (x,y) is in units from the current plot Place text in quotes with mouse: displays the plot window, puts up a cross-hair to be positioned with the mouse, and write the text onto the plot at the selected position when the left mouse button or any keyboard key is pressed Printing Figures and Saving Figure Files Plots can be printed using a gure window menu bar selection or with Matlab commands from the Command window. To print a plot using commands from the menu bar, make the Figure window the active window by clicking it with the mouse. Then select the Print menu item from the File menu. Using the parameters set in the Print Setup or Page Setup menu item, the current plot is sent to the printer. Matlab has its own printing commands that can be executed from the Command window. To print a Figure window, click it with the mouse or use the figure(n) command, where n is the gure number, to make it active, and then execute the print command: >> print % prints the current plot to the system printer 75 The orient command changes the print orientation mode, as follows: Command orient portrait orient landscape orient tall orient Description Prints vertically in middle of page (default) Prints horizontally, stretches to ll the page Prints vertically, stretches to ll the page Displays the current orientation Options to the print command provide for writing the plot to a le in many dierent formats. For example, to write the plot as a PostScript format le in the current directory: print -deps le where le is the name of the le to be written, usually having the le name extension .ps to remind you that it is a PostScript le. To view the contents of a PostScript le, type the following in a UNIX command window: gv le where le is the name of the PostScript le (including the .ps extension). To print a PostScript le, type the following in a UNIX command window: lp le Refer to on-line help for the print command for further information on other le formats and options. 4.3.2 Complex Plot Examples Consider three examples to summarize the concepts of complex numbers, algebra, and geometry, plus the handling of complex numbers and plots by Matlab. Rectangular Form Plot The Matlab commands for a rectangular form plot: z1 = 1.5 + 0.5j; z2 = 0.5 + 1.5j; z3 = z1 + z2; z4 = z1 * z2; z5 = conj(z1); z6 = j * z2; z7 = z2/z1; axis([-3 3 -3 3]); axis square; grid on; 76 hold on; plot(z1,b.); plot(z2,go); plot(z3,rx); plot(z4,m*); plot(z5,c+); plot(z6,kd); plot(z7,kp); text(real(z1)+0.1,imag(z1),z1); text(real(z2)+0.1,imag(z2),z2); text(real(z3)+0.1,imag(z3),z1+z2); text(real(z4)+0.1,imag(z4),z1*z2); text(real(z5)+0.1,imag(z5),z1*); text(real(z6)+0.1,imag(z6),j*z2); text(real(z7)+0.1,imag(z7),z2/z1); xlabel(Real Part); ylabel(Imaginary Part); title(Complex Numbers); The plot that results is shown in Figure 4.9. Complex Numbers 3 z1*z2 2 z2 1 z2/z1 Imaginary Part j*z2 0 z1* 1 z1 z1+z2 2 3 3 2 1 0 Real Part 1 2 3 Figure 4.9: Complex number plot Vector Form Plot Complex numbers can also be represented graphically as arrows, with the base at the origin of the complex plane and the arrow head at the location of the complex number. This represents the 77 complex number as a vector, which will be dened and described in greater detail later in the course. Matlab supports the plotting of complex numbers as vectors through the compass command. Consider the following example, in which two complex numbers z1 and z2 are added to form z3 and the three complex numbers are plotted using compass. z1 = 1.5 + 0.5j; z2 = 0.5 + 1.5j; z3 = z1 + z2; axis([-3 3 -3 3]); axis square; grid on; hold on; plot(z1,b.); plot(z2,go); plot(z3,rx); compass(z1); compass(z2); compass(z3); text(real(z1)+0.1,imag(z1),z1); text(real(z2)+0.1,imag(z2),z2); text(real(z3)+0.1,imag(z3)+0.1,z1+z2); xlabel(Real Part); ylabel(Imaginary Part); title(Compass Plot); The plot that results, shown in Figure 4.10, clearly shows the parallelogram property of the sum of two complex numbers. Polar Form Plot The magnitudes and angles of complex numbers can also be plotted in polar coordinates using the polar command. Consider the following example, in which two complex numbers z1 and z2 are multiplied to form z3 and the three complex numbers are plotted using polar. z1 = z2 = z3 = r1 = phi1 r2 = phi2 r3 = phi3 sqrt(3) + j; 1.5*(1 + j*sqrt(3)); z1 * z2; abs(z1); = angle(z1); abs(z2); = angle(z2); abs(z3); = angle(z3); 78 Compass Plot 3 2 z2 1 Imaginary Part z1 0 z1+z2 1 2 3 3 2 1 0 Real Part 1 2 3 Figure 4.10: Compass plot of complex number addition polar(phi3,r3,*); hold on text(real(z1)+0.1,imag(z1),z1) polar(phi2,r2,+) text(real(z2)+0.1,imag(z2),z2) polar(phi1,r1,o) text(real(z3)+0.1,imag(z3)-0.2,z1*z2) The plot that results, shown in Figure 4.11, clearly shows that the magnitudes multiply and the angles add under complex multiplication. Note that while Matlab usually uses radian units for angles, degrees are used in this plot. 79 90 120 6 z1*z2 60 4 150 2 z2 30 z1 180 0 210 330 240 270 300 Figure 4.11: Polar plot of complex number multiplication 80 Section 5 Arrays and Array Operations Scalars: Variables that represent single numbers, as considered to this point. Note that complex numbers are scalars, even though they have two components, the real part and the imaginary part. Arrays: Variables that represent more than one number. Each number is called an element of the array. Rather than than performing the same operation on one number at a time, array operations allow operating on multiple numbers at once. Row and Column Arrays: A row of numbers (called a row vector) or a column of numbers (called a column vector). Two-Dimensional Arrays: A two-dimensional table of numbers, called a matrix. Array Indexing or Addressing: Indicates the location of an element in the array. 5.1 Vector Arrays First consider one-dimensional arrays: row vectors and column vectors. Consider computing y = sin(x) for 0 x . It is impossible to compute y values for all values of x, since there are an innite number of values, so we will choose a nite number of x values. Consider computing y = sin(x), where x = 0, 0.1, 0.2, . . . , You can form a list, or array of the values of x, and then using a calculator you can compute the corresponding values of y, forming a second array y. These can be written down as: x y 0 0 0.1 .31 0.2 .59 0.3 .81 0.4 .95 0.5 1.0 0.6 .95 0.7 .81 0.8 .59 0.9 .31 0 x and y are ordered lists of numbers, i.e., the rst value or element of y is associated with the rst 81 value or element of x. Elements can be denoted by subscripts, e.g. x1 is the rst element in x, y5 is the fth element in y. The subscript is the index, address, or location of the element in the array. Vector Creation by Explicit List A vector in Matlab can be created by an explicit list, starting with a left bracket, entering the values separated by spaces (or commas) and closing the vector with a right bracket. >> x=[0 .1*pi .2*pi .3*pi .4*pi .5*pi .6*pi .7*pi .8*pi .9*pi pi] x= Columns 1 through 7 0 0.3142 0.6283 0.9425 1.2566 1.5708 1.8850 Columns 8 through 11 2.1991 2.5133 2.8274 3.1416 Matlab functions can be applied to vectors to compute a resulting vector: >> y=sin(x) y= Columns 1 through 7 0 0.3090 Columns 8 through 11 0.8090 0.5878 0.5878 0.3090 0.8090 0.0000 0.9511 1.0000 0.9511 The two vectors produced above are row vectors, each containing one row and 11 columns. They are each also known as a one-by-eleven array, a 1 11 array, or a row vector of length 11. Vector Addressing A vector element is addressed in Matlab with an integer index (also called a subscript) enclosed in parentheses. For example, to access the third element of x and the fth element of y: >> x(3) ans = 0.6283 >> y(5) ans = 0.9511 Colon notation: Addresses a block of elements The format for colon notation is: 82 (start:increment:end) where start is the starting index, increment is the amount to add to each successive index, and end is the ending index, where start, increment, and end must be integers. The increment can be negative, but negative indices are not allowed to be generated. If the increment is to be 1, a shortened form of the notation may be used: (start:end) 1:5 means start with 1 and count up to 5. >> x(1:5) ans = 0 0.3142 0.6283 0.9425 1.2566 7:end means start with 7 and count up to the end of the vector. >> x(7:end) ans = 1.8850 2.1991 2.5133 2.8274 3.1416 3:-1:1 means start with 3 and count down to 1. >> y(3:-1:1) ans = 0.5878 0.3090 0 Thus, a negative index can be used, but the indices generated must all be positive integers. 2:2:7 means start with 2, count up by 2, and stop at 7. >> x(2:2:7) ans = 0.3142 0.9425 1.5708 3:1 means start with 3, count up by 1, and stop at 1. This clearly doesnt make sense, so Matlab generates the error message: >> x(3:1) ans = Empty matrix: 1-by-0 A vector can be used to access elements of an vector in a desired order. To access elements 8, 2, 9, and 1 of y in order: >> y([8 2 9 1]) ans = 0.8090 0.3090 0.5878 0 83 Vector Creation Alternatives Explicit lists are often cumbersome ways to enter vector values. Alternatives: Combining: A vector can also be dened using another vector that has already been dened. For example: >> B = [1.5, 3.1]; >> S = [3.0 B] S= 3.0000 1.5000 3.1000 Changing: Values can be changed by referencing a specic address. For example, >> S(2) = -1.0; >> S S= 3.0000 -1.0000 3.1000 changes the second value in the vector S from 1.5 to 1.0. Extending: Additional values can be added using a reference to a specic address. For example, the following command: >> S(4) = 5.5; >> S S= 3.0000 -1.0000 3.1000 5.5000 extends the length of vector S from 3 to 4. Applying the following command >> S(7) = 8.5; >> S S= 3.0000 -1.0000 3.1000 5.5000 0 0 8.5000 extends the length of S to 7, and the values of S(5) and S(6) are automatically set to zero because no values were given for them. Colon notation: A generalized version of the colon notation used to index array elements can be used to generate array elements. The format for this notation is: (start:increment:end) where start, increment, and end can now be oating point numbers and there is no restriction that the values generated must be positive. Thus, to create x in the example above: 84 >> x=(0:0.1:1)*pi x= Columns 1 through 7 0 0.3142 Columns 8 through 11 2.1991 2.5133 0.6283 2.8274 0.9425 3.1416 1.2566 1.5708 1.8850 As in addressing, (0:5) creates a vector that starts at 0, increments by 1 (implied), and ends at 5. >> x = 0:5 x= 0 1 2 3 4 5 A particular choice of start and increment values may not lead to the generation of a last value equal to end. The last value generated must be less than or equal to end. For example: >> x = 0:2:9 x= 0 2 4 6 8 In this case, the next value to be generated would be 10, but this is not included in the output since it would be greater than 9, the value of end. linspace: This function generates a vector of uniformly incremented values, but instead of specifying the increment, the number of values desired is specied. This function has the form: linspace(start,end,number) The increment is computed internally, having the value: increment = For example: >> x=linspace(0,pi,11) x= Columns 1 through 7 0 0.3142 Columns 8 through 11 2.1991 2.5133 In this example: increment = = 0.1 11 1 end start number 1 0.6283 2.8274 0.9425 3.1416 1.2566 1.5708 1.8850 logspace: This function generates logarithmically spaced values and has the form 85 logspace(start_exponent,end_exponent,number) To create a vector starting at 100 = 1, ending at 102 = 100 and having 11 values: >> logspace(0,2,11) ans = Columns 1 through 7 1.0000 1.5849 2.5119 Columns 8 through 11 25.1189 39.8107 63.0957 3.9811 100.0000 6.3096 10.0000 15.8489 Vector Length To determine the length of a vector array: length(x): Returns the length of vector x. Example: >> x = [0 1 2 3 4 5] x= 0 1 2 >> length(x) ans = 6 3 4 5 Summary of Vector Addressing, Creation, and Length Command x=[2 2*pi sqrt(2) 2-3j] x=start:end x=start:increment:end linspace(start,end,number) logspace(start_exp,end_exp,number) length(x) Description Create row vector x containing elements specied Create row vector x starting with start, counting by one, ending at or before end create row vector x starting with start, counting by increment, ending at or before end Create row vector x starting with start, ending at end, having number elements Create row vector x starting with 10^(start_exp), ending at 10^(end_exp), having number elements Returns the length of vector x. Vector Orientation In the preceding examples, vectors contained one row and multiple columns and were therefore called row vectors. 86 A column vector, having one column and multiple rows, can be created by specifying it element by element, separating element values with semicolons: >> c = [1;2;3;4;5] c= 1 2 3 4 5 Thus, separating elements by spaces or commas species elements in dierent columns, whereas separating elements by semicolons species elements in dierent rows. The transpose operator () is used to transpose a row vector into a column vector >> a = 1:5 a= 1 2 >> b = a b= 1 2 3 4 5 3 4 5 For a complex vector, the transpose operator () produces the complex conjugate transpose (i.e. the sign on the imaginary part is changed as part of the transpose operation). The dot-transpose operator (.) transposes the vector but does not conjugate it. The two transpose operators are identical for real data. >> a = [1+j 2-3j -3+4j] a= 1.0000+ 1.0000i 2.0000- 3.0000i >> b = a b= 1.0000- 1.0000i 2.0000+ 3.0000i -3.0000- 4.0000i >> c = a. c= 1.0000+ 1.0000i 2.0000- 3.0000i -3.0000+ 4.0000i Vectors containing zeros and ones 87 -3.0000+ 4.0000i Two special functions in Matlab can be used to generate new vectors containing all zeros or all ones. zeros(m,1) zeros(1,n) ones(m,1) ones(1,n) Examples: >> x = zeros(3,1) x= 0 0 0 >> y = ones(1,3) y= 1 1 1 Returns Returns Returns Returns an an an an m-element n-element m-element n-element column vector of zeros row vector of zeros column vector of ones row vector of ones 5.2 Matrix Arrays A matrix array is two-dimensional, having both multiple rows and multiple columns. Creation of two-dimensional arrays follows that of row and column vectors: Begin with [, end with ] Spaces or commas are used to separate elements in a row. A semicolon or Enter is used to separate rows. >> f = [1 2 3; 4 5 6] f= 1 2 3 4 5 6 >> g = f g= 1 4 2 5 3 6 >> h = [1 2 3 456 7 8 9] h= 1 2 3 4 5 6 7 8 9 88 >> k = [1 2;3 4 5] ??? Number of elements in each row must be the same. Here f is an array having 2 rows and 3 columns, called a 2 by 3 matrix. Applying the transpose operator to f produces g, a 3 by 2 matrix. Array h is 3 by 3, created using Enter to start new rows. The incorrect attempt to construct k shows that all rows must contain the same number of columns. The transpose operator can be used to generate tables from vectors. For example, generate two vectors, x and y, then display the values such that x(1) and y(1) are on the same line, and so on, as follows: >> x = 0:4; >> y = 5:5:25; >> A = [x y] A= 0 5 1 10 2 15 3 20 4 25 Note that the matrix A has been created from the vectors x and y. Matrix Addressing To refer to the element in row 2 and column 3 of matrix f: >> f(2,3) ans = 6 Thus, the rst number in parentheses addresses the row and the second number (following a comma) addresses the column. Colon in place of a row or column reference represents the entire row or column. To create a column vector from the rst column of the array h above: >> h_1 = h(:,1) h_1 = 1 4 7 Submatrix creation: To create a submatrix using the colon operator, consider: 89 >> c = [1,0,0; 1,1,0; 1,-1,0; 0,0,2] c= 1 0 0 1 1 0 1 -1 0 0 0 2 >> c_1 = c(:,2:3) c_1 = 0 0 1 0 -1 0 0 2 >> c_2 = c(3:4,1:2) c_2 = 1 -1 0 0 Thus, c_1 contains all rows and columns 2 and 3 of c and c_2 contains rows 3 and 4 of columns 1 and 2 of c. Matrix size To determine the size of a matrix array: Command s = size(A) [r,c] = size(A) r = size(A,1) c = size(A,2) n=length(A) whos Examples: >> A = [1 2 3; 4 5 6] A= 1 2 3 4 5 6 >> s = size(A) s= 2 3 >> [r,c] = size(A) r= 2 c= 90 Description For an m n matrix A, returns the two-element row vector s = [m, n] containing the number of rows and columns in the matrix. Returns two scalars r and c containing the number of rows and columns in A, respectively. Returns the number of rows in A in the variable r. Returns the number of columns in A in the variable c. Returns the larger of the number of rows or columns in A in the variable n. Lists variables in the current workspace, together with information about their size, bytes, class, etc. (Long form of who. 3 >> r = size(A,1) r= 2 >> c = size(A,2) c= 3 >> length(A) ans = 3 >> whos Name Size A ans c r s 2x3 1x1 1x1 1x1 1x2 Bytes 48 8 8 8 16 Class double double double double double array array array array array Grand total is 11 elements using 88 bytes Matrices containing zeros and ones Two special functions in Matlab can be used to generate new matrices containing all zeros or all ones. zeros(n) zeros(m,n) zeros(size(A)) ones(n) ones(m,n) ones(size(A) Returns Returns Returns Returns Returns Returns an an an an an an n n array of zeros. m n array of zeros. array the same size as A containing all zeros. n n array of ones. m n array of ones. array the same size as A containing all ones. Examples of the use of zeros: >> A = zeros(3) A= 0 0 0 0 0 0 0 0 0 >> B = zeros(3,2) B= 0 0 0 0 0 0 >> C =[1 2 3; 4 2 5] C= 1 2 3 91 4 2 5 >> D = zeros(size(C)) D= 0 0 0 0 0 0 5.2.1 Array Operations Scalar-Array Mathematics Addition, subtraction, multiplication, and division of an array by a scalar simply apply the operation to all elements of the array. >> f = [1 2 3; 4 5 6] f= 1 2 3 4 5 6 >> g = 2*f -1 g= 1 3 5 7 9 11 Element-by-Element Array-Array Mathematics When two arrays have the same dimensions, addition, subtraction, multiplication, and division apply on an element-by-element basis. The notation for some operations is somewhat unconventional. Operation Addition Subtraction Multiplication Division Exponentiation Examples: >> A = [2 5 6]; >> B = [2 3 5]; >> C = A.*B C= 4 15 30 >> D = A./B D= 1.0000 1.6667 1.2000 >> E = A.^B E= 4 125 Algebraic Form a+b ab ab ab ab Matlab a+b a-b a.*b a./b a.^b 7776 92 >> F = 3.0.^A F= 9 243 729 Array-array mathematics on other than an element-by-element basis will be discussed later. 5.3 Array Plotting Capabilities The plot command introduced previously can be used to produce an xy plot of vector x against vector y, as linear plots with the x and y axes divided into equally spaced intervals. Other plots use a logarithmic scale (base 10) when a variable ranges over many orders of magnitude because the wide range of values can be graphed without compressing the smaller values. Generates a linear plot of the values of x (horizontal axis) and y (vertical axis). semilogx(x,y) Generates a plot of the values of x and y using a logarithmic scale for x and a linear scale for y. semilogy(x,y) Generates a plot of the values of x and y using a linear scale for x and a logarithmic scale for y. loglog(x,y) Generates a plot of the values of x and y using logarithmic scales for both x and y. Note that the logarithm of a negative value or of zero does not exist and if the data contains negative or zero values, a warning message will be printed by Matlab informing you that these data points have been omitted from the data plotted. Examples are shown in Figure 5.4 below. plot(x,y) Plot Linestyles An optional argument to the various plot commands above controls the linestyle. This argument is a character string consisting of one of the characters shown below. This character can be combined with the previously described characters that control colors and markers. Symbol : . Linestyle solid line dotted line dash-dot line dashed line Example 5.1 Vertical motion under gravity An object is thrown vertically upward with an initial speed v, under acceleration of gravity g. Neglecting air resistance, determine and plot the height of the object as a function of time, from time zero when the object is thrown until it returns to the ground. 93 The height of the object at time t is 1 h(t) = vt gt2 2 The object returns to the ground at time tg , when 1 h(tg ) = vtg gt2 = 0 2g or tg = 2v g If the initial velocity v is 60 m/s, the following script will compute and plot the vertical height as a function of the time of ight. % Vertical motion under gravity % Define input values g = 9.8; v = 60; % Calculate times t_g = 2*v/g; t = linspace(0,t_g,256); % Calculate values for h(t) h = v * t - g/2 * t.^2; % acceleration of gravity (m/s^2) % initial speed (m/s) % time (s) to return to ground % 256 element time vector (s) % height (m) % Plot h(t) plot(t, h, :), title(Vertical mtion under gravity), ... xlabel(time (s)), ylabel(Height (m)), grid The resulting plot is shown in Figure 5.1. Note that the option : in the plot command produces a dotted line in the plot. Multiple Curves Multiple curves can be plotted on the same graph by using multiple arguments in a plot command, as in plot(x,y,w,z), where the variables x, y, w and z are vectors. The curve of y versus x will be plotted, and then the curve corresponding to z versus w will be plotted on the same graph. 94 Vertical mtion under gravity 200 180 160 140 120 Height (m) 100 80 60 40 20 0 0 2 4 6 time (s) 8 10 12 14 Figure 5.1: Height-time graph of vertical motion under gravity Another way to generate multiple curves on one plot is to use a single matrix with multiple columns as the second argument of the plot command. Thus, the command plot(x,F), where x is a vector and F is a matrix, will produce a graph in which each column of F is a curve plotted against x. To distinguish between plots on the graph, the legend command is used. This command has the form: legend(string1,string2,string3,...) It places a box on the plot, with the curve type for rst curve labeled with the text string string1, the second curved labeled with the text string string2, and so on. The position of the curve can be controlled by adding a nal integer position argument pos to the legend command. Values of pos and the corresponding positions are: 0 = Automatic best placement (least conict with data) 1 = Upper right-hand corner (default) 2 = Upper left-hand corner 3 = Lower left-hand corner 4 = Lower right-hand corner -1 = To the right of the plot Example 5.2 Multiple plot of polynomial curves Consider plotting the two polynomial functions: f1 = x2 3x + 2 95 Multiple Function Plot 60 f1 f2 50 40 30 20 10 0 10 0 0.5 1 1.5 2 2.5 x 3 3.5 4 4.5 5 Figure 5.2: Plot with two polynomial curves f2 = 2x2 + x 3 % Generate plots of polynomials % x = 0:0.1:5; f(:,1) = x.^2 -3*x + 2; f(:,2) = 2*x.^2 +x - 3; plot(x,f),title(Multiple Function Plot),... xlabel(x), grid, legend(f1,f2) The resulting plot is shown in Figure 5.2 (the line colors dier, allowing the curves to be distinguished from one another). To plot two curves with separate y-axis scaling and labeling, the plotyy function can be used. This command is apparently not well developed, as it does not accept line style options and the legend command causes one of the curves to disappear. Example 5.3 Multiple plot of polynomial curves with separate y axes Again consider plotting the two polynomial functions from the previous example, but with separate y axes. 96 Multiple Axis Plot 12 60 10 50 8 40 6 30 4 20 2 10 0 0 2 0 0.5 1 1.5 2 2.5 x 3 3.5 4 4.5 5 10 Figure 5.3: Plot with two polynomial curves, separate y axes >> x = 0:0.1:5; >> f1 = x.^2 -3*x +2; >> f2 = 2*x.^2 +x -3; >> plotyy(x,f1,x,f2),title(Multiple Axis Plot),... xlabel(x),grid The resulting plot is shown in Figure 5.3 (the line colors dier, allowing the curves to be distinguished from one another). The y axis for function f1 is on the left and the y axis for function f2 is on the right. Multiple Figures As has been described, when the rst plot command is issued in a Matlab session, a new window named Figure No. 1 is created, containing the plot. A subsequent plot command will draw a new graph in this same window. The figure command allows the creation of multiple plot windows, allowing the display of multiple graphs. The command: figure(n) where n is a positive integer, will create a window named Figure No. n. Subsequent plot commands will draw graphs in this window, known as the active window. To reuse a Figure window for a new plot, it must be made the active or current gure. Clicking on the gure o choice with the mouse makes it the active current gure. The command figure(n) 97 makes the corresponding gure active or current. Only the active window is responsive to axis, hold, xlabel, ylabel, title, and grid commands. Figure windows can be deleted by closing them with a mouse, using the conventional method for the windowing system in use on your computer. The current window can be closed with the command: >> close The gure n window can be closed with the command: >> close(n) All gure windows are closed with the command: >> close(all) To erase the contents of a gure window without closing it, use the command: >> clf To erase the contents and also reset all properties, such as hold, to their default states: >> clf reset Subplots The subplot command allows you to split the graph window into subwindows. The possible splits are two subwindows (top and bottom or left and right) or four subwindows (two on top, two on the bottom). subplot(m,n,p): m by n grid of windows, with p specifying the current plot as the pth window. Example 5.4 Subplots of a polynomial function Consider plotting the polynomial y = 5x2 in subplots of dierent plot types. % Generate subplots of a polynomial % 98 x = 0:0.5:50; y = 5*x.^2; subplot(2,2,1),plot(x,y),... title(Polynomial - linear/linear (plot)),... ylabel(y),grid,... subplot(2,2,2),semilogx(x,y),... title(Polynomial - log/linear (semilogx)),... ylabel(y),grid,... subplot(2,2,3),semilogy(x,y),... title(Polynomial - linear/log (semilogy)),... xlabel(x),ylabel(y),grid,... subplot(2,2,4),loglog(x,y),... title(Polynomial - log/log (loglog)),... xlabel(x),ylabel(y),grid The resulting plot is shown in Figure 5.4. Polynomial linear/linear (plot) 14000 12000 10000 8000 y 6000 4000 2000 0 0 10 20 30 40 50 y 14000 12000 10000 8000 6000 4000 2000 0 1 10 10 0 Polynomial log/linear (semilogx) 10 1 10 2 10 5 Polynomial linear/log (semilogy) 10 5 Polynomial log/log (loglog) y 10 0 y 10 1 10 0 0 10 20 x 30 40 50 10 0 10 x 1 10 2 Figure 5.4: Plot with four polynomial subplots 99 Graphics Window Updates The graphics or gure window is updated, or rendered, after each graphics command. This can be a time-consuming task, which can be avoided by entering all of the graphics command for a given gure window on the same line. This has been done in the example above, where all of the graphics commands have eectively been placed on the same line, by separating commands with commas and using . . . to continue commands on the next line. 100 Section 6 Mathematical Functions and Applications Outline Signals Polynomials Partial fraction expansion Functions of two variables User-dened functions Plotting functions 6.1 Signal Representation, Processing, and Plotting As we have seen, one application of a one-dimensional array in Matlab is to represent a function by its uniformly spaced samples. One example of such a function is a signal, which represents a physical quantity such as voltage or force as a function of time, denoted mathematically as x(t). The value of x is known generically as the signal amplitude. Sinusoidal Signal A sinusoidal signal is a periodic signal, which satises the condition x(t) = x(t + nT ), n = 1, 2, 3, . . . where T is a positive constant known as the period. The smallest non-zero value of T for which this condition holds is the fundamental period T0 . Thus, the amplitude of the signal repeats every T0 . 101 Consider the signal x(t) = A cos(t + ) with signal parameters: A is the amplitude, which characterizes the peak-to-peak swing of 2A, with units of the physical quantity being represented, such as volts. t is the independent time variable, with units of seconds (s). is the angular frequency, with units of radians per second (rad/s), which denes the fundamental period T0 = 2/ between successive positive or negative peaks. is the initial phase angle with respect to the time origin, with units of radians, which denes the time shift = / when the signal reaches its rst peak. With so dened, the signal x(t) may also be written as x(t) = A cos (t ) When is positive, it is a time delay, that describes the time (greater than zero) when the rst peak is reached. When is negative, it is a time advance that describes the time (less than zero) when the last peak was achieved. This sinusoidal signal is shown in Figure 6.1. Figure 6.1: Sinusoidal signal A cos(t + ) with /2 < < 0. Consider computing and plotting the following cosine signal x(t) = 2 cos 2t Identifying the parameters: 102 Amplitude A = 2 Frequency = 2. The fundamental period T0 = 2/ = 2/2 = 1s. Phase = 0. A time-shifted version of this signal, having the same amplitude and frequency is y(t) = 2 cos[2(t 0.125)] where the time shift = 0.125s and the corresponding phase is = 2(0.125) = /4. A third signal is the related sine signal having the same amplitude and frequency z(t) = 2 sin 2t Preparing a script to compute and plot x(t), y(t), and z(t) over two periods, from t = 1s to t = 1s: % sinusoidal representation and plotting % t = linspace(-1,1,101); x = 2*cos(2*pi*t); y = 2*cos(2*pi*(t-0.125)); z = 2*sin(2*pi*t); plot(t,x,t,y,t,z),... axis([-1,1,-3,3]),... title(Sinusoidal Signals),... ylabel(Amplitude),... xlabel(Time (s)),... text(-0.13,1.75,x),... text(-0.07,1.25,y),... text(0.01,0.80,z),grid Thus, linspace has been used to compute the time row vector t having 101 samples or elements with values from 1 to 1. The three signals are computed as row vectors and plotted, with axis control, labels and annotation. The resulting plot is shown in Figure 6.2. Observe that x(t) reaches its peak value of 2 at time t = 0, which we can verify as being correct since we know cos 0 = 1. The signal y(t) is x(t) delayed by = 0.125s. The signal z(t) is 0 for t = 0, since sin 0 = 0. It reaches its rst peak at t = T0 /4 = 0.25s, as sin(2 0.25) = sin(/2) = 1. Phasor Representation of a Sinusoidal Signal An important and very useful representation of a sinusoidal signal is as a function of a complex exponential signal. 103 Sinusoidal Signals 3 2 x y 1 z Amplitude 0 1 2 3 1 0.8 0.6 0.4 0.2 0 Time (s) 0.2 0.4 0.6 0.8 1 Figure 6.2: Sinusoidal signals Recall ej = cos + j sin Thus, cos = Re ej As a result, the signal x(t) = A cos(t + ) can be represented as the real part of the complex exponential x(t) = Re Aej(t+) = Re Aej ejt We call Aej ejt the complex representation of x(t) and write x(t) Aej ejt meaning that the signal x(t) may be reconstructed by taking the real part of Aej ejt . In this representation, we call Aej the phasor or complex amplitude representation of x(t) and write x(t) Aej 104 meaning that the signal x(t) may be reconstructed from Aej by multiplying by ejt and taking the real part. The phasor representation of the sinusoid x(t) = A cos(t+) is shown in the complex plane in Figure 6.3. At t = 0, the complex representation produces the phasor Aej , where is approximately Figure 6.3: Rotating phasor /10. If time t increases to time t1 , then the complex representation produces Aej ejt1 From our discussion of complex numbers, we know that ejt1 rotates the phasor Aej through an angle t1 . Therefore, as we increase t from 0, we rotate the phasor from Aej , producing the circular trajectory around the circle of radius A shown in Figure 6.3. When t = 2/, then ejt = ej = 1. Therefore, every 2/ seconds, the phasor revisits any given position on the circle. The quantity Aej ejt is called a rotating phasor whose rotation rate is the frequency : d t = dt The rotation rate is also the frequency of the sinusoidal signal x(t) = A cos(t + ). The real part of the complex, or rotating phasor, representation Aej ejt is the desired signal x(t) = A cos(t + ). This real part is read o of the rotating phasor diagram as shown in Figure 6.4. Consider computing and plotting the phasor x(t) = ejt where = 2000 rad/s and t ranges from 2 103 s to 2 103 s, in steps of 0.02 103 s. Also to be plotted are y(t) = Re[x(t)] and z(t) = Im[x(t)]. The script le is 105 Figure 6.4: Reading a real signal from a complex, rotating phasor % Phasor computation and plot % t = (-2e-03:0.02e-03:2e-03); x = exp(j*2000*pi*t); y = real(x); z = imag(x); subplot(2,1,1),plot(x,:),... axis square,... title(exp(jwt)),... xlabel(Real),... ylabel(Imaginary),... subplot(2,1,2),plot(t,y,-,t,z,:),... title(Re[exp(jwt)] and Im[exp(jwt)] vs t w=1000*2*pi),... xlabel(Time (s)),grid on,... legend(Re[exp(j \omega t)],Im[exp(j \omega t)],-1) and the resulting plot is shown in Figure 6.5, where y(t) is plotted with a solid line and z(t) is plotted with a dotted line. Harmonic Motion A periodic motion that can be described by a sinusoidal function of time is called harmonic motion. A mechanism that produces such motion is a scotch yoke, shown in Figure 6.6. This mechanism is used in machines known as shakers, for testing the behavior of equipment subject to vibrations. The driving element is a rotating disk with a pin mounted a distance A from the center. The pin can slide in the slot of the element marked x-yoke. The motion of the x-yoke is restricted by a guided rod attached to it, so that this yoke can move only horizontally. A similar slotted element, marked y-yoke, is assembled above the x-yoke. The motion of the y-yoke is restricted by a guided rod that allows only vertical displacements. 106 exp(jwt) 1 0.5 Imaginary 0 0.5 1 1 0.5 0 Real 0.5 1 Re[exp(jwt)] and Im[exp(jwt)] vs t w=1000*2*pi 1 0.5 Re[exp(j t)] Im[exp(j t)] 0 0.5 1 2 1.5 1 0.5 0 0.5 Time (s) 1 1.5 x 10 2 3 Figure 6.5: The signals ejt , Re ejt , and Im ejt Figure 6.6: Scotch yoke mechanism 107 The motion is described mathematically by rst assuming that at time t = 0 the position of the pin relative to the x axis is dened by the angle . The x-coordinate of the point P 1 is x(0) = A cos() and the y-coordinate of the point P 2 is y(0) = A sin() If the angular speed of the disk is radians per second, then the x-coordinate of the point P 1 at time t is x(t) = A cos(t + ) and the y-coordinate of the point P 2 is y(t) = A sin(t + ) Using the phasor representation for this motion, we can dene z(t) = Aejt+ and we observe that x(t) is the real part and y(t) is the imaginary part of z(t). The point z can be considered to be the end of a vector with origin at the coordinate origin and magnitude A. This vector rotates with angular velocity , starting from angle . If y(t) is the displacement of the harmonic motion, the velocity is v(t) = dy = A cos(t + ) = A sin(t + + /2) dt and the acceleration is a(t) = d2 y(t) = 2 A sin(t + ) = 2 A sin(t + + ) dt2 We conclude that the velocity of the harmonic motion can be represented by a rotating vector leading the displacement by the phase /2, and the acceleration can be represented by another rotating vector, leading the displacement by the phase angle . Phasor Properties Positive and Negative Frequencies 108 An alternative phasor representation for the signal x(t) = A cos(t + ) is obtained by using the Euler identity cos = which yields x(t) = = A j(t+) e + ej(t+) 2 A j jt A j jt ee + e e 2 2 1 j e + ej 2 In this equation, the term A ej ejt is a rotating phasor that begins at the phasor value A ej 2 2 and rotates counterclockwise with frequency . The term A ej ejt is a rotating phasor that 2 begins at the (complex conjugate) phasor value A ej (for t = 0) and rotates clockwise with 2 (negative) frequency . The physically meaningful frequency for a cosine is , a positive quantity. A negative frequency is not physically meaningful, but just means that the direction of rotation for the rotating phasor is clockwise, not counterclockwise, in the complex exponential representation of the real sinusoid. Thus, the concept of negative frequency is just an artifact of the two-phasor representation. In the one-phasor representation, when we take the real part, the artifact does not arise. You are likely to encounter both the one-phasor and two-phasor representations, so you should be familiar with both. Adding Phasors The sum of two signals with common frequencies but dierent amplitudes and phases is A1 cos(t + 1 ) + A2 cos(t + 2 ). The rotating phasor representation for this sum is A1 ej1 + A2 ej2 ejt The new phasor is A1 ej1 + A2 ej2 and the corresponding real signal is x(t) = Re A1 ej1 + A2 ej2 ejt The new phasor is shown in Figure 6.7, where the parallelogram rule for adding complex numbers applies. 109 Figure 6.7: Adding phasors Beating Between Tones If you have heard two slightly mistuned musical instruments playing pure tones whose frequencies were close but not equal, you have sensed a beating phenomenon in which you perceive a single pure tone whose amplitude slowly varies periodically. The single perceived tone can be shown to have a frequency that is the average of the two mismatched frequencies, amplitude modulated by a tone whose beat frequency is half the dierence between the two mismatched frequencies. This eect is shown in Figure 6.8. To understand this phenomenon, begin with two pure tones whose frequencies are 0 + b and 0 b (for example, 0 = 2 1400 rad/s and b = 2 100 rad/s). The average frequency is 0 and the dierence frequency is 2b . You hear the sum of the two tones: x(t) = A1 cos [(0 + b )t + 1 ] + A2 cos [(0 b )t + 2 ] Assume that the amplitudes are equal, with A = A1 = A2 . The phases may be written as 1 = + with 1 1 = (1 + 2 ) and = (1 2 ) 2 2 Representing x(t) as a complex phasor x(t) = A Re ej[(0 +b )t++] + ej[(0 b )t+] = A Re ej(0 t+) ej(b t+) + ej(b t+) = 2A Re ej(0 t+) cos(b t + ) = 2A cos(0 t + ) cos(b t + ) 110 and 2 = This is an amplitude modulated waveform, in which a low frequency signal with beat frequency b modulates a high frequency signal with carrier frequency 0 rad/s. Over short periods of time, the modulating signal cos(b t + ) remains relatively constant while the carrier term cos(0 + ) produces many cycles of its pure tone. Thus, we perceive the pure tone at the carrier frequency 0 , having an amplitude that varies sinusoidally at the beat frequency b . The following is the script to simulate beating tones in which 0 = 21400 rad/s and b = 2100 rad/s, resulting in the plot shown in Figure 6.8. % beating sinusoidal tones % t = linspace(-1e-2,1e-2,1001); x = cos(2*pi*1500*t) + cos(2*pi*1300*t); m = 2*cos(2*pi*100*t); plot(t,m,b:,t,-m,b:,t,x,k),... axis([-0.01 0.01 -2.4 2.4]),... title(Beating between tones),... xlabel(Time (s)),... ylabel(Amplitude) Beating between tones 2 1.5 1 0.5 Amplitude 0 0.5 1 1.5 2 0.01 0.008 0.006 0.004 0.002 0 Time (s) 0.002 0.004 0.006 0.008 0.01 Figure 6.8: Beating between tones 6.2 Polynomials A polynomial is a function of a single variable that can be expressed in the general form A(s) = a1 sN + a2 sN 1 + a3 sN 2 + + aN s + aN +1 111 where the variable is s and the polynomial coecients are the N + 1 constants a1 , a2 , . . . , aN +1 . The polynomial is of degree N, the largest value used as an exponent. The general form of a degree 3 (cubic) polynomial is A(s) = a1 s3 + a2 s2 + a3 s + a4 and a specic example of a cubic polynomial is A(s) = s3 + 4s3 7s 10 Note that the notation used here is nonstandard, as the coecient of term sk is usually denoted as ak . However, the nonstandard notation is more compatible with the indexing of arrays in Matlab, as will be explained below. For information on Matlab functions supporting polynomial computations, type help polyfun. Polynomial Evaluation There are several ways to evaluate a polynomial for a set of values. Consider the cubic polynomial: A(s) = s3 + 4s2 7s 10 Scalar s: use scalar operations A = s^3 + 4*s^2 - 7*s - 10; Vector or matrix s: use array or element-by-element operations: A = s.^3 + 4*s.^2 - 7*s - 10; The size of the vector or matrix A will be the same as that of s. Using polyval(a,s): Evaluates a polynomial with coecients in vector a for the values in s. The result is a matrix the same size as s. Element a(1) corresponds to coecient a1 . Consider evaluating and plotting A(s) over the interval [-1,3]: s = linspace(-1,3,201); a = [1 4 -7 -10]; A = polyval(a,s); plot(s,A),... title(Polynomial Function A(s) = s^3 + 4s^2 -7s -10),... xlabel(s),... ylabel(A(s)) The resulting plot is shown in Figure 6.9. 112 Polynomial Function A(s) = s3 + 4s2 7s 10 35 30 25 20 15 A(s) 10 5 0 5 10 15 1 0.5 0 0.5 1 s 1.5 2 2.5 3 Figure 6.9: Plot of polynomial function A(s) Polynomial Operations By characterizing polynomial A(s) by coecients ak stored in vector a and polynomial B(s) by coecients bk stored in vector b, algebraic operations can be performed on the two polynomials. Addition: The coecients of the sum of two polynomials is the sum of the coecients of the two polynomials. The vectors containing the coecients must be of the same length. For example, to add A(s) = s4 3s3 s + 2 B(s) = 4s3 2s2 + 5s 16 C(s) = A(s) + B(s) = s4 + (4 3)s3 2s2 + (5 2)s + (2 16) = s4 + s3 2s2 + 4s 14 The script to perform this addition: >> a = [1 -3 0 -1 2]; >> b = [0 4 -2 5 -16]; >> c = a + b c= 1 1 -2 4 -14 113 Thus, C(s) = s4 + s3 2s2 + 4s 14 Scalar multiple: The coecient vector of the scalar multiple of a polynomial is simply the scalar times the coecient vector of the polynomial. To specify C(s) = 3A(s) for A(s) above, the following commands are used: >> a = [1 -3 0 -1 2]; >> c = 3*a c= 3 -9 0 -3 Thus C(s) = 3s4 9s3 3s + 6 Multiplication: Apply the function conv(a,b), which returns the coecient vector for the polynomial resulting from the product of polynomials represented by the coecients in a and b. The vectors a and b dont have to be of the same length. The resulting vector has length equal to the sum of the lengths of a and b minus one. Consider the following polynomial product, evaluated by hand using the rst-outer-innerlast (FOIL) method: F (s) = (s + 2)(s + 3) = s2 + 3s + 2s + 6 = s2 + 5s + 6 Using a Matlab script >> a = [1 2]; >> b = [1 3]; >> c = conv(a,b) c= 1 5 6 Now consider multiplying polynomials A(s) and B(s) above: C(s) = A(s)B(s) = (s4 3s3 s + 2)(4s3 2s2 + 5s 16) Recall from your algebra class that evaluation by hand is a tedious process. Using a Matlab script: >> a = [1 -3 0 -1 2]; >> b = [4 -2 5 -16]; >> c = conv(a,b) c= 4 -14 11 -35 6 58 -9 114 26 -32 Thus, C(s) = 4s7 14s6 + 11s5 35s4 + 58s3 9s2 + 26s 32 Division: You may have learned the long division method for evaluating the division of two polynomials. This method wont be reviewed here, except to remind you that two results are determined: the quotient and the remainder. The result, expressed mathematically, is N (s) R(s) = Q(s) + D(s) D(s) where N (s) is the numerator polynomial, D(s) is the denominator polynomial, Q(s) is the quotient polynomial, and R(s) is the remainder polynomial. The Matlab function to perform polynomial division: [q,r] = deconv(n,d) Returns quotient polynomial coecients q and remainder polynomial coecients r from numerator coecients n and denominator coecients d. Consider the evaluation of polynomial division in which the numerator is C(s) and the denominator is A(s), both from the multiplication example above. The result for the quotient should then be B(s) above and the remainder should be all zeroes. >> c = [4 -14 11 -35 >> a = [1 -3 0 -1 2]; >> [q r] = deconv(c,a) q= 4 -2 5 -16 r= 0 0 0 0 Now consider the following: H(s) = In Matlab: >> n = [1 5 11 13]; >> d = [1 2 4]; >> [q r] = deconv(n,d) q= 1 3 r= 0 0 1 1 Placing the result in mathematical form: H(s) = Q(s) + R(s) D(s) s+1 = s+3+ 2 s + 2s + 4 115 s3 + 5s2 + 11s + 13 N (s) = D(s) s2 + 2s + 4 58 -9 26 -32]; 0 0 0 0 Derivatives: You should be familiar with the rule for dierentiating a polynomial term dn as = nasn1 ds Applying this rule to the polynomial P (s) = s3 + 4s2 7s 10 produces the derivative d P (s) = 3s2 + 8s 7 ds Matlab provides function polyder for polynomial dierentiation. polyder(p) polyder(a,b) [n,d] = polyder(b,a) Returns the coecients of the derivative of the polynomial whose coecients are the elements of vector p. Returns the coecients of the derivative of the product polynomial A(s) B(s). Returns the derivative of the polynomial ratio B(s)/A(s), represented as N (s)/D(s). Conrming the example above using the rst form of polyder: >> p = [1 4 -7 -10]; >> d = polyder(p) d= 3 8 -7 The second form of polyder returns the coecients of the derivative of a product of two polynomials. This is equivalent to multiplying the two polynomials with conv and then dierentiating using the rst form of polyder. Consider applying the second form of polyder to the second example of polynomial multiplication above. The result can be conrmed by dierentiating C(s) above by hand. >> >> >> dc a = [1 -3 0 -1 2]; b = [4 -2 5 -16]; dc = polyder(a,b) = 28 -84 55 -140 174 -18 26 The derivative of a polynomial ratio is more dicult to evaluate by hand. Using the derivative notation f= df , ds recall the quotient rule for dierentiation f g = gf f g g2 116 Thus, for the polynomial ratio H(s) = the derivative is (s + 3) (s + 2) 1 1 dH(s) = = =2 2 2 ds (s + 3) (s + 3) s + 6s + 9 Applying the third form of polyder >> b = [1 2]; >> a = [1 3]; >> [q,d] = polyder(b,a) q= 1 d= 1 6 9 Note that the denominator polynomial d in the result is in expanded, rather than factored form. s+2 s+3 Roots of Polynomials In many engineering problems, there is a need to nd the roots of a polynomial P (s), which are the values of s for which P (s) = 0. When P (s) is of degree N , then there are exactly N roots, which may be repeated roots or complex roots. If the polynomial coecients (a1 , a2 , . . .) are real, then any complex roots will always occur in complex conjugate pairs. For degree one (linear) or two (quadratic), the roots are easily determined. The quadratic equation can be used for degree two, as described earlier. For polynomials of degree 3 and higher, numerical techniques are required to nd the roots. The Matlab function for nding the roots: roots(a) Returns as a vector the roots of the polynomial represented by the coecient vector a. Consider the denominator polynomial from the example above D(s) = (s + 3)2 = s2 + 6s + 9 >> d = [1 6 9]; >> roots(d) ans = -3 -3 117 This conrms that there are two roots at 3 in the denominator. Consider the cubic polynomial P (s) = s3 2s2 3s + 10 The commands to compute and display the roots: >> p = [1,-2,-3,10]; >> r = roots(p) r= 2.0000+ 1.0000i 2.0000- 1.0000i -2.0000 Note that there are 3 roots for the degree 3 polynomial, with a complex conjugate pair of roots and a real root. To verify that these values are roots, evaluate the polynomial at the roots: >> P= polyval(p,r) P= 1.0e-013 * -0.0355+ 0.1377i -0.0355- 0.1377i 0.0711 While P (r) is not exactly zero, due to the limitations on numerical accuracy, for each root it is of the order of 1014 . The roots can be used to express the polynomial in factored form. For example: P (s) = s3 2s2 3s + 10 = (s r1 )(s r2 )(s r3 ) = (s 2 j)(s 2 + j)(s + 2) The coecients of the polynomial can be determined from the roots using the poly function: poly(r) Returns as a row vector the coecients of the polynomial whose roots are contained in the vector r. For example: >> a = poly(r) a= 1.0000 -2.0000 -3.0000 10.0000 Example 6.1 Finding the depth of a well using roots of a polynomial 118 Consider the problem of nding the depth of a well by dropping a stone and measuring the time t to hear a splash. This time is composed of the time t1 of free fall from release to reaching the water and the time t2 that the sound takes to travel from the water surface to the ear of the person dropping the stone. Let g denote the acceleration of gravity, d the well depth (approximately equal to the distance between the hand or the ear of the person and the water surface), and c the speed of sound in air. The depth d, the distance traveled by the stone during time t1 is d= or t1 = 2d/g g2 t 21 The same distance traveled by the sound during t2 is d = c t2 or t2 = d/c The total time is t = t1 + t2 = 2d/g + d/c Squaring the equation above and rearranging the terms d2 2(tc + c2 /g)d + c2 t2 = 0 The depth d is the solution (roots) of the quadratic polynomial equation above. If the measured time t was 2.5s and the speed of sound c in air at atmospheric pressure and 20 Celsius is 343m/s, the following Matlab script can be used to calculate the depth d. % % % t g c Well depth problem Define input values = 2.5; % time to hear splash (s) = 9.81; % acceleration of gravity (m/s^2) = 343; % speed of sound in air (m/s) % Calculate polynomial coefficients a(1) = 1; a(2) = -2*(t*c + c^2/g); 119 a(3) = (c*t)^2; % Find roots corresponding to depth depth = roots(a) The displayed results from this script: depth = 1.0e+004 * 2.5672 0.0029 As is the case in many problems involving roots of a quadratic equation, one of the solutions is not physically reasonable. In this problem, the rst root gives an impossibly large well depth, while the second root gives a reasonable depth. Investigating this solution with Matlab: >> d = depth(2) d= 28.6425 >> t1 = sqrt(2*d/g) t1 = 2.4165 >> t2 = d/c t2 = 0.0835 >> t = t1 + t2 t= 2.5000 Thus, the depth is 28.6m, conrming the time of 2.5s. 6.3 Partial Fraction Expansion A rational function is a ratio of polynomials having the form H(s) = B(s) b1 sm + b2 sm1 + + bm s + bm+1 = A(s) a1 sn + a2 sn1 + + an s + an+1 For m < n, H(s) is known as a proper rational function and for m n, it is known as an improper rational function. Denoting the roots of the denominator by r1 , r2 , . . . rn , A(s) can be written in factored form as A(s) = a1 (s r1 )(s r2 ) (s rn ) 120 and H(s) can be written as B(s) a1 (s r1 )(s r2 ) (s rn ) H(s) = Partial fraction expansion is a technique to express H(s) as a sum of terms. Expansion of Proper Rational Functions For a proper rational function (m < n), there are three dierent cases of partial fraction expansion that must be considered: 1. Distinct (nonrepeated) real roots. 2. Distinct complex roots. 3. Repeated roots. Distinct Real Roots When the roots r1 , r2 , . . . , rn are distinct and real, then by partial fraction expansion H(s) can be expressed as H(s) = c2 cn c1 + + + s r1 s r2 s rn where the constants ci are called the residues, which can be computed using the residue command: [c,r] = residue(b,a) nds the residues c and roots r of a partial fraction expansion of the ratio of two polynomials B(s)/A(s). Vectors b and a specify the coecients of the numerator and denominator polynomials in descending powers of s. The residues are returned in the column vector c and the roots in column vector r. Example: s+2 + 4s2 + 3s H(s) = s3 >> b = [1 2]; >> a = [1 4 3 0]; >> [c,r] = residue(b,a) c= -0.1667 -0.5000 0.6667 121 r= -3 -1 0 H(s) = 0.5000 0.6667 0.1667 + s (3) s (1) s0 1/2 2/3 1/6 + = s+3 s+1 s Distinct Complex Roots Partial fraction expansion applies as well to distinct complex roots. Note that if root r1 is complex, then the complex conjugate r1 is also a root. It can also be shown that the residue c2 corresponding to the root r2 is equal to the conjugate c of the residue corresponding to the root r1 . 1 Example: s2 2s + 1 s3 + 3s2 + 4s + 2 H(s) = >> b = [1 -2 1]; >> a = [1 3 4 2]; >> [c,r] = residue(b,a) c= -1.5000+ 2.0000i -1.5000- 2.0000i 4.0000 r= -1.0000+ 1.0000i -1.0000- 1.0000i -1.0000 H(s) = 1.5 + j2 1.5 j2 4 + + s+1j s+1+j s+1 Repeated Roots Again consider the general case where m < n and H(s) = B(s) A(s) 122 Suppose that root r1 of A(s) is repeated p times and the other np roots (denoted rp+1 , rp+2 , . . . , rn ) are distinct. Then H(s) has the partial fraction expansion H(s) = c2 cp cp+1 cn c1 + + + + + + 2 p s r1 (s r1 ) (s r1 ) s rp+1 s rn Example: H(s) = 5s 1 s3 3s 2 >> b = [5 -1]; >> a = [1 0 -3 -2]; >> [c,r] = residue(b,a) c= 1.0000 -1.0000 2.0000 r= 2.0000 -1.0000 -1.0000 H(s) = 1 1 2 + s 2 s + 1 (s + 1)2 Expansion of Improper Rational Functions Again consider the rational function B(s) A(s) H(s) = with the degree of B(s) greater than or equal to the degree of A(s) (m n). By polynomial division, H(s) can be written in the form R(s) A(s) H(s) = Q(s) + where the quotient Q(s) is a polynomial in s with degree m n, and the remainder R(s) is a polynomial in s with degree strictly less than n. Thus, R(s)/A(s) is a proper rational function that can be expanded using partial fraction expansion. The expansion of an improper rational function in Matlab is computed with a variation of the residue function: 123 [c,r,q] = residue(b,a) nds the residues, roots and quotient of a partial fraction expansion of the ratio of two polynomials B(s)/A(s). The coecients of the quotient Q(s) are returned in the row vector q. The number of roots is n = length(a)-1 = length(c) = length(r). The quotient coecient vector is empty if length(b) < length(a), otherwise length(q) = length(b)-length(a)+1. Example: s3 + 2s 4 s2 + 4s 2 H(s) = >> b = [1 0 2 -4]; >> a = [1 4 -2]; >> [c,r,q] = residue(b,a) c= 20.6145 -0.6145 r= -4.4495 0.4495 q= 1 -4 H(s) = s 4 + 20.6145 0.6145 s + 4.4495 s 0.4495 Recovering the Rational Function The coecients of the rational function can be recovered from the residues, the roots, and the coecients of the quotient term using yet another form of the residue function. [b,a] = residue(c,r,q), with three input arguments and two output arguments, converts the partial fraction expansion back to the polynomials with coecients in b and a. Consider the continuation of the example above. >> [b,a] = residue(c,r,q) b= 1.0000 0 2.0000 a= 1 4 -2 s2 + 2s 4 s2 + 4s 2 -4.0000 H(s) = 124 6.4 Functions of Two Variables To evaluate a function of two variables f (x, y), rst dene a two-dimensional grid in the xy plane, then evaluate the function at the grid points to determine points on the three-dimensional surface. This is shown in Figure 6.10, where the surface values z = f (x, y) are plotted above the grid of xy values. Figure 6.10: Function of two variables plotted in three dimensions A two-dimensional grid in the xy plane is dened in Matlab by two vectors, one containing the x-coordinates at all the points in the grid, and the other containing the y-coordinates. For example, to dene a grid in x varying from 2 to 2 in increments of 1 and a grid in y varying from 1 to 2 in increments of 1, using colon notation: >> x = -2:2 x= -2 -1 >> y = -1:2 y= -1 0 0 1 2 1 2 The meshgrid function generates two matrices that dene the underlying grid for a two-dimensional function. [X,Y] = meshgrid(x,y) Transforms the domain specied by vectors x and y into arrays X and Y that can be used for the evaluation of functions of two variables and 3-D surface plots. The rows of the output array X are copies of the vector x and the columns of the output array Y are copies of the vector y. If x has length n and y has length m, then X and Y are m n arrays. Abbreviation for [X,Y] = meshgrid(x,x) [X,Y] = meshgrid(x) For example: >> [X,Y] = meshgrid(x,y) 125 X= -2 -2 -2 -2 Y= -1 0 1 2 -1 0 1 2 -1 0 1 2 -1 0 1 2 -1 0 1 2 -1 -1 -1 -1 0 0 0 0 1 1 1 1 2 2 2 2 After the underlying grid matrices have been dened, the corresponding function values can be computed. For example, for the following function f (x, y) = ye(x 2 +y 2 ) the function values would be computed as Z = Y.* exp(-(X.^2 + Y.^2)); The operations are element-by-element, so the value Z(1,1) is computed from X(1,1) and Y(1,1), and so on. Note that Z must be computed from X and Y and not from x and y, a common error. Three-dimensional plots Among several ways to plot a three-dimensional (3D) surface in Matlab, a mesh plot and a surface plot will be described, followed by a description of a contour plot. For further information on 3D plots, type help graph3d. mesh(x_pts,y_pts,Z) Generates an open mesh plot of the surface dened by matrix Z. The arguments x_pts and y_pts can be vectors dening the ranges of the values of the x- and y-coordinates, or they can be matrices dening the underlying grid of x- and ycoordinates. Generates a shaded mesh plot of the surface dened by matrix Z. The arguments x_pts and y_pts can be vectors dening the ranges of the values of the x- and y-coordinates, or they can be matrices dening the underlying grid of x- and ycoordinates. surf(x_pts,y_pts,Z) For example, to plot the function f (x, y) in the example above, the following commands are executed % Mesh and surface plots of a function of two variables % x = -2:0.1:2; y = -1:0.1:2; 126 [X Y] = meshgrid(x,y); Z = Y.*exp(-(X.^2 + Y.^2)); subplot(2,1,1),mesh(X,Y,Z),... title(Mesh Plot),xlabel(x),... ylabel(y),zlabel(z),... subplot(2,1,2),surf(X,Y,Z),... title(Surface Plot),xlabel(x),... ylabel(y),zlabel(z) Note that the xy grid has been made ner by incrementing both x and y by 0.1. Also note that the arguments X and Y could have been replaced by x and y in both the mesh and surf commands. The resulting plots are shown in Figure 6.11. Note that you need to know something about the Mesh Plot 0.5 0 z 0.5 2 1 0 1 y Surface Plot 2 1 x 0 1 2 0.5 0 z 0.5 2 1 0 1 y 2 1 x 0 1 2 Figure 6.11: Mesh and surface plots of a function of two variables properties of the two-dimensional function f (x, y) to know what range of values on x and y that you want it to be plotted. A contour plot is an elevation or topographic map consisting of curves representing equal elevations or values of z, called contours of constant elevation. 127 contour(x,y,Z) Generates a contour plot of the surface dened by the matrix Z. The arguments x and y are vectors dening the ranges of values of the x- and y-coordinates. The number of contour lines and their values are chosen automatically. Generates a contour plot of the surface dened by the matrix Z. The arguments x and y are vectors dening the ranges of values of the x- and y-coordinates. The vector v denes the values to use for the contour lines. Generates an open mesh plot of the surface dened by the matrix Z. The arguments x_pts and y_pts can be vectors dening the ranges of values of the x- and y-coordinates or they can be matrices dening the underlying grid of x- and y-coordinates. In addition, a contour plot is generated below the mesh plot. contour(x,y,Z,v) meshc(x_pts,y_pts,Z) The commands to produce the contour plot shown in Figure 6.12 from the function f (x, y) considered in the examples above: contour(x,y,Z),... title(Contour Plot),xlabel(x),... ylabel(y),grid Contour Plot 2 1.5 1 0.5 y 0 0.5 1 2 1.5 1 0.5 0 x 0.5 1 1.5 2 Figure 6.12: Contour plot of a function of two variables The commands to produce the mesh/contour plot shown in Figure 6.13 from the function f (x, y) considered in the examples above: meshc(X,Y,Z),... 128 title(Mesh/Contour Plot),xlabel(x),... ylabel(y),zlabel(z) Mesh/Contour Plot 0.5 z 0 0.5 2 1.5 1 0.5 0 0.5 y 1 2 1 x 0 1 2 Figure 6.13: Mesh/contour plot of a function of two variables 6.5 User-Dened Functions For more information, type help function in Matlab. If you nd that you are often building a function from several Matlab commands, you can develop a user-dened function that can can be used in a same way as the built-in Matlab functions. A user-dened function is similar to a script le in that it is a text le having a .m extension and it is thus called a function M-le. Their variables are local, meaning that their values are available only within the function. They are the building blocks of larger scripts, facilitating a modular approach to the development of scripts. For example, consider writing a function to compute the sine of an angle, with the angle in degrees rather than radians. The user-dened function is the following: function y = sind(x) % SIND Sine in degrees % SIND(X) is the sine of the elements of X, in degrees y = sin(x*pi/180); This function is written to a le named sind.m. Matlab programs and scripts can refer to this function in the same way that they refer to functions such as sqrt and abs. An example of the use of this to simplify a command in the script written for Example 4.2 is: 129 >> AC = 245/sind(30) AC = 490.0000 The rules for writing an M-le function are the following: 1. Function denition line: The rst line of a function has the following syntax: function [output variables] = function_name(input variables); Thus, it must contain the word function, followed by the output variables, an equal sign, and the name of the function. The input variables, called arguments, of the function follow the function name and are enclosed in parentheses. This line distinguishes the function le from a script le. The denition line for the example above is: function y = sind(x) The output variable is y, the function name is sind, and the input argument is x. 2. Function call: A user-dened function is called by the name of the M-le in which it is dened, not by the name given the function in the rst line of the le. Thus, if the function script above were renamed dsin.m, but the script itself were unchanged, then it would have to be called by the name dsin, as follows: >> y = dsin(30) y= 0.5000 An attempt to call it by the function name sind results in an error message: >> y = sind(30) ??? Undefined function or variable sind. To avoid confusion, use the same name for the function and the M-le. 3. Comments: The rst few lines should be comments, as they will be displayed if help is requested for the function name. The rst comment line is referenced by the lookfor command. Each comment line must start with a percent character (%). For the example above: >> help sind SIND Sine in degrees SIND(X) is the sine of the elements of X, in degrees 4. Information returned: The only information returned from the function is contained in the output variables (also called output arguments). A statement must always be included that assigns a value to the output variables specied in the function denition line. These output variables will be arrays. Thus, while we thought of the function sind as operating on a scalar and returning a scalar, it can also operate on an array and return an array: 130 >> lengths = 100*sind([30 60 90; 120 150 180]) lengths = 50.0000 86.6025 100.0000 86.6025 50.0000 0.0000 Note that output variables are optional. This allows a function to be written to perform an operation such as toggling diary, but not to return any information. 5. Communication: A function communicates with the Matlab workspace only through the variables passed to it and through the output variables it creates. Intermediate variables within the function do not appear in, or interact with, the Matlab workspace. Thus, each function has its own workspace separate from the Matlab workspace. Variables in the function M-le that are not output are input variables are said to be local to the function. 6. Multiple outputs: To return more than one output variable, they must be listed in a vector, separated by commas, as in the following example that will return three variables: function [distance, velocity, accel] = position(x) Since the function is returning multiple arguments, it must be used as follows: >> [d v a] = position(A) 7. Multiple inputs: When there are multiple input arguments, they must be separated by commas. For example: function c = hypot(a,b) 8. Semicolons: The use of semicolons (;) at the end of commands in a function script have the same purpose that they serve an any other Matlab command, suppressing display of the results of the command. In most cases, it is not desirable to display the results of commands internal to a function. Many of the commands available in Matlab are written as function M-les. You can nd the locations of these les using the which command. For example: >> which linspace /usr/pkg/matlab52/toolbox/matlab/elmat/linspace.m You could display this M-le using the type command and take ideas from the function script for use in writing your own function scripts. Application: Minimizing a Function of One Variable To nd the minimum of a function of a single variable: 131 x = fminbnd(F,x1,x2) [x,fval] = fminbnd(...) For example: >> fminbnd(cos,0,4) ans = 3.1416 Returns a value of x that is the local minimizer of F(x) in the interval [x1, x2], where F is a string containing the name of the function. Also returns the value of the objective function, fval, computed in F, at x. To use this command to nd the minimum of more complicated functions, it is convenient to dene the function in a function M-le. For example, consider the polynomial function y = 0.025x5 0.0625x4 0.333x3 + x2 Dening the function le fp5.m: function y = fp5(x) % FP5, fifth degree polynomial function y = 0.025*x.^5 - 0.0625*x.^4 - 0.333*x.^3 + x.^2 Observe in Figure 6.5 that this function has two minima in the interval 1 x 4. The minimum near x = 3 is called a relative or local minimum because it forms a valley whose lowest point is higher than the minimum at x = 0. The minimum at x = 0 is the true minimum and is also called the global minimum. Searching for the minimum over the interval 1 x 4: >> xmin = fminbnd(fp5,-1,4) xmin = 2.0438e-006 The resulting value for xmin is essentially 0, the true minimum point. Searching for the minimum over the interval 0.1 x 2.5: >> xmin = fminbnd(fp5,0.1,2.5) xmin = 0.1001 This misses the true minimum point, as it is not included in the specied interval. Also, fminbnd can give incorrect answers. If the interval is 1 x 4: >> xmin = fminbnd(fp5,1,4) xmin = 2.8236 132 4.5 4 3.5 3 2.5 y 2 1.5 1 0.5 0 1 0.5 0 0.5 1 1.5 x 2 2.5 3 3.5 4 Figure 6.14: Plot of the function y = 0.025x5 0.0625x4 0.333x3 + x2 The result corresponds to the valley shown in the plot, but which is not the minimum point on this interval, which is at the boundary x = 1. The fminbnd function rst looks for a minimum point corresponding to a zero slope; if it nds one, it stops. If it does not nd one, it looks at the function values at the boundaries of the specied interval for x. In this example, a zero-slope minimum was found, so the true minimum at the boundary was missed. 6.6 Plotting Functions As described previously, a function such as a polynomial can be evaluated and then plotted with the plot command. This can also be done in one step with the fplot command. fplot(fun,lims) plots the function specied by the string fun between the x-axis limits specied by lims = [xmin xmax]. Using lims = [xmin xmax ymin ymax] also controls the y-axis limits. fun must be the name of an M-le function or a string with variable x, such as sin(x), diric(x,10) or [sin(x),cos(x)]. The function fun(x) must return a row vector for each element of vector x. Consider the plotting the following function, known as the sinc or sampling function: sinc(x) = sin(x) x This is a function available in Matlab, which can be plotted with the command: fplot(sinc,[-10 10]),ylabel(sinc(x)),xlabel(x) This generates the plot shown in Figure 6.15 133 1 0.8 0.6 0.4 sinc(x) 0.2 0 0.2 0.4 10 8 6 4 2 0 x 2 4 6 8 10 Figure 6.15: Sinc signal 134 Section 7 Data Analysis Outline Maximum and minimum Sums and products Statistical analysis Random number generation The purpose of data analysis is to determine some properties and characteristics of the data, which may contain measurement errors or other random inuences. For the examples to be considered in this section, generate, save, and plot the two data vectors, data1 data2 and as shown below. The commands used to generate this data will be explained later. The two data vectors, data_1 and data_2 are plotted in Figure 7.1. data1 = 2*rand(1,500) + 2; data2 = randn(1,500)+3; save data subplot(2,1,1),plot(data1),... axis([0 500 0 6]),... title(Random Numbers - data1),... subplot(2,1,2),plot(data2),... title(Random Numbers - data2),... xlabel(Index, k) Other data analysis functions available in Matlab are described with the command help datafun. 135 Random Numbers data1 6 5 4 3 2 1 0 0 50 100 150 200 250 300 350 400 450 500 Random Numbers data2 6 5 4 3 2 1 0 0 50 100 150 200 250 Index, k 300 350 400 450 500 Figure 7.1: Random sequences 7.1 Maximum and Minimum Returns largest value in vector x, or the row vector of largest elements of each column in matrix x Returns maximum values y and corresponding indices k of the rst maximum value from each column of x. Returns a matrix the same size as x and y, with each element being the maximum value from the corresponding positions in x and y. Returns smallest value in vector x, or the row vector of smallest elements of each column in matrix x Returns minimum values y and corresponding indices k of rst minimum value from each column of x. Returns a matrix the same size as x and y, with each element being the minimum value from the corresponding positions in x and y. max(x) [y,k] = max(x) max(x,y) min(x) [y,k] = min(x) min(x,y) Determining the maximum and minimum of a vector: >> v = [3 -2 4 -1 5 0]; >> max(v) ans = 5 >> [vmin, kmin] = min(v) vmin = 136 -2 kmin = 2 The maximum value is found to be 5, the minimum value 2, and the index of the minimum value is 2. Thus, vmin = v(kmin) = v(2) = -2. For the random data vectors data1 and data2: >> max(data1) ans = 3.9985 >> min(data1) ans = 2.0005 >> [max2,kmax2] = max(data2) max2 = 5.7316 kmax2 = 256 >> [min2,kmin2] = min(data2) min2 = 0.3558 kmin2 = 268 These results can be approximately conrmed from Figure 7.1. For a matrix, the min and max functions return a row vector of the minimum or maximum elements of each column of the matrix. >> B = [-1 B= -1 -3 1 >> min(B) ans = -3 >> max(B) ans = 1 1 7 0; -3 5 5 8; 1 4 4 -8] 1 5 4 7 5 4 0 8 -8 1 4 -8 5 7 8 Example 7.1 Minimum cost tank design A cylindrical tank with a hemispherical top, as shown in Figure 7.2, is to be constructed that will hold 5.00 105 L when lled. Determine the tank radius R and height H to minimize the tank 137 cost if the cylindrical portion costs $300/m2 of surface area and the hemispherical portion costs$400/m2 . Figure 7.2: Tank conguration Mathematical model: Cylinder volume: Vc = R2 H Hemisphere volume: Vh = 2 R3 3 Cylinder surface area: Ac = 2RH Hemisphere surface area: Ah = 2R2 Assumptions: Tank contains no dead air space. Concrete slab with hermetic seal is provided for the base. Cost of the base does not change appreciably with tank dimensions. Computational method: Express total volume in meters cubed (note: 1m3 = 1000L) as a function of height and radius Vtank = Vc + Vh For Vtank = 5 105 L = 500m3 : 2 500 = R2 H + R3 3 Solving for H: H= 500 2R 2 R 3 Express cost in dollars as a function of height and radius C = 300Ac + 400Ah = 300(2RH) + 400(2R2 ) 138 Method: compute H and then C for a range of values of R, then nd the minimum value of C and the corresponding values of R and H. To determine the range of R to investigate, make an approximation by assuming that H = R. Then from the tank volume: 2 5 Vtank = 500 = R3 + R3 = R3 3 3 Solving for R: R= From Matlab: >> Rest = (300/pi)^(1/3) Rest = 4.5708 We will investigate R in the range 3.0 to 7.0 meters. Computational implementation: Matlab script to determine the minimum cost design: % % % R H C Tank design problem Compute H & C as functions of R = 3:0.001:7.0; = 500./(pi*R.^2) - 2*R/3; = 300*2*pi*R.*H + 400*2*pi*R.^2; 300 1 3 % Generate trial radius values R % Height H % Cost % Plot cost vs radius plot(R,C),title(Tank Design),... xlabel(Radius R, m),... ylabel(Cost C, Dollars),grid % Compute and display minimum cost, corresponding H & R [Cmin kmin] = min(C); disp(Minimum cost (dollars):) disp(Cmin) disp(Radius R for minimum cost (m):) disp(R(kmin)) disp(Height H for minimum cost (m):) disp(H(kmin)) 139 Running the script: Minimum cost (dollars): 9.1394e+004 Radius R for minimum cost (m): 4.9240 Height H for minimum cost (m): 3.2816 Note that the radius corresponding to minimum cost (Rmin = 4.9240 is close to the approximate value, Rest = 4.5708 that we computed to assist in the selection of a range of R to investigate. The plot of cost C versus radius R is shown in Figure 7.3. 1.15 x 10 5 Tank Design 1.1 Cost C, Dollars 1.05 1 0.95 0.9 3 3.5 4 4.5 5 Radius R, m 5.5 6 6.5 7 Figure 7.3: Tank design problem: cost versus radius 7.2 Sums and Products If x is a vector with N elements denoted x(n), n = 1, 2, . . . , N , then: The sum y is the scalar y= N n=1 x(n) = x(1) + x(2) + + x(N ) 140 The product y is the scalar y= N n=1 x(n) = x(1) x(2) x(N ) The cumulative sum y is the vector having elements y(k), k = 1, 2, . . . , N y(k) = k n=1 x(n) = x(1) + x(2) + + x(k) The cumulative product y is the vector having elements y(k), k = 1, 2, . . . , N y(k) = k n=1 x(n) = x(1) x(2) x(k) If X is a matrix with M rows and N columns with elements denoted x(m, n), m = 1, 2, . . . , M, n = 1, 2, . . . , N , then The column sum y is the vector having elements y(n), n = 1, 2, . . . , N y(n) = M m=1 x(m, n) = x(1, n) + x(2, n) + + x(M, n) The column product y is the vector having elements y(n), n = 1, 2, . . . , N y(n) = M m=1 x(m, n) = x(1, n) x(2, n) x(M, n) The cumulative column sum Y is the matrix with M rows and N columns with elements denoted y(k, n), k = 1, 2, . . . , M, n = 1, 2, . . . , N y(k, n) = k m=1 x(m, n) = x(1, n) + x(2, n) + + x(k, n) The cumulative column product Y is the matrix with M rows and N columns with elements denoted y(k, n), k = 1, 2, . . . , M, n = 1, 2, . . . , N y(k, n) = k m=1 x(m, n) = x(1, n) x(2, n) x(k, n) The Matlab functions implementing these mathematical functions are the following. 141 sum(x) prod(x) cumsum(x) cumprod(x) Returns the sum of the elements in vector x, or the row vector of the sum of elements of each column in matrix x Returns the product of the elements in vector x, or the row vector of the product of elements of each column in matrix x Returns a vector the same size as x containing the cumulative sum of the elements in vector x, or a matrix the same size as x containing cumulative sums of values from the columns of x. Returns a vector the same size as x containing the cumulative product of the elements in vector x, or a matrix the same size as x containing cumulative products of values from the columns of x. Example 7.2 Summation The following is a summation identity N n=1 n= N (N + 1) 2 This identity can be checked with Matlab, for example, for N = 8: >> N = 8; >> n =1:8; >> S = sum(n) S= 36 >> N*(N+1)/2 ans = 36 Example 7.3 Factorial The factorial of n is expressed and dened as n! = n (n 1) (n 2) 1 This can be computed as prod(1:n), as in the following examples: >> nfact = prod(1:4) nfact = 24 >> nfact = prod(1:70) nfact = 1.1979e+100 142 7.3 Statistical Analysis Many engineering measurements have an element of randomness. Statistics is a branch of applied mathematics involving the analysis, interpretation, and presentation of data including some degree of randomness or uncertainty. In descriptive statistics, the important features or properties of a set of data are summarized or described. Continuous random variable: A variable vector x, whose elements can have any of a continuum of values for each measurement, or observation. Population: all possible measurements of a random variable Sample: nite number of measurements. Assume that N measurements have been made of a random variable, denoted as the vector x, with elements x(n), n = 1, . . . , N . Frequency distribution of a random variable: indicates the relative frequency with which specic values of this random variable occur in the population. Histogram: frequency distribution of sample data, indicating the frequency with which sample values fall within ranges of values, called bins. Range of values: xmin x xmax Number of bins: m xmax xmin Bin width: x = m Histogram value in bin k: N (k), the number of samples with value xmin + (k 1)x x xmin + kx. This is also known as the absolute frequency. The Matlab commands for generating and plotting a histogram are; 143 Command N = hist(x) N = hist(x,m) N = hist(x,xc) N = histc(x,edges) [N,xc] = hist(...) hist (...) Description Returns the row vector histogram N of the values in x using 10 bins. If x is matrix, returns a histogram matrix N operating on the columns of x. Returns the row vector histogram N of the values in x using m bins. Returns the row vector histogram N of the values in x using bin centers specied by xc. Counts the number of values in vector x that fall between the elements in the edges vector (which must contain monotonically non-decreasing values). N is a length(edges) vector containing these counts. N(k) will count the value x(i) if edges(k) >= x(i) > edges(k+1). The last bin will count any values of x that match edges(end). Values outside the values in edges are not counted. Use -inf and inf in edges to include all non-NaN values. Also returns the position of the bin centers in xc. Without output arguments produces a histogram bar plot of the results. For example, the following commands produce the 25-bin histograms shown in Figure 7.4, operating on the random data vectors data1 and data2. % Histogram plots % subplot(2,1,1),hist(data1,25),... title(Histogram of data1),... xlabel(x),... ylabel(N),... subplot(2,1,2),hist(data2,25),... title(Histogram of data2),... xlabel(x),... ylabel(N) Note the dierences in the nature of the two distributions. Random data data1 is called a uniform distribution, as it has roughly equal, or uniform, frequency in all bins. Random data data2 has a bell-shaped distribution, called a Gaussian distribution or Normal distribution. Relative frequency histogram: The histogram value in bin k is normalized by the total number, N , of data samples: N (k)/N . The hist function is limited in its ability to produce relative frequency histograms. It is better to generate such plots with the bar function, dened as follows: 144 Histogram of data1 35 30 25 20 N 15 10 5 0 2 2.2 2.4 2.6 2.8 3 x 3.2 3.4 3.6 3.8 4 Histogram of data2 60 50 40 N 30 20 10 0 0 1 2 3 x 4 5 6 Figure 7.4: Absolute frequency histograms bar(X,Y) bar(Y) bar(x,y bar(X,Y,width) Draws the columns of the M-by-N matrix Y as M groups of N vertical bars. The vector X must be monotonically increasing or decreasing. Uses the default value of X=1:M. For vector inputs, length(y) bars are drawn, with each bar centered on a value of x. Species the width of the bars. Values of width 1 produce overlapped bars. The default value is width=0.8 For example, the following commands produce relative histogram plots shown in Figure 7.5 for the random data vectors data1 and data2. % Relative frequency histogram plots % n1 = length(data1); [freq1,x1] = hist(data1,25); rfreq1 = freq1/n1; n2 = length(data2); [freq2,x2] = hist(data2,25); rfreq2 = freq2/n2; subplot(2,1,1),bar(x1,rfreq1),... title(Relative histogram of data1), grid,... xlabel(x), ylabel(relative frequency),... subplot(2,1,2),bar(x2,rfreq2),... title(Relative histogram of data2), grid,... xlabel(x), ylabel(relative frequency) 145 Relative histogram of data1 0.07 0.06 relative frequency 0.05 0.04 0.03 0.02 0.01 0 2 2.2 2.4 2.6 2.8 3 x 3.2 3.4 3.6 3.8 4 Relative histogram of data2 0.12 0.1 relative frequency 0.08 0.06 0.04 0.02 0 0 1 2 3 x 4 5 6 Figure 7.5: Relative frequency histograms Measures of central tendency The mean and median describe the middle, or center, of the range of the random variable. The sample mean of vector x having N elements is x x= 1 N N n=1 x(m) The population mean (mu) is the value of x for an innite number of measurements, denoted by = lim x N where the right hand side is read as the limit of x bar as N approaches innity. Median: Middle value of a set of random samples, sorted by value (rank ordered). If there is an even number of samples, then the median is the average of the two middle sample values. Command mean(x) median(x) sort(x) Description Returns the sample mean of the elements of the vector x. Returns a row vector of the sample means of the columns of matrix x. Returns the median of the elements of the vector x. Returns a row vector of the medians of the columns of matrix x. Returns a vector with the values of vector x in ascending order. Returns a matrix with each column of matrix x in ascending order. 146 For example: >> mean(data1) ans = 2.9940 >> sum(data1)/length(data1) ans = 2.9940 >> mean(data2) ans = 2.9768 >> median(data1) ans = 3.0285 >> median(data2) ans = 3.0574 These results can be conrmed by observing the plots of the data in Figure 7.1, in which the data values can be seen to be centered on 3.0. For the two data sets considered, the values of the mean and median are very close. This is not necessarily the case for other data sets. Also note that the results above show that mean(data1) = sum(data1)/length(data1). Measures of variation Measures of variation: indicate the degree of deviation of random samples from the measure of central tendency. Referring again to the plots of our random data sets data1 and data2 in Figure 7.1, observe that data2 has greater variation from the mean. The sample standard deviation of vector x having N elements is N 1 s= (x(n) x)2 N 1 n=1 1 2 The sample variance s2 is the square of the standard deviation. std(x) Returns the sample standard deviation of the elements of the vector x. Returns a row vector of the sample standard deviations of the columns of matrix x. For example: >> std(data1) 147 ans = 0.5989 >> std(data2) ans = 0.9408 Thus, the variation of data2 is greater than that of data1, as we concluded from observation of the plotted data values. 7.4 Random Number Generation Many engineering problems require the use of random numbers in the development of a solution. In some cases, the random numbers are used to develop a simulation of a complex problem. The simulation can be tested over and over to analyze the results, with each test representing a repetition of the experiment. Random numbers also used to represent noise sequences, such as those heard on a radio. Uniform Random Numbers Random numbers are characterized by their frequency distributions. Uniform random numbers have a constant or uniform distribution over their range between minimum and maximum values. The rand function in Matlab generates uniform random numbers distributed over the interval [0,1]. A state vector is used to generate a random sequence of values. A given state vector always generates the same random sequence. Thus, the sequences are known more formally as pseudorandom sequences. This generator can generate all the oating point numbers in the closed interval [253 , 1 253 ]. Theoretically, it can generate over 21492 values before repeating itself. Command rand(n) rand(m,n) rand s = rand(state) rand(state,s) rand(state,0) rand(state,J) rand(state,sum(100*clock)) Description Returns an nn matrix x of random numbers distributed uniformly in the interval [0,1]. Returns an mn matrix x of random numbers distributed uniformly in the interval [0,1]. Returns a scalar whose value changes each time it is referenced. rand(size(A)) is the same size as A. Returns a 35-element vector s containing the current state of the uniform generator. Resets the state to s. Resets the generator to its initial state. Resets the generator to its J-th state for integer J. Resets the generator to a dierent state each time. The following commands generate and display two sets of ten random numbers uniformly distributed between 0 and 1; the dierence between the two sets is caused by the dierent states. rand(state,0) 148 set1 = rand(10,1); rand(state,123) set2 = rand(10,1); [set1 set2] The results displayed by these commands are the following, where the rst column gives values of set1 and the second column gives values of set2: 0.9501 0.2311 0.6068 0.4860 0.8913 0.7621 0.4565 0.0185 0.8214 0.4447 0.0697 0.2332 0.7374 0.7585 0.6368 0.6129 0.3081 0.2856 0.0781 0.9532 Note that as desired, the two sequences are dierent. To generate sequences having values in the interval [xmin , xmax ], rst generate a random number r in the interval [0, 1], multiply by (xmax xmin ), producing a number in the interval [0, (xmax xmin ], then add xmin . To generate a row vector of 500 elements, the command needed is x = (xmax - xmin)*rand(1,500) + xmin The sequence data1, plotted in Figure 7.1, was generated with the command: data1 = 2*rand(1,500) + 2; Thus, xmax xmin = 2 and xmin = 2, so xmax = 4 and the range of the data sequence is (2,4), which can be conrmed from Figure 7.1. Example 7.4 Flipping a coin When a fair coin is ipped, the probability of getting heads or tails is 0.5 (50%). If we want to represent tails by 0 and heads by 1, we can rst generate a uniform random number in the range [0,2). The probability of a result in the range [0,1) is 0.5 (50%), which can be mapped to 0 by truncation of this result. Similarly, the probability of a result in the range [1,2) is 0.5 (50%), which can be mapped to 1 by truncation. The trunction function in Matlab is floor. A script to simulate an experiment to ip a coin 50 times and display the results is the following: 149 % Coin flipping simulation % % Coin flip: n = 50; coin = floor(2*rand(1,n)) % number of flips % vector of n flips: 0: tails, 1:heads % Histogram computation and display: xc = [0 1]; % histogram centers y = hist(coin,xc); % absolute frequency bar(xc,y), ylabel(frequency),... xlabel(0: tails, 1: heads),... title(Absolute Frequency Histogram for 50 Coin Flips) The output displayed by the script: coin = Columns 1 Columns 1 Columns 0 Columns 0 Columns 0 1 through 12 1 0 0 13 through 24 0 0 0 25 through 36 1 1 1 37 through 48 1 0 1 49 through 50 0 1 0 0 0 0 1 0 1 1 1 0 1 1 0 0 0 1 1 0 1 0 0 0 1 1 1 1 0 1 1 0 0 The histogram plot is shown in Figure 7.6. Note that in this trial, there were 26 tails and 24 heads, close to the expected result. Since this is a random process, repeated trials will give dierent results. Example 7.5 Rolling dice When a fair die (singular of dice) is rolled, the number uppermost is equally likely to be any integer from 1 to 6. The following statement will generate a vector of 10 random numbers in the range 1-6: die = floor( 6 * rand(1,10) + 1 ) The following are the results of two such simulations: 2 5 3 2 4 2 5 5 2 5 1 6 3 3 3 6 1 3 4 5 150 Absolute Frequency Histogram for 50 Coin Flips 30 25 20 frequency 15 10 5 0 0 0: tails, 1: heads 1 Figure 7.6: Histogram of 50 Coin Flips Gaussian Random Numbers A random distribution that is an appropriate model for data sets from many engineering problems is the Gaussian or Normal distribution. This is the distribution having a bell shape, with a high relative frequency at the mean, decreasing away from the mean, but extending indenitely in both directions. The Gaussian distribution is specied by two parameters: (1) the population mean , and (2) the population standard deviation , with the distribution function given by f (x) = 1 2 2 e(x) /2 2 This distribution is plotted in Figure 7.7, where it is observed to be symmetrical about the mean, dropping o rapidly away from the mean. The distribution can be normalized by performing a change of scale that converts the units of measurement into standard units z= x as shown in Figure 7.7. It can be shown that approximately 68% of the values will fall within one standard deviation of the mean (1 < z < 1), 95% will fall within two standard deviations of the mean (2 < z < 2), and 99% will fall within three standard deviations of the mean (3 < z < 3). The Matlab functions for generating Gaussian random numbers are: 151 Figure 7.7: Normalized Gaussian distribution Command randn(n) randn(m,n) randn s = randn(state) randn(state,s) randn(state,0) randn(state,J) randn(state,sum(100*clock)) Description Returns an nn matrix x of Gaussian random numbers with a mean of 0 and a standard deviation of 1. Returns an mn matrix x of Gaussian random numbers with a mean of 0 and a standard deviation of 1. Returns a scalar whose value changes each time it is referenced. rand(size(A)) is the same size as A. Returns a 2-element vector s containing the current state of the uniform generator. Resets the state to s. Resets the generator to its initial state. Resets the generator to its J-th state for integer J. Resets the generator to a dierent state each time. To generate a row vector x of 500 Gaussian random variables with mean m and standard deviation s: x = m + s*randn(1,500); The sequence data2, plotted in Figure 7.1, was generated with the command: data2 = rand(1,500) + 3; Thus, m = 3 and s = 1. The mean value can be conrmed from the plot in Figure 7.1 and the mean value computed in Section 7.3. The standard deviation can be conrmed by the value computed in Section 7.3. The computed values are not identical to the desired values because the computed values are due to a nite number of random samples. The computed and desired values become closer as the number of samples increases. Example 7.6 Noisy signal simulation Many sensors, which convert physical quantities into electrical signals, produce measurement errors, resulting in what is known as noisy signals. Additive Gaussian random numbers are often an accurate model of the errors. The signal model is x=s+n 152 where s is the noise-free signal of interest, n is the Gaussian measurement error, and x is the noisy signal. A measure of the signal quality is the signal to noise ratio (SNR), dened by SNR = 10 log10 = 20 log10 2 s 2 n s n where s is the standard deviation of the signal and n is the standard deviation of the noise. Consider simulating a noisy signal consisting of a sine wave with additive Gaussian measurement noise. The variance of a sine wave can be shown to be 2 s = A 2 where A is the sinusoidal amplitude. For A = 1 and Gaussian noise standard deviation n = 0.1, the SNR is SNR = 10 log10 0.5 = 10 log10 50 = 17.0 0.12 The following script simulates this noisy signal. In the next section, methods to reduce the aect of the noise will be considered. % Noisy signal generation and analysis t = linspace(0,10,512); % time base s = sin(2*pi/5*t); % signal n = 0.1*randn(size(t)); % noise x = s + n; % noisy signal disp(Signal to Noise Ratio (SNR), dB) SNR = 20*log10(std(s)/std(n)) % signal to noise ratio, dB plot(t,x), xlabel(Time (s)),... ylabel(Signal amplitude),... title(Noisy signal) The computed SNR is Signal to Noise Ratio (SNR), dB SNR = 16.9456 This is very close to the value computed above. The reason for the dierence is the nite number of samples used to estimate the noise standard deviation. The resulting noise signal is shown in Figure 7.8. 153 Noisy signal 1.5 1 Signal amplitude 0.5 0 0.5 1 0 1 2 3 4 5 Time (s) 6 7 8 9 10 Figure 7.8: Simulated noisy sinusoidal signal 154 Section 7 Data Analysis Outline Maximum and minimum Sums and products Statistical analysis Random number generation The purpose of data analysis is to determine some properties and characteristics of the data, which may contain measurement errors or other random inuences. For the examples to be considered in this section, generate, save, and plot the two data vectors, data1 and data2 as shown below. The commands used to generate this data will be explained later. The two data vectors, data_1 and data_2 are plotted in Figure 7.1. data1 = 2*rand(1,500) + 2; data2 = randn(1,500)+3; save data subplot(2,1,1),plot(data1),... axis([0 500 0 6]),... title(Random Numbers - data1),... subplot(2,1,2),plot(data2),... title(Random Numbers - data2),... xlabel(Index, k) Other data analysis functions available in Matlab are described with the command help datafun. 135 Random Numbers data1 6 5 4 3 2 1 0 0 50 100 150 200 250 300 350 400 450 500 Random Numbers data2 6 5 4 3 2 1 0 0 50 100 150 200 250 Index, k 300 350 400 450 500 Figure 7.1: Random sequences 7.1 Maximum and Minimum Returns largest value in vector x, or the row vector of largest elements of each column in matrix x Returns maximum values y and corresponding indices k of the rst maximum value from each column of x. Returns a matrix the same size as x and y, with each element being the maximum value from the corresponding positions in x and y. Returns smallest value in vector x, or the row vector of smallest elements of each column in matrix x Returns minimum values y and corresponding indices k of rst minimum value from each column of x. Returns a matrix the same size as x and y, with each element being the minimum value from the corresponding positions in x and y. max(x) [y,k] = max(x) max(x,y) min(x) [y,k] = min(x) min(x,y) Determining the maximum and minimum of a vector: >> v = [3 -2 4 -1 5 0]; >> max(v) ans = 5 >> [vmin, kmin] = min(v) vmin = 136 -2 kmin = 2 The maximum value is found to be 5, the minimum value 2, and the index of the minimum value is 2. Thus, vmin = v(kmin) = v(2) = -2. For the random data vectors data1 and data2: >> max(data1) ans = 3.9985 >> min(data1) ans = 2.0005 >> [max2,kmax2] = max(data2) max2 = 5.7316 kmax2 = 256 >> [min2,kmin2] = min(data2) min2 = 0.3558 kmin2 = 268 These results can be approximately conrmed from Figure 7.1. For a matrix, the min and max functions return a row vector of the minimum or maximum elements of each column of the matrix. >> B = [-1 B= -1 -3 1 >> min(B) ans = -3 >> max(B) ans = 1 1 7 0; -3 5 5 8; 1 4 4 -8] 1 5 4 7 5 4 0 8 -8 1 4 -8 5 7 8 Example 7.1 Minimum cost tank design A cylindrical tank with a hemispherical top, as shown in Figure 7.2, is to be constructed that will hold 5.00 105 L when lled. Determine the tank radius R and height H to minimize the tank 137 cost if the cylindrical portion costs $300/m2 of surface area and the hemispherical portion costs$400/m2 . Figure 7.2: Tank conguration Mathematical model: Cylinder volume: Vc = R2 H Hemisphere volume: Vh = 2 R3 3 Cylinder surface area: Ac = 2RH Hemisphere surface area: Ah = 2R2 Assumptions: Tank contains no dead air space. Concrete slab with hermetic seal is provided for the base. Cost of the base does not change appreciably with tank dimensions. Computational method: Express total volume in meters cubed (note: 1m3 = 1000L) as a function of height and radius Vtank = Vc + Vh For Vtank = 5 105 L = 500m3 : 2 500 = R2 H + R3 3 Solving for H: H= 500 2R 2 R 3 Express cost in dollars as a function of height and radius C = 300Ac + 400Ah = 300(2RH) + 400(2R2 ) 138 Method: compute H and then C for a range of values of R, then nd the minimum value of C and the corresponding values of R and H. To determine the range of R to investigate, make an approximation by assuming that H = R. Then from the tank volume: 2 5 Vtank = 500 = R3 + R3 = R3 3 3 Solving for R: R= From Matlab: >> Rest = (300/pi)^(1/3) Rest = 4.5708 We will investigate R in the range 3.0 to 7.0 meters. Computational implementation: Matlab script to determine the minimum cost design: % % % R H C Tank design problem Compute H & C as functions of R = 3:0.001:7.0; = 500./(pi*R.^2) - 2*R/3; = 300*2*pi*R.*H + 400*2*pi*R.^2; 300 1 3 % Generate trial radius values R % Height H % Cost % Plot cost vs radius plot(R,C),title(Tank Design),... xlabel(Radius R, m),... ylabel(Cost C, Dollars),grid % Compute and display minimum cost, corresponding H & R [Cmin kmin] = min(C); disp(Minimum cost (dollars):) disp(Cmin) disp(Radius R for minimum cost (m):) disp(R(kmin)) disp(Height H for minimum cost (m):) disp(H(kmin)) 139 Running the script: Minimum cost (dollars): 9.1394e+004 Radius R for minimum cost (m): 4.9240 Height H for minimum cost (m): 3.2816 Note that the radius corresponding to minimum cost (Rmin = 4.9240 is close to the approximate value, Rest = 4.5708 that we computed to assist in the selection of a range of R to investigate. The plot of cost C versus radius R is shown in Figure 7.3. 1.15 x 10 5 Tank Design 1.1 Cost C, Dollars 1.05 1 0.95 0.9 3 3.5 4 4.5 5 Radius R, m 5.5 6 6.5 7 Figure 7.3: Tank design problem: cost versus radius 7.2 Sums and Products If x is a vector with N elements denoted x(n), n = 1, 2, . . . , N , then: The sum y is the scalar y= N n=1 x(n) = x(1) + x(2) + + x(N ) 140 The product y is the scalar y= N n=1 x(n) = x(1) x(2) x(N ) The cumulative sum y is the vector having elements y(k), k = 1, 2, . . . , N y(k) = k n=1 x(n) = x(1) + x(2) + + x(k) The cumulative product y is the vector having elements y(k), k = 1, 2, . . . , N y(k) = k n=1 x(n) = x(1) x(2) x(k) If X is a matrix with M rows and N columns with elements denoted x(m, n), m = 1, 2, . . . , M, n = 1, 2, . . . , N , then The column sum y is the vector having elements y(n), n = 1, 2, . . . , N y(n) = M m=1 x(m, n) = x(1, n) + x(2, n) + + x(M, n) The column product y is the vector having elements y(n), n = 1, 2, . . . , N y(n) = M m=1 x(m, n) = x(1, n) x(2, n) x(M, n) The cumulative column sum Y is the matrix with M rows and N columns with elements denoted y(k, n), k = 1, 2, . . . , M, n = 1, 2, . . . , N y(k, n) = k m=1 x(m, n) = x(1, n) + x(2, n) + + x(k, n) The cumulative column product Y is the matrix with M rows and N columns with elements denoted y(k, n), k = 1, 2, . . . , M, n = 1, 2, . . . , N y(k, n) = k m=1 x(m, n) = x(1, n) x(2, n) x(k, n) The Matlab functions implementing these mathematical functions are the following. 141 sum(x) prod(x) cumsum(x) cumprod(x) Returns the sum of the elements in vector x, or the row vector of the sum of elements of each column in matrix x Returns the product of the elements in vector x, or the row vector of the product of elements of each column in matrix x Returns a vector the same size as x containing the cumulative sum of the elements in vector x, or a matrix the same size as x containing cumulative sums of values from the columns of x. Returns a vector the same size as x containing the cumulative product of the elements in vector x, or a matrix the same size as x containing cumulative products of values from the columns of x. Example 7.2 Summation The following is a summation identity N n=1 n= N (N + 1) 2 This identity can be checked with Matlab, for example, for N = 8: >> N = 8; >> n =1:8; >> S = sum(n) S= 36 >> N*(N+1)/2 ans = 36 Example 7.3 Factorial The factorial of n is expressed and dened as n! = n (n 1) (n 2) 1 This can be computed as prod(1:n), as in the following examples: >> nfact = prod(1:4) nfact = 24 >> nfact = prod(1:70) nfact = 1.1979e+100 142 7.3 Statistical Analysis Many engineering measurements have an element of randomness. Statistics is a branch of applied mathematics involving the analysis, interpretation, and presentation of data including some degree of randomness or uncertainty. In descriptive statistics, the important features or properties of a set of data are summarized or described. Continuous random variable: A variable vector x, whose elements can have any of a continuum of values for each measurement, or observation. Population: all possible measurements of a random variable Sample: nite number of measurements. Assume that N measurements have been made of a random variable, denoted as the vector x, with elements x(n), n = 1, . . . , N . Frequency distribution of a random variable: indicates the relative frequency with which specic values of this random variable occur in the population. Histogram: frequency distribution of sample data, indicating the frequency with which sample values fall within ranges of values, called bins. Range of values: xmin x xmax Number of bins: m xmax xmin Bin width: x = m Histogram value in bin k: N (k), the number of samples with value xmin + (k 1)x x xmin + kx. This is also known as the absolute frequency. The Matlab commands for generating and plotting a histogram are; 143 Command N = hist(x) N = hist(x,m) N = hist(x,xc) N = histc(x,edges) [N,xc] = hist(...) hist (...) Description Returns the row vector histogram N of the values in x using 10 bins. If x is matrix, returns a histogram matrix N operating on the columns of x. Returns the row vector histogram N of the values in x using m bins. Returns the row vector histogram N of the values in x using bin centers specied by xc. Counts the number of values in vector x that fall between the elements in the edges vector (which must contain monotonically non-decreasing values). N is a length(edges) vector containing these counts. N(k) will count the value x(i) if edges(k) >= x(i) > edges(k+1). The last bin will count any values of x that match edges(end). Values outside the values in edges are not counted. Use -inf and inf in edges to include all non-NaN values. Also returns the position of the bin centers in xc. Without output arguments produces a histogram bar plot of the results. For example, the following commands produce the 25-bin histograms shown in Figure 7.4, operating on the random data vectors data1 and data2. % Histogram plots % subplot(2,1,1),hist(data1,25),... title(Histogram of data1),... xlabel(x),... ylabel(N),... subplot(2,1,2),hist(data2,25),... title(Histogram of data2),... xlabel(x),... ylabel(N) Note the dierences in the nature of the two distributions. Random data data1 is called a uniform distribution, as it has roughly equal, or uniform, frequency in all bins. Random data data2 has a bell-shaped distribution, called a Gaussian distribution or Normal distribution. Relative frequency histogram: The histogram value in bin k is normalized by the total number, N , of data samples: N (k)/N . The hist function is limited in its ability to produce relative frequency histograms. It is better to generate such plots with the bar function, dened as follows: 144 Histogram of data1 35 30 25 20 N 15 10 5 0 2 2.2 2.4 2.6 2.8 3 x 3.2 3.4 3.6 3.8 4 Histogram of data2 60 50 40 N 30 20 10 0 0 1 2 3 x 4 5 6 Figure 7.4: Absolute frequency histograms bar(X,Y) bar(Y) bar(x,y bar(X,Y,width) Draws the columns of the M-by-N matrix Y as M groups of N vertical bars. The vector X must be monotonically increasing or decreasing. Uses the default value of X=1:M. For vector inputs, length(y) bars are drawn, with each bar centered on a value of x. Species the width of the bars. Values of width 1 produce overlapped bars. The default value is width=0.8 For example, the following commands produce relative histogram plots shown in Figure 7.5 for the random data vectors data1 and data2. % Relative frequency histogram plots % n1 = length(data1); [freq1,x1] = hist(data1,25); rfreq1 = freq1/n1; n2 = length(data2); [freq2,x2] = hist(data2,25); rfreq2 = freq2/n2; subplot(2,1,1),bar(x1,rfreq1),... title(Relative histogram of data1), grid,... xlabel(x), ylabel(relative frequency),... subplot(2,1,2),bar(x2,rfreq2),... title(Relative histogram of data2), grid,... xlabel(x), ylabel(relative frequency) 145 Relative histogram of data1 0.07 0.06 relative frequency 0.05 0.04 0.03 0.02 0.01 0 2 2.2 2.4 2.6 2.8 3 x 3.2 3.4 3.6 3.8 4 Relative histogram of data2 0.12 0.1 relative frequency 0.08 0.06 0.04 0.02 0 0 1 2 3 x 4 5 6 Figure 7.5: Relative frequency histograms Measures of central tendency The mean and median describe the middle, or center, of the range of the random variable. The sample mean of vector x having N elements is x x= 1 N N n=1 x(m) The population mean (mu) is the value of x for an innite number of measurements, denoted by = lim x N where the right hand side is read as the limit of x bar as N approaches innity. Median: Middle value of a set of random samples, sorted by value (rank ordered). If there is an even number of samples, then the median is the average of the two middle sample values. Command mean(x) median(x) sort(x) Description Returns the sample mean of the elements of the vector x. Returns a row vector of the sample means of the columns of matrix x. Returns the median of the elements of the vector x. Returns a row vector of the medians of the columns of matrix x. Returns a vector with the values of vector x in ascending order. Returns a matrix with each column of matrix x in ascending order. 146 For example: >> mean(data1) ans = 2.9940 >> sum(data1)/length(data1) ans = 2.9940 >> mean(data2) ans = 2.9768 >> median(data1) ans = 3.0285 >> median(data2) ans = 3.0574 These results can be conrmed by observing the plots of the data in Figure 7.1, in which the data values can be seen to be centered on 3.0. For the two data sets considered, the values of the mean and median are very close. This is not necessarily the case for other data sets. Also note that the results above show that mean(data1) = sum(data1)/length(data1). Measures of variation Measures of variation: indicate the degree of deviation of random samples from the measure of central tendency. Referring again to the plots of our random data sets data1 and data2 in Figure 7.1, observe that data2 has greater variation from the mean. The sample standard deviation of vector x having N elements is N 1 s= (x(n) x)2 N 1 n=1 1 2 The sample variance s2 is the square of the standard deviation. std(x) Returns the sample standard deviation of the elements of the vector x. Returns a row vector of the sample standard deviations of the columns of matrix x. For example: >> std(data1) 147 ans = 0.5989 >> std(data2) ans = 0.9408 Thus, the variation of data2 is greater than that of data1, as we concluded from observation of the plotted data values. 7.4 Random Number Generation Many engineering problems require the use of random numbers in the development of a solution. In some cases, the random numbers are used to develop a simulation of a complex problem. The simulation can be tested over and over to analyze the results, with each test representing a repetition of the experiment. Random numbers also used to represent noise sequences, such as those heard on a radio. Uniform Random Numbers Random numbers are characterized by their frequency distributions. Uniform random numbers have a constant or uniform distribution over their range between minimum and maximum values. The rand function in Matlab generates uniform random numbers distributed over the interval [0,1]. A state vector is used to generate a random sequence of values. A given state vector always generates the same random sequence. Thus, the sequences are known more formally as pseudorandom sequences. This generator can generate all the oating point numbers in the closed interval [253 , 1 253 ]. Theoretically, it can generate over 21492 values before repeating itself. Command rand(n) rand(m,n) rand s = rand(state) rand(state,s) rand(state,0) rand(state,J) rand(state,sum(100*clock)) Description Returns an nn matrix x of random numbers distributed uniformly in the interval [0,1]. Returns an mn matrix x of random numbers distributed uniformly in the interval [0,1]. Returns a scalar whose value changes each time it is referenced. rand(size(A)) is the same size as A. Returns a 35-element vector s containing the current state of the uniform generator. Resets the state to s. Resets the generator to its initial state. Resets the generator to its J-th state for integer J. Resets the generator to a dierent state each time. The following commands generate and display two sets of ten random numbers uniformly distributed between 0 and 1; the dierence between the two sets is caused by the dierent states. rand(state,0) 148 set1 = rand(10,1); rand(state,123) set2 = rand(10,1); [set1 set2] The results displayed by these commands are the following, where the rst column gives values of set1 and the second column gives values of set2: 0.9501 0.2311 0.6068 0.4860 0.8913 0.7621 0.4565 0.0185 0.8214 0.4447 0.0697 0.2332 0.7374 0.7585 0.6368 0.6129 0.3081 0.2856 0.0781 0.9532 Note that as desired, the two sequences are dierent. To generate sequences having values in the interval [xmin , xmax ], rst generate a random number r in the interval [0, 1], multiply by (xmax xmin ), producing a number in the interval [0, (xmax xmin ], then add xmin . To generate a row vector of 500 elements, the command needed is x = (xmax - xmin)*rand(1,500) + xmin The sequence data1, plotted in Figure 7.1, was generated with the command: data1 = 2*rand(1,500) + 2; Thus, xmax xmin = 2 and xmin = 2, so xmax = 4 and the range of the data sequence is (2,4), which can be conrmed from Figure 7.1. Example 7.4 Flipping a coin When a fair coin is ipped, the probability of getting heads or tails is 0.5 (50%). If we want to represent tails by 0 and heads by 1, we can rst generate a uniform random number in the range [0,2). The probability of a result in the range [0,1) is 0.5 (50%), which can be mapped to 0 by truncation of this result. Similarly, the probability of a result in the range [1,2) is 0.5 (50%), which can be mapped to 1 by truncation. The trunction function in Matlab is floor. A script to simulate an experiment to ip a coin 50 times and display the results is the following: 149 % Coin flipping simulation % % Coin flip: n = 50; coin = floor(2*rand(1,n)) % number of flips % vector of n flips: 0: tails, 1:heads % Histogram computation and display: xc = [0 1]; % histogram centers y = hist(coin,xc); % absolute frequency bar(xc,y), ylabel(frequency),... xlabel(0: tails, 1: heads),... title(Absolute Frequency Histogram for 50 Coin Flips) The output displayed by the script: coin = Columns 1 Columns 1 Columns 0 Columns 0 Columns 0 1 through 12 1 0 0 13 through 24 0 0 0 25 through 36 1 1 1 37 through 48 1 0 1 49 through 50 0 1 0 0 0 0 1 0 1 1 1 0 1 1 0 0 0 1 1 0 1 0 0 0 1 1 1 1 0 1 1 0 0 The histogram plot is shown in Figure 7.6. Note that in this trial, there were 26 tails and 24 heads, close to the expected result. Since this is a random process, repeated trials will give dierent results. Example 7.5 Rolling dice When a fair die (singular of dice) is rolled, the number uppermost is equally likely to be any integer from 1 to 6. The following statement will generate a vector of 10 random numbers in the range 1-6: die = floor( 6 * rand(1,10) + 1 ) The following are the results of two such simulations: 2 5 3 2 4 2 5 5 2 5 1 6 3 3 3 6 1 3 4 5 150 Absolute Frequency Histogram for 50 Coin Flips 30 25 20 frequency 15 10 5 0 0 0: tails, 1: heads 1 Figure 7.6: Histogram of 50 Coin Flips Gaussian Random Numbers A random distribution that is an appropriate model for data sets from many engineering problems is the Gaussian or Normal distribution. This is the distribution having a bell shape, with a high relative frequency at the mean, decreasing away from the mean, but extending indenitely in both directions. The Gaussian distribution is specied by two parameters: (1) the population mean , and (2) the population standard deviation , with the distribution function given by f (x) = 1 2 2 e(x) /2 2 This distribution is plotted in Figure 7.7, where it is observed to be symmetrical about the mean, dropping o rapidly away from the mean. The distribution can be normalized by performing a change of scale that converts the units of measurement into standard units z= x as shown in Figure 7.7. It can be shown that approximately 68% of the values will fall within one standard deviation of the mean (1 < z < 1), 95% will fall within two standard deviations of the mean (2 < z < 2), and 99% will fall within three standard deviations of the mean (3 < z < 3). The Matlab functions for generating Gaussian random numbers are: 151 Figure 7.7: Normalized Gaussian distribution Command randn(n) randn(m,n) randn s = randn(state) randn(state,s) randn(state,0) randn(state,J) randn(state,sum(100*clock)) Description Returns an nn matrix x of Gaussian random numbers with a mean of 0 and a standard deviation of 1. Returns an mn matrix x of Gaussian random numbers with a mean of 0 and a standard deviation of 1. Returns a scalar whose value changes each time it is referenced. rand(size(A)) is the same size as A. Returns a 2-element vector s containing the current state of the uniform generator. Resets the state to s. Resets the generator to its initial state. Resets the generator to its J-th state for integer J. Resets the generator to a dierent state each time. To generate a row vector x of 500 Gaussian random variables with mean m and standard deviation s: x = m + s*randn(1,500); The sequence data2, plotted in Figure 7.1, was generated with the command: data2 = rand(1,500) + 3; Thus, m = 3 and s = 1. The mean value can be conrmed from the plot in Figure 7.1 and the mean value computed in Section 7.3. The standard deviation can be conrmed by the value computed in Section 7.3. The computed values are not identical to the desired values because the computed values are due to a nite number of random samples. The computed and desired values become closer as the number of samples increases. Example 7.6 Noisy signal simulation Many sensors, which convert physical quantities into electrical signals, produce measurement errors, resulting in what is known as noisy signals. Additive Gaussian random numbers are often an accurate model of the errors. The signal model is x=s+n 152 where s is the noise-free signal of interest, n is the Gaussian measurement error, and x is the noisy signal. A measure of the signal quality is the signal to noise ratio (SNR), dened by SNR = 10 log10 = 20 log10 2 s 2 n s n where s is the standard deviation of the signal and n is the standard deviation of the noise. Consider simulating a noisy signal consisting of a sine wave with additive Gaussian measurement noise. The variance of a sine wave can be shown to be 2 s = A 2 where A is the sinusoidal amplitude. For A = 1 and Gaussian noise standard deviation n = 0.1, the SNR is SNR = 10 log10 0.5 = 10 log10 50 = 17.0 0.12 The following script simulates this noisy signal. In the next section, methods to reduce the aect of the noise will be considered. % Noisy signal generation and analysis t = linspace(0,10,512); % time base s = sin(2*pi/5*t); % signal n = 0.1*randn(size(t)); % noise x = s + n; % noisy signal disp(Signal to Noise Ratio (SNR), dB) SNR = 20*log10(std(s)/std(n)) % signal to noise ratio, dB plot(t,x), xlabel(Time (s)),... ylabel(Signal amplitude),... title(Noisy signal) The computed SNR is Signal to Noise Ratio (SNR), dB SNR = 16.9456 This is very close to the value computed above. The reason for the dierence is the nite number of samples used to estimate the noise standard deviation. The resulting noise signal is shown in Figure 7.8. 153 Noisy signal 1.5 1 Signal amplitude 0.5 0 0.5 1 0 1 2 3 4 5 Time (s) 6 7 8 9 10 Figure 7.8: Simulated noisy sinusoidal signal 154 Section 8 Selection Programming A selection statement allows a question to be asked or a condition to be tested to determine which steps are to be performed next. The question or condition is dened using relational and logical operators, which will be described prior to introducing the selection statements. Outline Relational and logical operators Control ow Loops Selection statements in user-dened functions Update processes Applied problem solving: speech signal analysis 8.1 Relational and Logical Operators For relational and logical expressions: Inputs: True is any nonzero number False is 0 (zero) True is 1 (one) False is 0 (zero) Outputs: An output array variable assigned to a relational or logical expression is identied as logical. That is, the result contains numerical values 1 and 0, that can be used in mathematical statements, but also allow logical array addressing. 155 Relational Operators For more information: help ops. Relational Operator < <= > >= == = Description less than less than or equal greater than greater than or equal equal not equal Relational operators can be used to compare two arrays of the same size or to compare an array to a scalar. In the second case, the scalar is compared with all elements of the array and the result has the same size as the array. Examples: >> A=1:9, B=8-A A= 1 2 B= 7 6 >> tf1 = A <=4 tf1 = 1 1 >> tf2 = A > B tf2 = 0 0 >> tf3 = (A==B) tf3 = 0 0 >> tf4 = B-(A>2) tf4 = 7 6 3 5 4 4 5 3 6 2 7 1 8 0 9 -1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 0 1 0 0 0 0 0 4 3 2 1 0 -1 -2 tf1 nds elements of A that are less than or equal to 4. Ones appear in the result where A 4 and zeroes appear where A> 4. tf2 nds elements of A that are greater than those in B. tf3 nds elements of A that are equal to those in B. Note that = and == mean two dierent things: == compares two variables and returns ones where they are equal and zeros where they are not; =, on the other hand, is used to assign the output of an operation to a variable. tf4 nds where A>2 and subtracts the resulting vector from B. This shows that since the output of logical operations are numerical arrays of ones and zeros, they can be used in mathematical operations. 156 Note that = and == mean two dierent things: == compares two variables and returns ones where they are equal and zeros where they are not; on the other hand, = is used to assign the output of an operation to a variable. Example 8.1 Avoiding division by zero Consider the following: >> x=(-3:3)/3 x= -1.0000 -0.6667 >> sin(x)./x -0.3333 0 0.3333 0.6667 1.0000 Warning: Divide by zero. ans = 0.8415 0.9276 0.9816 NaN 0.9816 0.9276 0.8415 Computing the function sin(x)/x gives a warning because the fourth element in x is zero. Since sin(0)/0 is undened, the result for that element in the result is NaN (meaning Not a Number). The following will replace the zero in x with the special number eps, which is approximately 2.21016 , resolving the divide-by-zero problem. >> x = x + (x==0)*eps x= -1.0000 -0.6667 >> sin(x)./x ans = 0.8415 0.9276 -0.3333 0.0000 0.3333 0.6667 1.0000 0.9816 1.0000 0.9816 0.9276 0.8415 Logical Operators Logical operators provide a way to combine or negate relational expressions. Logical Operator & | Description and or not A fourth logical operator is implemented as a function: xor(A,B) Exclusive or: Returns ones where either A or B is True (nonzero); returns False (zero) where both A and B are False (zero) or both are True (nonzero). 157 Denitions of the logical operators, with 0 representing False and 1 representing True: A 0 0 1 1 B 0 1 0 1 A 1 1 0 0 A|B 0 1 1 1 A&B 0 0 0 1 xor(A,B) 0 1 1 0 The precedence from highest to lowest is relational operators, followed by logical operators , &, and |. Parentheses can be used to change the precedence and should be used liberally to clarify the operations. Examples: >> A=1:9 A= 1 >> tf1 = tf1 = 0 >> tf2 = tf2 = 1 >> tf3 = tf3 = 0 >> tf4 = tf4 = 1 2 A>4 0 ~(A>4) 3 4 5 6 7 8 9 0 0 1 1 1 1 1 1 1 (A>2)&(A<6) 1 0 0 0 0 0 0 1 1 xor((A>2),(A<6)) 1 0 0 1 0 0 0 0 0 1 1 1 1 tf1 nds where A is greater than 4. tf2 negates tf1, nding where A is not greater than 4. tf3 nds where A is greater than 2 and less than 6. tf4 nd where the exclusive or of (A>2) and (A<6), which is observed to be the same as the logical inverse of tf3. Example 8.2 Generating discontinuous signals The logical operators allow the generation of arrays representing signals with discontinuities or signals that are composed os segments of other signals. The basic idea is to multiply those values in a array that are to be retained with ones, and multiply all other values with zeros. Consider the discontinuous signal: x(t) = sin(t) sin(t) > 0 0 sin(t) < 0 158 The following script computes and plots this signal over the range t = [0,10] s. t = linspace(0,10,100); % create x vector x = sin(t); % compute sine vector x = x.*(x>0); % set negative values of sin(t) to zero % % Plot x and label plot(t,x),xlabel(Time (s)),ylabel(Amplitude),... title(Discontinuous signal), axis([0 10 -0.1 1.1]) The plot that is generated is shown in Figure 8.1. Discontinuous signal 1 0.8 0.6 Amplitude 0.4 0.2 0 0 1 2 3 4 5 Time (s) 6 7 8 9 10 Figure 8.1: Plot of a discontinuous signal Relational and Logical Functions Matlab provides several useful relational and local functions that operate on scalars, vectors, and matrices. The following is a partial list of these functions. 159 Function any(x) all(x) find(x) isnan(x) isfinite(x) isinf(x) isempty(x) Description Returns a scalar that is 1 (true) if any element in the vector x is nonzero; otherwise, the scalar is 0 (false). Returns a row vector containing a 1 (true) in each element for which any element of the corresponding column of matrix x is nonzero, and a 0 (false) otherwise. Returns a scalar that is 1 (true) if all elements in the vector x are nonzero; otherwise, the scalar is 0 (false). Returns a row vector containing a 1 (true) in each element for which all elements of the corresponding column of matrix x are nonzero, and a 0 (false) otherwise. Returns a vector containing the indices of the nonzero elements of a vector x. Returns a vector containing the indices of the nonzero elements of x(:), which is a single-column vector formed from the columns of matrix x. Returns an array with ones where the elements of x are NaN and zeros where they are not. Returns an array with ones where the elements of x are nite and zeros where they are not. For example, isfinite([pi NaN Inf -Inf]) is [1 0 0 0]. Returns an array with ones where the elements of x are +Inf or -Inf and zeros where they are not. Returns 1 if x is an empty array and 0 otherwise. Example 8.3 Height and speed of a projectile The height and speed of a projectile (such as a thrown ball) launched with a speed of v0 at an angle to the horizontal are given by h(t) = v0 t sin 0.5gt2 v(t) = 2 v0 2v0 gt sin + g 2 t2 where g is the acceleration due to gravity. The projectile will strike the ground when h(t) = 0, which gives the time to return to the ground, tg = 2(v0 /g) sin . For = 40 , v0 = 20 m/s, and g = 9.81 m/s2 , determine the times when the height is no less than 6m and the speed is simultaneously no greater than 16 m/s. This problem can be solved by using the find command to determine the times at which the logical expression (h >= 6 & (v <= 16) is ture. % Set the values for initial speed, gravity, and angle v0 = 20; g = 9.81; theta = 40*pi/180; % Compute the time to return to the ground t_g = 2*v0*sin(theta)/g; 160 % Compute the arrays containing time, height, and speed. t = [0:t_g/200:t_g]; h = v0*t*sin(theta) - 0.5*g*t.^2; v = sqrt(v0^2 - 2*v0*g*sin(theta)*t + g^2*t.^2); % Determine when the height is no less than 6, % and the speed is no greater than 16. u = find(h>6 & v <= 16); % Compute the corresponding times t_1 = t(u(1)) t_2 = t(u(end)) The beginning of the time interval, t1 is the value of t indexed by the rst element of u and the end of the time interval, t2 is the value of t indexed by the last element of u. The results are: t_1 = 0.8518 t_2 = 1.7691 This problem could have been solved by plotting h(t) and v(t), but the accuracy of the results would be limited by our ability to pick points o the graph. In addition, the graphical approach is more time-consuming. 8.2 Flow Control Selection statements that test the results of relational or logical functions or operators are the decision-making structures that allow the ow of command execution to be controlled. For more information: help lang. Simple if Statement The general form of a simple if statement is: if end If the logical expression is true, the commands between the if statement and the end statement are executed. If the logical expression is false, the ow of execution jumps immediately to the end statement without executing the statements between the if statement and the end statement. 161 logical expression commands To help in reading and understanding an if structure, it is good style to indent the statements within the structure. Example: if d < 50 count = count + 1; disp(d); end Assuming that d is a scalar, then if d is less than 50, count is incremented by 1 and the value of d is displayed; otherwise, these two statements are skipped. If d is not a scalar, then count is incremented by 1 and d is displayed only if every element of d is less than 50. Nested if Statements if statements may be nested, as shown in the following example: if d < 50 count = count + 1; disp(d); if b > d b = 0; end end Assuming rst that b and d are scalars, then if d<50, count is incremented by 1 and d is displayed. In addition, if b>d, then b is set to 0. If d is not less than 50, we skip immediately to the second end statement. else and elseif Clauses else clause: allows one set of statements to be executed if a logical expression is true and a dierent set if the logical expression is false. For example, if variable interval is less than one, set the value of xinc to interval/10; otherwise, set the value of xinc to 0.1. if interval < 1 xinc = interval/10; else xinc = 0.1; end 162 When several levels of if-else statements are nested, it may be dicult to determine which logical expressions must be true (or false) to execute each set of statement. In these cases, the elseif clause is often used to clarify the program logic, as shown in the example below. if temperature > 100 disp(Too hot - equipment malfunctioning.) elseif temperature > 90 disp(Normal operating range.) elseif temperature > 50 disp(Below desired operating range.) else disp(Too cold - turn off equipment.) end In this example, temperature between 90 and 100 are in the normal operating range; temperatures outside this range generate an appropriate message. Example 8.4 Testing variable type The if command can be used to write a function M-le to take a single input argument and then report, in text, whether that argument is scalar, vector, or matrix. No argument is returned. function testvar(x) % Display text indicating whether x is a % scalar, vector, or matrix [m,n] = size(x); if m==n & m==1 disp( Argument is a scalar) elseif m==1 | n==1 disp( Argument is a vector) else disp( Argument is a matrix) end A test of this function: >> a=2; b=[2 3]; c=[4 5; 6 7]; >> testvar(a) Argument is a scalar >> testvar(b) Argument is a vector >> testvar(c) Argument is a matrix 163 Switch Selection Structure The switch selection structure provides an alternative to using the if, elseif, and else commands. Anything programmed using if structures can also be programmed using switch structures. The advantage of the switch structure is that in some situations, it yields code that is more readable. The syntax is switch expression case test expression 1 commands case {test expression 2, test expression 3} commands otherwise commands end The expression result is compared in turn to the result of each case test expression. If they are equal, then the commands following the case command are executed and processing continues with the command following the end statement. If expression is a character string, then a string comparison is made with the case test expression. Multiple test expressions can be listed, comma separated, enclosed in braces {}. Only the rst matching case is executed, If no match occurs, the statements following the otherwise statement are executed. However, the otherwise statement is optional. If it is absent, execution continues with the command following the end statement if no match exists. Each case test expression statement must be on a single line. Consider an example that was previously implemented with an if, else structure: switch interval < 1 case 1 xinc = interval/10; case 0 xinc = 0.1; end For this example, the resulting code is no more readable than in the previous implementation. An example using string comparisons: x = 6.1; units = ft; % convert x to meters switch units case {inch,in} 164 y = x*0.0254; case{feet,ft} y = x*0.3048; case{meter,m} y = x; case{centimeter,cm} y = x/100; case{millimeter,mm} y = x/1000; otherwise disp([Unknown units: units]) y = NaN; end Executing this example gives a nal value of y = 1.8593. 8.3 Loops A loop is a structure that allows a group of commands to be repeated. for Loop A for loop repeats a group of commands a xed, predetermined number of times. A for loop has the following structure: for end The commands between the for and end statements are executed once for every column in the expression, beginning with the rst column and stepping through to the last column. At each step, known as an iteration, the appropriate column of the expression is assigned to the variable. Thus, on step n, column n of the expression is assigned to the variable, which then can be operated on by one of the commands in the loop. Rules for writing and using a for loop include: 1. If the expression results in an empty matrix, the loop will not be executed. Control will pass directly to the statement following the end statement. 2. If the result of the expression is a scalar, the loop will be executed once, with the variable equal to the value of the scalar. 3. If the result of the expression is a vector, then each time through the loop, the variable will contain the next value in the vector. variable=expression commands 165 4. A for loop cannot be terminated by reassigning the loop variable within the loop. 5. Upon completion of a for loop, the variable contains the last value used. 6. The colon operator can be used to dene the expression using the following format for index = initial:increment:limit Example 8.5 Use of a for statement in a function Consider writing a user-dened function that searches a matrix input argument for the element with the largest value and returns the indices of that element. function [r,c] = indmax(x) % INDMAX returns the row and column indices of % the maximum-valued element in x [m n] = size(x); xmax = x(1,1); r=1; c=1; for k=1:m for l=1:n if x(k,l)> xmax xmax = x(k,l); r=k; c=l; end end end Testing this function: >> A=3; B=[2 4 3]; C=[3 2; 6 1]; >> [r c]=indmax(A) r= 1 c= 1 >> [r c]=indmax(B) r= 1 c= 2 >> [r c]=indmax(C) r= 2 c= 1 166 Avoiding Loops In general, loops should be avoided in Matlab, as they can signicantly increase the execution time of a program. Matlab has the capability to increase the size of vectors and matrices dynamically, as can be required by a for loop. For example, a vector can be of length N at the N th iteration of a loop, and at iteration N + 1, Matlab can increase the length of the vector from N to N + 1. However, this dynamic storage allocation is not accomplished eciently in terms of computation time. For example, consider the following script for generating a sine wave: % sinusoid of length 5000 for t=1:5000 y(t) = sin(2*pi*t/10); end This results in a scalar t, with nal value 5000, and a vector y, a sine wave with 10 samples per cycle, with 5000 elements. Each time the command inside the for loop is executed, the size of the variable y must be increased by one. The execution time on a certain PC is 7.91 seconds. Then consider a second script: % sinusoid of length 10000 for t=1:10000 y(t) = sin(2*pi*t/10); end Clearing the Matlab workspace and running this script to produce vector y having 10000 elements requires an execution time on the same PC of 28.56 seconds, nearly four times the execution time of the rst script. Forcing Matlab to allocate memory for y each time through the loop takes time, causing the execution time to grow geometrically with the vector length. To maximize speed, arrays should be preallocated before a for loop is executed. To eliminate the need to increase the length of y each time through the loop, the script could be rewritten as: % sinusoid of length 10000 with vector preallocation y = zeros(1,10000); for t=1:10000 y = sin(2*pi*t/10); end In this case, y was arbitrarily dened to be a vector of length 10000, with values recomputed in the loop. The computation time in this case was 2.03 seconds. Execution time can be decreased even further by vectorizing the algorithm, applying an operation to an entire vector instead of a single element at a time. Applying this approach to our example of sinusoid generation: 167 % better method of sinusoid generation t = 1:10000; y = sin(2*pi*t/10); This script produces the vector t with 10,000 elements and then the vector y with 10,000 elements, which is the as same y from the second script above. However, again using the same PC, this script executes in 0.06 second, as opposed to 16.04 seconds for the second for loop script, or 1.92 seconds with a for loop and vector preallocation. while Loop A while loop repeats a group of commands as long as a specied condition is true. A while loop has the following structure: while end If all elements in expression are true, the commands between the while and end statements are executed. The expression is reevaluated and if all elements are still true, the commands are executed again. If any element in the expression is false, control skips to the statement following the end statement. The variables modied within the loop should include the variables in the expression, or the value of the expression will never change. If the expression is always true (or is a value that is nonzero), the loop becomes an innite loop. Example 8.6 Computing the special value eps Consider the following example of one way to compute the special Matlab value eps, which is the smallest number that can be added to 1 such that the result is greater than 1 using nite precision: num = 0; EPS = 1; while (1+EPS)>1 EPS=EPS/2; num = num + 1; end num = num - 1 EPS = 2*EPS Uppercase EPS was used so that the Matlab value eps was not overwritten. EPS starts at 1 and is continually divided in two, as long as (1+EPS)>1 is True (nonzero). EPS eventually gets so small that adding EPS to 1 is no longer greater than 1. The loop then terminates, and EPS is multiplied by 2 because the last division by 2 made it too small by a factor of two. The results: num = 168 expression commands 52 EPS = 2.2204e-016 While Loops and Break The while loop allows the execution of a set of commands an indenite number of times. The break command is used to terminate the execution of the loop. Example 8.7 Use of a while loop to evaluate quadratic polynomials Consider writing a script to ask the user to input the scalar values for a, b, c, and x and then returns the value of ax2 + bx + c. The program repeats this process until the user enters zero values for all four variables. % Script to compute ax^2 +bx + c disp(Quadratic ax^2+bx+c evaluated) disp(for user input a, b, c, and x) a=1; b=1; c=1; x=0; while a~=0 | b~=0 | c~=0 | x~=0 disp(Enter a=b=c=x=0 to terminate) a = input(Enter value of a: ); b = input(Enter value of b: ); c = input(Enter value of c: ); x = input(Enter value of x: ); if a==0 & b==0 & c==0 & x==0 break end quadratic = a*x^2 + b*x + c; disp(Quadratic result:) disp(quadratic) end Note that this script will display multiple quadratic expression values if arrays are entered for a, b, c, or x. Consider modifying the script to check each input and breaking if any input is not a scalar. 8.4 Selection Statements in User-Dened Functions 1. User-dened functions must appropriately handle any input data and produce output variables of the proper dimensions and values. This often requires the use of selection statements 169 to determine the dimensions and values of the input. 2. The function error displays a character string in the Command window, aborts function execution, and returns control to the keyboard. This function is useful for agging improper command usage, as in the following portion of a function: if length(val)>1 error(val must be a scalar.) end 3. The functions nargin and nargout can be used in functions to determine the number of input arguments and the number of output arguments used to call the function. This provides information for handling cases when the function is to use a variable number of arguments. For example, if a default value of 1 is to be returned for the output variable err if the function call includes no input arguments, the function can include the following: if nargin==0 err=1; end If the function is written to use a xed number of input or output arguments, but the wrong number has been supplied in the use of the funciton, Matlab provides the needed error handling, without the need for the nargin and nargout functions. 4. Function M-les terminate execution and return when they reach the end of the M-le, or alternatively, when the command return is encountered. Example 8.8 Step signal The step signal is zero for negative time, stepping up to one for positive time u(t) = 0 t<0 1 t0 A user-dened function to implement the step signal: function u = step(t) % STEP unit step function % u = step(t) % u = 0 for t<0 % u = 1 for t>=0 u = zeros(size(t)); u(find(t>=0))=1; The output array u is rst set to an array of zeros with the same dimensions at that of the input argument t. Then the elements of u corresponding to values of t greater than or equal to 0 are set to 1, with the use of the find function. 170 8.5 Update Processes Many problems in science and engineering involve modelling a process where the main variable is updated over a period of time. In many situations, the updated value is a function of the current value. A comprehensive study of update processes is beyond the scope of this class, but a simple example can be considered. A can of soda at temperature 25 C is placed in a refrigerator, where the ambient temperature F is 10 C. We want to determine how the temperature of the soda changes over a period of time. A standard way of approaching this type of problem is to subdivide the time interval into a number of small steps, each of duration t. If Ti is the temperature at the beginning of step i, the following model can be used to determine Ti+1 : Ti+1 = Ti + Kt(F Ti ) where K is the conduction coecient, a parameter that depends on the insulating properties of the can and the thermal properties of the soda. Assume that units are chosen so that time is in minutes and that an interval t = 1 minute provides sucient accuracy. A script to compute, display, and plot this update process for K = 0.05: % Define input values K = 0.05; F = 10; % Define vector variables t = 0:100; T = zeros(1,101); T(1) = 25; % Update to compute T for i = 1:100; T(i+1) = T(i) + K * (F - T(i)); end % Conduction coefficient % Refrigerator temperature (degrees C) % Time variable (min) % Preallocate temperature vector % Initial soda temperature (degrees C) % Time in minutes % Compute T % Display results every 10 minutes, plot every minute disp([ t(1:10:101) T(1:10:101) ]) plot(t,T),grid,xlabel(Time (min)),ylabel(Temperature (degrees C)),... title(Cooling curve) The statement T = zeros(1,101) preallocates a vector for the temperature of the soda. The script would work without this statement, but it would be much slower, as T would have to be redimensioned during each repeat of the for loop, in order to make space for a new element each time. The for loop computes the values for T(2),...,T(101), ensuring that temperature T(i) corresponds to t(i). 171 The displayed output of the script is: 0 10.0000 20.0000 30.0000 40.0000 50.0000 60.0000 70.0000 80.0000 90.0000 100.0000 25.0000 18.9811 15.3773 13.2196 11.9277 11.1542 10.6910 10.4138 10.2477 10.1483 10.0888 The resulting graph is shown in Figure 8.2. Note that the initial slope of the temperature is steep and that temperature of the soda changes rapidly. As the dierence between the soda temperature and the refrigerator temperature becomes smaller, that change in soda temperature slows. Cooling curve 25 Temperature (degrees C) 20 15 10 0 10 20 30 40 50 Time (min) 60 70 80 90 100 Figure 8.2: Soda cooling curve Exact Solution This cooling problem has an exact mathematical solution, which is determined by expressing the update relationship as a dierential equation and then solving the resulting dierential equation. The result is T (t) = F + (T0 F )eKt 172 where T0 is the initial temperature. This is a decaying exponential, which you an conrm as having the same properties as the cooling curve shown in Figure 8.2. 8.5.1 Signal Filtering Related to update processes are methods for ltering signals to reduce noise components. As described in Example 7.6, the noise is often modeled as a random measurement error. The noise can be reduced by averaging several successive signal samples. Denoting a noisy input signal sample by x(k) and a ltered output signal sample by y(k), a 3-point moving average lter is represented by y(k) = 1 [x(k) + x(k 1) + x(k 2)] 3 The output signal sample at index k is the average of the input signal samples at index k and the two previous indices. To test this ltering method, consider the following script in which a noisy signal is simulated and ltered by a moving average. % Noisy signal generation and filtering t = linspace(0,10,512); % time base s = sin(2*pi/5*t); % signal n = 0.1*randn(size(t)); % noise, std dev 0.1 x = s + n; % signal + noise disp(Input Signal to Noise Ratio (SNR), dB) SNRin = 20*log10(std(s)/std(n)) % input SNR, dB y = zeros(size(t)); % initialize output signal % filtering computation y(1) = x(1); y(2) = (x(2)+x(1))/2; for k = 3:length(t); y(k) = (x(k)+x(k-1)+x(k-2))/3; end % analysis of filtered signal disp(Output Signal to Noise Ratio (SNR), dB) SNRout = 20*log10(std(s)/std(y-s)) % output SNR, dB % plot signals subplot(2,1,1),plot(t,x), xlabel(Time (s)),... ylabel(Signal amplitude),... title(Input signal) subplot(2,1,2),plot(t,y), xlabel(Time (s)),... 173 ylabel(Signal amplitude),... title(Output signal) Note that in computing the rst two ltered signal samples, two previous input signal samples are not available, so shortened averages are required. Signal quality is measured by signal to noise ratio (SNR), where the noise remaining in the ltered signal is the dierence between the ltered signal and the original noise-free signal. The improvement in the signal is apparent in Figure 8.3. The displayed output below shows that the SNR has been improved by about 4.3 dB. Input Signal to Noise Ratio (SNR), dB SNRin = 17.5063 Output Signal to Noise Ratio (SNR), dB SNRout = 21.8317 Input signal 1.5 1 Signal amplitude 0.5 0 0.5 1 1.5 0 1 2 3 4 5 Time (s) Output signal 1.5 1 Signal amplitude 0.5 0 0.5 1 1.5 0 1 2 3 4 5 Time (s) 6 7 8 9 10 6 7 8 9 10 Figure 8.3: 3-point moving average signal ltering If more input signal samples were averaged, the noise would be further reduced. However, this would also lead to distortion of the original signal. Thus, ltering functions must be designed to compromise between noise reduction and signal distortion. More eective ltering functions and better implementations of ltering in Matlab are available, but these are beyond the scope of this presentation, which is intended as a short introduction to ltering. 174 8.6 Applied Problem Solving: Speech Signal Analysis Consider the design of a system to recognize the spoken words for the ten digits: zero, one, two, . . ., nine. A rst step in the design is to analyze data sequences collected with a microphone to see if there are some statistical measurements that would allow recognition of the digits. The statistical measurements should include the mean, variance, average magnitude, average power and number of zero crossings. If the data sequence is x(n), n = 1, . . . , N , these measurements are dened as follows: 1 N N mean = = x(n) n=1 1 2 1 standard deviation = = N N N n=1 (x(n) )2 average magnitude = 1 N |x(n)| n=1 average power = 1 N N n=1 (x(n))2 The number of zero crossings is the number of times that x(k) and x(k + 1) dier in sign, or the number of times that x(k) x(k + 1) < 0. Matlab functions for reading and writing Microsoft wave (.wav) sound les include: Function x = wavread(wavefile) Description Reads a wave sound le specied by the string wavefile, returning the sampled data in x. The .wav extension is appended if no extension is given. Amplitude values are in the range [-1,+1]. Returns the sample rate (fs) in Hertz and the number of bits per sample (bits) used to encode the data in the le. Writes a wave sound le specied by the string wavfile. The data should be arranged with one channel per column. Amplitude values outside the range [-1,+1] are clipped prior to writing. Species the sample rate fs of the data in Hertz. [x,fs,bits]=wavread(wavefile) wavwrite(x,wavefile) wavwrite(x,fs,wavefile) The following is a script (digit.m) to prompt the user to specify the name of a wave le to be read and then to compute and display the statistics and to plot the sound sequence and a histogram of the sequence. To compute the number of zero crossings, a vector prod is generated whose rst 175 element is x(1)*x(2), whose second element is x(2)*x(3), and so on, with the last value equal to the product of the next-to-the-last element and the last element. The find function is used to determine the locations of prod that are negative, and length is used to count the number of these negative products. The histogram has 51 bins between 1.0 and 1.0, with the bin centers computed using linspace. % Script to read a wave sound file named "digit.wav" % and to compute several speech statistics % file = input(Enter name of wave file as a string: ); [x,fs,bits] = wavread(file); n = length(x); % fprintf(\n) fprintf(Digit Statistics \n\n) fprintf(samples: %.0f \n,n) fprintf(sampling frequency: %.1f \n,fs) fprintf(bits per sample: %.0f \n,bits) fprintf(mean: %.4f \n,mean(x)) fprintf(standard deviation: %.4f \n, std(x)) fprintf(average magnitude: %.4f \n, mean(abs(x))) fprintf(average power: %.4f \n, mean(x.^2)) prod = x(1:n-1).*x(2:n); crossings = length(find(prod<0)); fprintf(zero crossings: %.0f \n, crossings) subplot(2,1,1),plot(x),... axis([1 n -1.0 1.0]),... title(Data sequence of spoken digit),... xlabel(Index), grid,... subplot(2,1,2),hist(x,linspace(-1,1,51)),... axis([-0.6,0.6,0,5000]),... title(Histogram of data sequence),... xlabel(Sound amplitude),... ylabel(Number of samples),grid For a hand example to be used to test the script to be developed for this problem, consider the sequence: [0.25 0.82 0.11 0.02 0.15] Using a calculator, we compute the following: 1 mean = = (0.25 + 0.82 0.11 0.02 + 0.15) = 0.218 5 standard deviation = = 1 (0.25 )2 + (0.82 )2 + (0.11 )2 5 176 + (0.02 )2 + (0.15 )2 1 2 = 0.365 average magnitude = 1 (|0.25| + |0.82| + | 0.11| + | 0.02| + |0.15|) = 0.270 5 average power = 1 0.252 + 0.822 + (0.11)2 + (0.02)2 + 0.152 = 0.154 5 number of zero crossings = 2 The Matlab commands to compute and write a wave le test.wav containing the test sequence: >> x = [0.25 0.82 -0.11 -0.02 0.15] >> wavwrite(x,test.wav) Executing the script digit.m on the test data: >> digit Enter name of wave file as a string: test.wav Digit Statistics samples: 5 sampling frequency: 8000.0 bits per sample: 16 mean: 0.2180 standard deviation: 0.3648 average magnitude: 0.2700 average power: 0.1540 zero crossings: 2 Observe that the results from the script agree with the hand-calculated values, giving us some condence in the script. Executing the script on a wave le zero.wav created using the Sound Recorder program on a PC to record the spoken word zero as a wave le: >> digit Enter name of wave file as a string: zero.wav Digit Statistics samples: 13493 sampling frequency: 11025.0 177 bits per sample: 8 mean: -0.0155 standard deviation: 0.1000 average magnitude: 0.0690 average power: 0.0102 zero crossings: 271 The resulting plots are shown in Figure 8.4. Data sequence of spoken digit 1 0.5 0 0.5 1 2000 4000 6000 Index 8000 10000 12000 Histogram of data sequence 5000 Number of samples 4000 3000 2000 1000 0 0.5 0.4 0.3 0.2 0.1 0 0.1 Sound amplitude 0.2 0.3 0.4 0.5 Figure 8.4: Data sequence and histogram for the spoken word zero Observe that the characteristics of the change with time (index), as dierent sounds are being made to produce the word. An eective word recognition algorithm must capture this change in sounds, but the design of such an algorithm is beyond the material covered in this course. Note that the histogram has the appearance of a Gaussian or bell-shaped curve, with a mean near zero and a standard deviation of about 0.35, while the data sequence looks much dierent than the Gaussian random data sequence shown in Figure 7.1. You can experiment with the PC Sound Recorder program to record wave les for other spoken digits and then process them with digit. In recording these les, you will need to make sure that the number of data samples in the recorded le is less than 16,384 if you are using the Student Version of Matlab. This requires using the Telephone Quality recording option and you may need to delete the silent sections at the beginning and end of the record. You can also create wave les using Matlab and then listen to the signals in these les using PC Sound Recorder. For example, to compute and write a wave le containing 16000 samples in 2 seconds of an 800Hz sinusoid: >> t = linspace(0,2,16000); 178 >> y = cos(2*pi*800*t); >> wavwrite(y,cosine.wav) 179 Section 9 Vectors, Matrices and Linear Algebra A matrix is a two-dimensional array, as introduced in Section 7.2, where matrix addressing and methods of array creation were described. In Section 7.3, scalar-array mathematics and element-byelement array mathematics were described. In this chapter, we present a set of matrix operations and functions that apply to the matrix as a unit, as opposed to individual elements in the matrix. Review of Matrix Denitions 1. Scalar: A matrix with one row and one column. A = [3.5] 2. Vector: A matrix with one row (a row vector) or one column, ( a column vector). B1 = [ 1.5 3.1 ] B2 = 2.5 3.1 3. Matrix: An example of a matrix with four rows and three columns: C= 1 0 0 1 1 0 1 1 0 0 0 2 The elements in a matrix are denoted by cij , where i is the row index and j is the column index. For example c43 refers to the element in row 4 and column 3, and thus, c43 = 2 in the example above. If needed to avoid misinterpretation, the indexes can be separated by a comma, as in ci,j and c4,3 . In Matlab, subscripts are indicated by parentheses, as in C(4,3). 180 9.1 Vectors We usually think of vectors as column vectors, written as x= x1 x2 . . . xn To indicate that x is a vector of n real numbers, we write x Rn Geometrically, Rn is n-dimensional space, and the notation x Rn means that x is a point in that space, specied by the n coordinates x1 , x2 , . . . xn . Figure 9.1 shows a vector in R3 , drawn as an arrow from the origin to the point x. Figure 9.1: A vector in R3 Vector Addition Vectors with the same number of elements can be added and subtracted in a very natural way: x+y = x1 + y1 x2 + y2 x3 + y3 . . . xn + yn , xy = x1 y1 x2 y2 x3 y3 . . . xn yn 181 Inner or Dot Product The inner product (x, y), also referred to as the dot product x y, is a scalar sum of products (x, y) = x y = x1 y1 + x2 y2 + x3 y3 + + xn yn For example, for 4 x = 1 , 3 the inner product is (x, y) = 4 (2) + (1) 5 + 3 2 = 7 Properties: 1. (x, y) = (y, x) 2. (ax, y) = a(x, y) = (x, ay) 3. (x, y + z) = (x, y) + (x, z) In Matlab, the inner product is computed with the dot function: dot(x,y) dot(A,B) Example: >> x = [4 -1 3] x= 4 -1 3 >> y = [-2 5 2] y= -2 5 2 >> d = dot(x,y) d= -7 182 Returns the scalar dot product of the vectors x and y, which must be of the same length. Returns a row vector of the dot products for the corresponding columns of matrices A and B. 2 y = 5 , 2 Euclidean Norm The length of a vector is called the norm of the vector. From Euclidean geometry, the distance between two points is the square root of the sum of the squares of the distances in each dimension. Thus, the notation and denition of the Euclidean norm is ||x|| = x2 + x2 + + x2 n 1 2 Note that the norm can be dened in terms of the inner product: ||x|| = (x, x) For example, the norm of x above is ||x|| = 42 + (1)2 + 32 = 5.1 In Matlab, norm(x) returns the Euclidean norm of row vector or column vector x. For example, for x and y as dened above: >> nx = norm(x) nx = 5.0990 >> ny = norm(y) ny = 5.7446 Triangle Inequality This inequality, also called the Cauchy-Bernoulli-Schwarz (CBS) inequality, states that the absolute value of the inner product of vectors x and y is less than or equal to the norm of x times the norm of y, with equality if and only if y = x |(x, y)| ||x|| ||y|| Conrming this using the examples above: >> abs(dot(x,y)) ans = 7 >> norm(x)*norm(y) ans = 29.2916 183 To conrm the equality condition, dene z = 5x >> z = 5*x z= 20 -5 15 >> abs(dot(x,z)) ans = 130 >> norm(x)*norm(z) ans = 130 Unit Vectors The unit vector ux is a vector of length one pointing in the same direction as x ux = 1 x ||x|| >> ux = x/norm(x) ux = 0.7845 -0.1961 0.5883 >> norm(ux) ans = 1 Angle Between Vectors The angle between two vectors x and y is cos = (x, y) ||x|| ||y|| >> theta = acos(dot(x,y)/(norm(x)*norm(y))) theta = 1.8121 >> thetad = (180/pi)*theta thetad = 103.8261 184 Orthogonality When the angle between two vectors is /2 (90 ), the vectors are said to be orthogonal. From the equation for the angle between two vectors (x, y) = 0 x and y are orthogonal Projection The projection of vector x onto y is found by dropping a perpendicular from the head of x onto the line representing y, as shown in Figure 9.2. The point where the perpendicular intersects y (or an extension of y) is the projection of x onto y, or the component of x in the direction of y. Denoting the projection vector by z z= (x, y) (x, y) y y= (y, y) ||y||2 Figure 9.2: Projection of x onto y >> z = (dot(x,y)/norm(y)^2)*y z= 0.4242 -1.0606 -0.4242 Example 9.1 Ship course Figure 9.1 shows a ship sailing on bearing 315 (NW) with a speed of 20 knots. The local current due to the tide is 2 knots in the direction 67.5 (ENE). The ship also drifts at 0.5 knots under wind in the direction 180 . The following script is written to do the following: Represent the ship velocity as a vector S, the current velocity as a vector C, and the wind-drift velocity as a vector W. Calculate the true-velocity vector T (velocity over the ocean bottom). North represents the rst component and East the second component of the vectors. 185 Figure 9.3: Ship course components Calculate the true ship speed, that is, the ship speed over the ocean bottom. Calculate the true ship course, that is, the direction of sailing over the ocean bottom, measured in degrees clockwise from north. disp(Ship velocity vector:) S = 20*[cos(315*pi/180) sin(315*pi/180)] disp(Current velocity vector:) C = 2*[cos(67.5*pi/180) sin(67.5*pi/180)] disp(Wind drift velocity vector:) W = 0.5*[-1 0] disp(True velocity vector:) T=S+C+W disp(Ship speed (knots):) speed = norm(T) course = atan2(T(2),T(1))*180/pi; if course < 0 course = 360 + course; end disp(Ship course (degrees)) course % ship vector % current vector % wind vector % ship course, degrees % correct course if negative The function atan2 was used to compute the course heading, as it provides a four-quadrant result. However, it produces results in the range to , which have to be converted to degrees in the range 0 to 360. The displayed results: Ship velocity vector: S= 14.1421 -14.1421 Current velocity vector: 186 C= 0.7654 1.8478 Wind drift velocity vector: W= -0.5000 0 True velocity vector: T= 14.4075 -12.2944 Ship speed (knots): speed = 18.9401 Ship course (degrees) course = 319.5248 9.2 Matrices A matrix is denoted by a boldfaced capital letter having elements in the corresponding lower-case letter having double subscripts for the row and column. An m n (read m by n) matrix, having m rows and n columns A= a11 a21 a31 . . . a12 a22 a32 . . . a13 a23 a33 . . . ... ... ... a1n a2n a3n . . . am1 am2 am3 . . . amn Scalar element aij is located in row i and column j. A square matrix has the same number of row and columns (m = n). The main diagonal of a matrix consists of the elements aii (the two subscripts are equal). The main diagonal runs diagonally down and away from the top left corner of the matrix, but it does not end in the lower right corner unless the matrix is square. Transpose The transpose of matrix A is another matrix B such that the element in row j and column i is bji = aij , for 1 i m and 1 j n. The notation for transpose is B = AT = A . In Matlab, transpose is denoted by A. A more intuitive way of describing the transpose operation is to say that it ips the matrix about its main diagonal so that rows become columns and columns become rows. For example: >> A = [2 1; 5 4; 7 9] 187 A= 2 5 7 >> B = A B= 2 1 1 4 9 5 4 7 9 Identity Matrix An identity matrix is a matrix with ones on the main diagonal and zeros elsewhere. The following is an identity matrix with four rows and four columns I= 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 To generate an identity matrix in Matlab: eye(n) eye(m,n) eye(size(a)) Returns an n n identity matrix. Returns an m n matrix with ones on the main diagonal and zeros elsewhere. Returns a matrix with ones on the main diagonal and zeros elsewhere that is the same size as A. >> I = eye(3) I= 1 0 0 1 0 0 0 0 1 The variable name i should not be used for an identity matrix in Matlab, because i will not represent 1 in statements that follow. Matrix Addition and Scalar Multiplication Two matrices of the same dimensions may be added or subtracted in the same way as vectors, by adding or subtracting the corresponding elements. The equation C = A B means that for each i and j, cij = aij bij . 188 Scalar multiplication of a matrix multiplies each element of the matrix by the scalar: A = a11 a21 a31 . . . a12 a22 a32 . . . a13 a23 a33 . . . ... ... ... a1n a2n a3n . . . am1 am2 am3 . . . amn Matrix Multiplication The matrix multiplication of mn matrix A and np matrix B yields mp matrix C, denoted by C = AB Element cij is the inner product of row i of A and column j of B cij = n k=1 (9.1) aik bkj (9.2) To better understand this operation, represent a matrix as a collection of vectors. For example, each row of A can be represented as being a vector aT 1 aT 2 A= . . . aT m = [a11 [a21 [am1 a12 a22 . . . a1n ] a2n ] amn ] (9.3) am2 Alternatively, each column of B can be represented as a vector B = b1 | | | | b2 bp = | | b11 b21 . . . bn1 b12 b22 . . . bn2 b1p b2p . . . bnp (9.4) Matrix-matrix multiplication can now be dened in terms of inner products of these vectors. An n-element column vector x is an n 1 matrix, whose transpose xT is a 1 n matrix, or n-element row vector x= x1 x2 . . . xn ; xT = x1 x2 xn 189 Consider the matrix multiplication xT y, where x and y are n-element column vectors x y= T x1 x2 xn y1 y2 . . . yn Applying equation (9.1), let A = xT , and B = y, with m = p = 1, and the result from equation (9.2) is the scalar xT y = n k=1 xk yk = x1 y1 + x2 y2 + + xn yn Observe that this is the inner product of x and y xT y = (x, y) Applying this result to equation (9.1) and using representation of matrices A and B as the collections of vectors dened in equations (9.3) and (9.4) cij = aT bj = (ai , bj ) i Thus, C = AB = aT b1 aT b2 aT bp m m m aT b1 1 aT b1 2 . . . aT b2 1 aT b2 2 . . . aT bp 1 aT bp 2 . . . For example, consider the following matrices 25 1 0 3 1 10 2 B = 1 4 2 52 1 A= where m n = 2 3 and n p = 3 3. Computing elements of C: c11 = 251 1 1 = 2 1 + 5 (1) + 1 5 = 2 5 c23 = 0 3 1 2 2 = 0 2 + 3 (2) 1 1 = 7 1 190 Similarly, the rest of the elements can be computed to yield 2 22 5 8 10 7 C = AB = To determine if a matrix product exists and if so, the size of the resulting matrix, write the matrix sizes side-by-side (m n) (n p) = (m p) The two center sizes (columns of A and rows of B) must be equal for the product to exist. For the example above (2 3) (3 3) = (2 3) Thus AB exists and is of size 2 3. If we wanted to compute BA, we again write the sizes side-by-side (3 3) (2 3) The two inner numbers are not the same, so BA does not exist. Note that this implies that matrix multiplication does not commute AB = BA In Matlab matrix multiplication is denoted by an asterisk. >> A = [2,5,1; 0,3,-1] A= 2 5 1 0 3 -1 >> B = [1,0,2; -1,4,-2; 5,2,1] B= 1 0 2 -1 4 -2 5 2 1 >> C = A*B C= 2 22 -5 -8 10 -7 >> C = B*A ??? Error using ==> * Inner matrix dimensions must agree. 191 Another matrix multiplication involving two vectors is the outer product, which is the product of a row vector and a column vector. If x and y are n-element column vectors, then the product xyT is a n 1 matrix multiplying an 1 n matrix, yielding an n n result xy = T x1 x2 . . . xn y1 y2 y n = x1 y1 x2 y1 . . . x1 y2 x2 y2 . . . x 1 yn x 2 yn . . . xm y1 xm y2 xm yn In the outer product, the inner products that dene its elements are between the one-dimensional row vectors of x and the one-dimensional column vectors of yT . If I is an n n identity matrix and A is an n n matrix, then IA = AI = A Conrming >> A = [1 2 3;4 5 6;7 8 9] A= 1 2 3 4 5 6 7 8 9 >> I = eye(3) I= 1 0 0 0 1 0 0 0 1 >> I*A ans = 1 2 3 4 5 6 7 8 9 >> A*I ans = 1 2 3 4 5 6 7 8 9 Matrix Inverse Two scalars are called inverses when their product is 1, such as 5 and 0.2. Two square matrices are inverses of one another if their product is the identity matrix AB = I B = A1 192 Multiplying the rst form by B from the left B(AB) = BI = B Multiplying in a dierent order (BA)B = B We can then conclude BA = I Thus, while matrix multiplication is not commutative in general, this is a special case where it is commutative. If A and B are inverses of each other, then AB = BA = I The inverse for an ill-conditioned or singular matrix does not exist. The rank of a matrix is the number of independent equations represented by the rows of the of the matrix. If the rank of a matrix is equal to the number of its rows, the matrix is nonsingular and its inverse exists. Matlab functions for matrix rank and inverse: rank(A) inv(A) Returns the rank of the matrix A, which is the number of independent rows of A. Returns the inverse of the matrix A, which must be square and have a rank equal to the number of rows. If the inverse does not exist, an error message is printed. Matrix inverse can also be computed using the expression A^(-1). Example: >> A = [2,1; 4,3] A= 2 1 4 3 >> rank(A) ans = 2 >> B = inv(A) B= 1.5000 -0.5000 -2.0000 1.0000 >> C = A*B C= 193 1 0 0 1 >> D = A^(-1) D= 1.5000 -0.5000 -2.0000 1.0000 The following commands determine the size and rank of a matrix A and then either compute the inverse of A or print a message indicating that the matrix inverse does not exist: [nr,nc] = size(A); if nr ~= nc disp(Inverse does not exist, matrix is not square) elseif rank(A) ~= nr disp(Inverse does not exist, rank not equal to number of rows) else disp(Matrix inverse:) disp(inv(A)) end As shown in this example, it is good programming practice to avoid performing computations that would generate Matlab error messages by including error checks that provide more specic messages. Matrix Powers If A is a matrix, then A.^2 is the expression that squares each element in the matrix. Thus, A.^2 = 2 2 a2 m1 am2 . . . amn a2 11 a2 21 . . . a2 12 a2 22 . . . ... ... a2 1n a2 2n . . . To square the matrix, that is, to compute A*A, the expression A^2 can be used. Then A^4 is equivalent to A*A*A*A. To perform a matrix multiplication between two matrices, the number of rows in the rst matrix must be the same as the number of columns in the second matrix; therefore, to raise a matrix to a power, the matrix must be square. Determinant A determinant is a scalar computed from a square matrix. While there are many applications of determinants, a full explanation is beyond the intent of this presentation. For a 2 2 matrix A, the determinant is |A| = a11 a22 a12 a21 194 Thus, for A= the determinant is |A| = 1 5 3 (1) = 8 For a 3 3 matrix A, the determinant is |A| = a11 (a22 a33 a23 a32 ) a12 (a21 a33 a23 a31 ) + a13 (a21 a32 a22 a31 ) = a11 a22 a33 a11 a23 a32 a12 a21 a33 + a12 a23 a31 + a13 a21 a32 a13 a22 a31 Thus, for 130 B = 1 5 2 121 the determinant is |B| = 1(5 4) 3(1 2) + 0(1 5) = 1 + 9 = 10 If the determinant of a matrix is zero, the matrix is said to be singular and its inverse does not exist. The Matlab function det(A) computes the determinant of the square matrix A. Examples: >> A = [1 A= 1 -1 >> detA = detA = 8 >> B = [1 B= 1 -1 1 >> detB = detB = 10 3; -1 5] 3 5 det(A) 13 1 5 3 0; -1 5 2; 1 2 1] 3 0 5 2 2 1 det(B) 195 9.3 Solutions to Systems of Linear Equations The following is an example of a system of three linear equations with three unknowns (x1 , x2 , x3 ): 3x1 +2x2 x3 = 10 x1 +3x2 +2x3 = 5 x1 x2 x3 = 1 This system of equations can be rewritten in matrix form as 3 2 1 x1 10 3 2 x2 = 5 1 1 1 1 1 x3 or Ax = b where 3 2 1 3 2 A = 1 1 1 1 x1 x = x2 x3 10 b= 5 1 The system of equations can also be expressed using row vectors for B and x. For example, the set of equations above can be written as xA = b by dening 3 1 1 3 1 b = A= 2 1 2 1 x= x1 x2 x3 1 5 1 Note that the matrix A in this equation is the transpose of the matrix A in the original matrix equation. The system of equations is nonsingular if the matrix A containing the coecients of the equations is nonsingular. The rank of A must be equal to the number of rows of A and the determinant |A| must be nonzero for the system of equations to be nonsingular. To avoid errors, evaluate the rank of A (using the rank function) to determine that the system is nonsingular before attempting to compute the solution. Two methods for solving a nonsingular system are given below. 196 Solution by Matrix Inverse Premultiply the system of equations Ax = b by A1 A1 Ax = A1 b Because A1 A is equal to the identity matrix I Ix = A1 b or x = A1 b Example: >> A = [3 2 -1; -1 3 2; 1 -1 -1] A= 3 2 -1 -1 3 2 1 -1 -1 >> b = [10; 5; -1] b= 10 5 -1 >> x = inv(A)*b x= -2.0000 5.0000 -6.0000 >> A*x ans = 10.0000 5.0000 -1.0000 If the system of equations is expressed in the form xA = b 197 where x and b are row vectors and A is a matrix, the equation can be postmultiplied by A1 to yield xAA1 = bA1 Because AA1 is equal to the identity matrix I, xI = bA1 or x = bA1 This is computed in Matlab with the command x = b*inv(A) Solution by Matrix Division In Matlab, a system of simultaneous equations can be solved by matrix division. The solution to the equation Ax = b can be computed using left division x = A\b; Example: >> A = [3,2,-1; -1,3,2; 1,-1,-1] A= 3 2 -1 -1 3 2 1 -1 -1 >> b = [10;5;-1] b= 10 5 -1 >> x = A\b x= 198 -2.0000 5.0000 -6.0000 >> check = A*x check = 10.0000 5.0000 -1.0000 This is the preferred method of solving a system of simultaneous linear equations, as the numerical method used, known as Gauss elimination, has better performance than the method of matrix inverse. If the system of equations is represented by the equation xA = b, where x and b are row vectors, the solution can be computed using matrix right division, as in x = b/A. Geometric Interpretations of Linear Equations in Two Unknowns 1. Intersection: The two equations 3x1 4x2 = 5 6x1 10x2 = 2 can be solved for x2 to yield x2 = (3x1 5)/4 x2 = (6x1 2)/10 These equations are plotted in Figure 9.4. The linear equations describe straight lines that intersect at x1 = 7, x2 = 4, which is the solution of the two equations. Conrming the solution: >> A = [3 -4; 6 -10]; >> det(A) ans = -6 >> b = [5; 2]; >> x = A\b x= 7 4 Note that since |A| = 0, a solution exists. 2. One Line: A set of two equations can describe the same line, which can be interpreted to have an innite number of intersections and thus, no unique solution. The matrix A in this case can be shown to be singular. 199 7 6 5 4 x1=7, x2=4 3 x2 2 x =(6x 2)/10 2 1 1 0 x =(3x 5)/4 2 1 1 2 0 1 2 3 4 5 x 6 7 8 9 10 1 Figure 9.4: The graphs of two equations intersect at the solution For example the two equations 3x1 4x2 = 5 6x1 8x2 = 10 can be solved for x2 , with each yielding 3 5 x2 = x1 4 4 There is no unique solution because the second equation is simply the rst equation multiplied by two. The graphs of these two equations are identical. All that can be said is that the solution must satisfy x2 = 3(x1 5)/4, which describes an innite number of solutions. Conrming this with Matlab: >> A = [3 -4; 6 -8]; >> det(A) ans = 0 >> rank(A) ans = 1 The determinant is 0 and the rank is less that the dimension of the matrix, so the problem is singular and no unique solution exists. 200 3. Parallel Lines: When the two lines are parallel, there can be no intersection, and thus no solution exists. The set of equations 3x1 4x2 = 5 6x1 8x2 = 3 can be solved for x2 , giving x2 = (3x1 5)/4 x2 = (6x1 3)/8 Note that slopes of the two lines are both equal to 3/4, so the lines are parallel, as shown in Figure 9.5. Since they do not intersect, no solution exists. The matrix A is the same as that in the one-line singular case above, where this matrix was found to be singular. 8 7 6 5 4 x2 x2 =(6x1 3)/8 3 2 1 0 x2 =(3x1 5)/4 1 2 0 1 2 3 4 5 x 6 7 8 9 10 1 Figure 9.5: Parallel graphs indicate that no solution exists An ill-conditioned set of equations is a set that is close to being singular (for example, two equations whose graphs are close to being parallel). The solution to an ill-conditioned set of equations depends on the accuracy with which the calculations are made. No computer can represent a number with innitely many signicant gures, and so a given set of equations can appear to be singular if the accuracy required to solve them is greater than the number of signicant gures used by the software. 201 9.4 Applied Problem Solving: Robot Motion Illustrated in Figure 9.6 is a robot arm having two links connected by two joints: a shoulder, or base, joint, and an elbow joint. There is a motor at each joint and the joint angles are 1 and 2 . The (x1 , x2 ) coordinates of the hand at the end of the arm are given by x1 = L1 cos 1 + L2 cos(1 + 2 ) x2 = L1 sin 1 + L2 sin(1 + 2 ) where L1 and L2 are the lengths of the links. Figure 9.6: Robot arm The problem is to determine how to control the joint angles by the motors to move the hand from one position to another. The arm is to start from rest at a known position and move to a desired position. It must start and stop with zero velocity and acceleration. The following polynomial expressions are to be used for controlling the motion by generating commands to be sent to the 202 joint motor controllers 1 (t) = 1 (0) + a1 t5 + a2 t4 + a3 t3 + a4 t2 + a5 t 2 (t) = 2 (0) + b1 t5 + b2 t4 + b3 t3 + b4 t2 + b5 t where 1 (0) and 2 (0) are the initial angles at time t = 0 and coecient vectors a = [a1 a2 a3 a4 a5 ]T and b = [b1 b2 b3 b4 b5 ]T are to be determined to provide the desired motion. The choice of the degree of the polynomials will be explained as the equations of motion are described. For desired initial coordinates (x1 , x2 ) at time t = 0 and desired nal coordinates at time t = tf , the required values for angles 1 (0), 2 (0), 1 (tf ), and 2 (tf ) can be found using trigonometry. For given values of 1 (0), 1 (tf ), and tf , matrix equations are to be set up and solved for coecient vector a. Similarly, for given values of 2 (0), 2 (tf ), and tf , matrix equations are to be set up and solved for coecient vector b. These results are to be used to plot the path of the hand. The remaining constraints in the arm motion are that the velocity and acceleration of the links are to be zero at the known starting location and the desired nal location. This implies that the angular velocity and angular acceleration of the two angles are to be zero at time t = 0 and t = tf . The angular velocity of the rst link at time t is the derivative of the angle 1 (t) with respect to time 1 (t) = 5a1 t4 + 4a2 t3 + 3a3 t2 + 2a4 t + a5 The velocity at t = 0 is 1 (0) = a5 = 0 and thus, coecient a5 = 0. The angular acceleration is the second derivative of the angle 1 (t) with respect to time 1 (t) = 20a1 t3 + 12a2 t2 + 6a3 t + 2a4 The acceleration at t = 0 is 1 (0) = 2a4 = 0 and thus, coecient a4 = 0. Writing the three constraints on 1 (t) and its derivatives at time t = tf in matrix form: 20t3 f 5t4 f t5 f 12t2 f 4t3 f t4 f 1 (tf ) 1 (0) a1 3t2 a2 = 0 f 0 a3 6tf 203 t3 f Similarly, for 2 (t): 20t3 f 5t4 f t5 f 12t2 f 4t3 f t4 f 2 (tf ) 2 (0) b1 3t2 b2 = 0 f b3 0 6tf t3 f Note that there are three equations and three unknowns for each angle and its two derivatives. This is the reason for choosing a fth-degree polynomial to control the motion. The lower degree three terms (t2 , t1 , t0 ) have coecients with value zero to meet the constraints at t = 0. This leaves the higher degree three terms (t5 , t4 , t3 ) and corresponding three unknown coecients. Adding additional higher degree terms would require additional constraint equations to be able to compute the values of the corresponding coecients. Assuming the following initial and nal values and link lengths: tf = 2s 1 (0) = 19 2 (0) = 44 1 (tf ) = 43 2 (tf ) = 151 L1 = 4 feet L2 = 3 feet which correspond to a starting hand location of (6.5, 0) and a destination location of (0, 2), a script to compute the motion control coecients and to compute and plot the path of the hand is: % Robot arm motion script % % Initial values, angles in degrees tf = 2; theta10 = -19*pi/180; theta1tf = 43*pi/180; theta20 = 44*pi/180; theta2tf = 151*pi/180; % % Equations for a coefficients T=[ tf^5 tf^4 tf^3 5*tf^4 4*tf^3 3*tf^2 20*tf^3 12*tf^2 6*tf ]; c = [ theta1tf-theta10; 0; 0 ]; disp(Coefficients for theta1 motion:) a = T\c % % Equations for b coefficients d = [ theta2tf-theta20; 0; 0 ]; disp(Coefficients for theta2 motion:) 204 b = T\d % % Equations of motion L1 = 4; L2 = 3; t = linspace(0,2,401); tq = [ t.^5; t.^4; t.^3 ]; theta1 = theta10 + a*tq; theta2 = theta20 + b*tq; x1 = L1*cos(theta1) + L2*cos(theta1 + theta2); x2 = L1*sin(theta1) + L2*sin(theta1 + theta2); % % Plot path of hand plot(x1,x2),... xlabel(x_1),... ylabel(x_2),... title(Path of robot hand),... text(4.3,0,t=0s: (x_1,x_2) = (6.5,0)),... text(0.2,2,t=2s: (x_1,x_2) = (0,2)) Executing the script: >> robot Coefficients for theta1 motion: a= 0.2029 -1.0145 1.3526 Coefficients for theta2 motion: b= 0.3502 -1.7508 2.3344 The plot of the hand motion is shown in Figure 9.7. 205 Path of robot hand 4 3.5 3 2.5 2 x 2 t=2s: (x ,x ) = (0,2) 12 1.5 1 0.5 0 t=0s: (x1,x2) = (6.5,0) 0.5 0 1 2 3 x1 4 5 6 7 Figure 9.7: Calculated path of robot hand 206 Section 10 Curve Fitting and Interpolation Curve tting: Finding a function (curve) y = f (x) that best ts a set of measured x values and corresponding y values. Interpolating: Estimating the value of yi corresponding to a chosen xi having value between the measured x values. 10.1 Minimum Mean-Square Error Curve Fitting The commonly-used measure of t in curve tting is mean-squared error (MSE). In minimum mean-squared error (MMSE) curve tting (also called least-square curve tting), the parameters of a selected function are chosen to minimize the MSE between the curve and the measured values. Polynomial regression is a form of MMSE curve tting in which the selected function is a polynomial and the parameters to be chosen are the polynomial coecients. Linear regression is a form of polynomial regression in which the polynomial is of rst degree. The two parameters of a linear equation, representing a straight line curve, are chosen to minimize the average of the squared distances between the line and the measured data values. Linear Regression To illustrate linear regression, consider the following set of temperature measurements: Time (s) 0 1 2 3 4 5 Temperature (deg F) 0 20 60 68 77 110 207 Observing the plot of these data points shown in Figure 10.1, it can be seen that a good estimate of a line passing through the points is y = 20x. The script used to generate this plot: x = 0:5; y = [0 20 60 68 77 110]; yhat = 20*x err = yhat-y MSE = mean(err.^2) RMSE = sqrt(MSE) plot( x,y,o, x,yhat),title(Linear Estimate),... xlabel(time, s),ylabel(Temperature, degrees F),... grid,axis([-1,6,-2,120]), legend(measured, estimated,4) Linear Estimate 120 100 80 Temperature, degrees F 60 40 20 measured estimated 0 1 0 1 2 time, s 3 4 5 6 Figure 10.1: A linear estimate A measure of the quality of the t of the linear estimate to the data is the mean squared error (MSE) or the root mean squared error (RMSE) 1 N N k=1 MSE = (k yk )2 y RMSE = MSE where N is the number of measured data values (xk , yk ), with estimated values yk = 20xk . Note that the units of MSE are the square of the units of the measured quantity yk . The units of RMSE are the same as those of the measured quantity. 208 For the plotted data >> MSE = mean((yhat-y).^2) MSE = 95.5000 >> RMSE = sqrt(MSE) RMSE = 9.7724 Note that the absolute values of the errors in the estimates range from zero to 20, so an RMSE value of about 10 seems reasonable. The best t is the line y = a1 x + a2 having coecients a1 and a2 that produce the smallest mean squared error (MSE). MSE is expressed as 1 N 1 N N k=1 N k=1 MSE = = (k yk )2 y (a1 xk + a2 yk )2 The MSE is minimum when its partial derivatives with respect to each of the coecients are zero. MSE a1 1 N 2 N 2 N N k=1 N k=1 = = = 2(a1 xk + a2 yk )xk a1 x2 + a2 xk xk yk k N k=1 x2 a1 + k N k=1 xk a2 N k=1 xk yk =0 MSE a2 = = 1 N 2 N N k=1 2(a1 xk + a2 yk ) N k=1 xk a1 + N a2 N k=1 yk =0 Ignoring the constant factors 2/N and writing the two equations above in matrix form with the 209 terms in the unknown coecients a1 and a2 on the left hand side x2 k k=1 N xk k=1 N N k=1 xk N a1 a2 xk yk k=1 = N yk k=1 N This is a pair of linear equations in two unknowns that can be solved using the methods discussed in the previous section of these notes. The values of a1 and a2 determined in this way represent the straight line with the minimum mean squared error. The Matlab command to compute the best linear t to a set of data is polyfit(x,y,1), which returns a coecient vector of the straight line tting the data. For the data above: x = 0:5; y = [0 20 60 68 77 110]; a = polyfit(x,y,1) yhat = polyval(a,x); err = yhat - y MSE = mean(err.^2) RMSE = sqrt(MSE) plot(x,y,o,x,yhat),title(Linear Regression),... xlabel(time, s),ylabel(Temperature, degrees F),... grid,axis([-1,6,-2,120]),legend(measured,estimated,4) Running this script: a= 20.8286 err = 3.7619 MSE = 59.4698 RMSE = 7.7117 3.7619 4.5905 -14.5810 -1.7524 10.0762 -2.0952 Thus, the best linear t is y = 20.8286x + 3.7619 and the root mean squared error is 7.71, lower than that of the previous estimate. The resulting plot is shown in Figure 10.2. The linear regression curve can also be used to interpolate the measured data to estimate values of y corresponding to values of x between the measured values. For example: 210 Linear Regression 120 100 80 Temperature, degrees F 60 40 20 measured estimated 0 1 0 1 2 time, s 3 4 5 6 Figure 10.2: Linear regression curve t >> yi = polyval(a,2.5) yi = 55.8333 Polynomial Regression The method discussed above for linear regression can be extended to an n-th degree polynomial curve t, using a method known as polynomial regression to nd the minimum mean squared error values of the polynomial coecients. The n-th degree polynomial in one variable is f (x) = a1 xn + a2 xn1 + a3 xn2 + + an x + an+1 The Matlab command to compute a polynomial regression is a = polyfit(x,y,n} This provides an n-th degree least-squares polynomial curve t to the vectors x and y, which must be of the same length. It returns a coecient vector a of length n+1. As the degree increases, the MSE decreases and the number of points that fall on the curve increases. If a set of n + 1 points is used to determine the n-th degree polynomial, all n + 1 points will fall on the polynomial curve. Regression analysis cannot determine a unique solution if the number of points is equal to or less than the degree of the polynomial model. Consider a set of 11 data points and nd the best 2-nd and 10-th degree polynomial curve ts to this data, using the following script: 211 x = 0:0.1:1; y = [-0.447 1.978 3.28 6.16 7.08 7.34 7.66 9.56 9.48 9.30 11.2]; a2 = polyfit(x,y,2); disp(a2 coefficients:) disp(a2) a10 = polyfit(x,y,10); disp(a10 coefficients:) format short e disp(a10) xi = linspace(0,1,101); yi2 = polyval(a2,xi); yi10 = polyval(a10,xi); plot(x,y,o,xi,yi2,--,xi,yi10),... xlabel(x), ylabel(y),... title(2nd and 10th Degree Polynomial Curve Fit),... legend(Measured,2nd Degree,10th Degree,4) Executing this script: a2 coefficients: -9.8108 20.1293 -0.0317 a10 coefficients: -4.6436e+005 2.2965e+006 -4.8773e+006 5.8233e+006 -4.2948e+006 2.0211e+006 -6.0322e+005 1.0896e+005 -1.0626e+004 4.3599e+002 -4.4700e-001 The plot produced is shown in Figure 10.3. Thus, the best quadratic (n = 2) curve t is y = 9.8108x2 + 20.1293x 0.0317 As the polynomial degree increases, the approximation becomes less smooth since higher-order polynomials can be dierentiated more times before then become zero. Note the values of the 10th degree polynomial coecients a10 compared to those of the quadratic t. Note also the seven orders of magnitude dierence between the smallest (-4.4700e-001) and the largest (5.8233e+006) 212 2nd and 10th Degree Polynomial Curve Fit 16 14 12 10 8 y 6 4 2 Measured 2nd Degree 10th Degree 0 0.1 0.2 0.3 0.4 0.5 x 0.6 0.7 0.8 0.9 1 0 2 Figure 10.3: Polynomial Curve Fitting coecients and the alternating signs on the coecients. This example clearly demonstrates the diculties with higher-degree polynomials. 10.2 Applied Problem Solving: Hydraulic Engineering The following application is from Matlab for Engineering Applications by William J. Palm III (McGraw-Hill, 1999). Torricellis principle of hydraulic resistance states that the volume ow rate f of a liquid through a restriction, such as an opening or a valve, is proportional to the square root of the pressure drop p across the restriction f =c p where c is a constant. In many applications the weight of liquid in a tank above the valve causes the pressure drop. In this case, the ow rate is proportional to the square root of the the volume V in the tank f =r V where r is a constant. This expression can be generalized to f = rV e 213 and an attempt can be made to experimentally verify that e = 0.5. Consider applying this principle to the ow of coee out of a coee pot. To gather some experimental measurements, place a 15-cup coee pot under a water faucet and ll to the 15-cup line. With the outlet valve open, adjust the ow rate through the faucet so that the water level remains constant at 15 cups, and measure the time for one cup to ow out of the pot. Repeat this measurement with the pot lled to the levels shown in the following table: Volume V (cups) 15 12 9 6 Fill time t (s) 6 7 8 9 Use this data to verify Torricellis principle for the coee pot and obtain a relation between the ow rate and the number of cups in the pot. Torricellis principle is a power function and if we apply the base 10 logarithm to both sides, we obtain log10 f = e log10 V + log10 r This equation has the form y = a1 x + a2 where y = log10 f , x = log10 V , a1 = e, and a2 = log10 r. Thus, if we plot log10 f versus log10 V , we should obtain a straight line. The values for f are obtained from the reciprocals of the given data for t. That is, f = 1/t cups per second. We can nd the power function that ts the data with the command a = polyfit(log10(V), log10(f), 1) The rst element a1 of vector a will be e and the second element a2 will be log10 r. We can nd r from r = 10a2 . The Matlab script is: % Modeling of hydraulic resistance in a coffee pot % % Measured data vol = [6, 9, 12, 15]; % coffee pot volume (cups) time = [9, 8, 7, 6]; % time to fill one cup (seconds) flow = 1./time; % flow rate (cups/seconds) % % Fit a straight line to log transformed data a = polyfit(log10(vol),log10(flow),1); 214 disp(Exponent e) e = a(1) disp(Constant r) r = 10^a(2) % % Plot the data and the fitted curve on loglog and linear plots x = [6:0.01:40]; y = r*x.^e; subplot(2,1,1),loglog(x,y,vol,flow,o),grid,... xlabel(Volume (cups - log scale)),... ylabel(Flow Rate (cups/sec - log scale)),... axis([5 15 .1 .3]), legend(Model,Experimental) subplot(2,1,2),plot(x,y,vol,flow,o),grid,xlabel(Volume (cups)),... ylabel(Flow Rate (cups/sec)),axis([5 15 .1 .3]),... legend(Model,Experimental) The displayed output is: Exponent e e= 0.4331 Constant r r= 0.0499 The derived relation is f = 0.0499V 0.433 Because the exponent is 0.433, not 0.5, our model does not agree exactly with Torricellis principle, but it is close. Note that the plot in Figure 10.4 shows that the data points do not lie exactly on the tted straight line. In this application it is dicult to measure the time to ll one cup with an accuracy greater than an integer second, so this inaccuracy could have caused our result to disagree with that predicted by Torricelli. 10.3 Interpolation In interpolation, data points (x(k), y(k)) for k = 1, 2, . . . , N have been acquired, with the x values in ascending order so that x(k) < x(k + 1). For a chosen value, an estimate is sought for yi corresponding to a chosen xi value that lies within the data range (x(1) xi x(N )). Thus, the objective is not to t the entire range of data with a single curve, as the case for curve tting, but rather to use the data to estimate a yi value corresponding to xi . The estimate is local in the sense that it is oomputed only from data points having x(k) values near that of xi . 215 Flow Rate (cups/sec log scale) Model Experimental 10 1 10 Volume (cups log scale) 1 Flow Rate (cups/sec) 0.25 Model Experimental 0.2 0.15 0.1 5 6 7 8 9 10 11 Volume (cups) 12 13 14 15 Figure 10.4: Flow rate and volume for a coee pot In linear interpolation, the estimate is based on a straight line connecting neighboring pairs of data points. In cubic-spline interpolation, the estimate is based on joining neighboring data points with a cubic (third-degree) polynomial. Linear Interpolation The method of linear interpolation is illustrated in Figure 10.5, which shows measured data points (x(k), y(k)) and (x(k + 1), y(k + 1)) for which the chosen xi lies in the range x(k) xi x(k + 1). The equation of the straight line between these data points is yi = y(k) + m(xi x(k)) where m is the slope of the line m= y(k + 1) y(k) x(k + 1) x(k) Substituting this slope into the equation of the line, the estimated or interpolated value is yi = y(k) + y(k + 1) y(k) (xi x(k)) x(k + 1) x(k) Given a set of data points, it is conceptually straightforward to interpolate for a new point between two of the given points. However, the interpolation takes several steps, because it is rst necessary 216 Figure 10.5: Linear interpolation to nd the two values in the data between which the desired point falls. When these two values are found, the interpolation equation above can be applied. These steps are performed by the Matlab interpolation function interp1. yi = interp1(x,y,xi) Returns vector yi of the length of xi, containing the interpolated y values corresponding to xi using linear interpolation. The vector x, which must have values in ascending order, species the points at which the data y is given. Vectors x and y must be of the same length. The values of xi must be within the range of the x values. To illustrate use of linear interpolation, consider again the temperature data from the previous section. The range of x in this data is 0.0 to 5.0, so interpolated values can be found for data in this range. Computing linearly interpolated values for x = 2.6 and x = 4.9: >> >> >> yi x = 0:5; y = [0 20 60 68 77 110]; yi = interp1(x,y,[2.6 4.9]) = 64.8000 106.7000 If the second argument of the interp1 function is a matrix, the function returns a row vector with the same number of columns, and each value returned will be interpolated from its corresponding 217 column of data. Assume the following data has been acquired: Time, s 0 1 2 3 4 5 Temp 1 0 20 60 68 77 110 Temp 2 0 25 62 67 82 103 Temp 3 0 52 90 91 93 96 Storing this data in a matrix and interpolating at the time 2.6 seconds: >> x = (0:5); >> y(:,1) = [0,20,60,68,77,110]; >> y(:,2) = [0,25,62,67,82,103]; >> y(:,3) = [0,52,90,91,93,96]; >> temps = interp1(x,y,2.6) temps = 64.8000 65.0000 90.6000 Cubic-Spline Interpolation A cubic spline is a third-degree polynomial computed to provide a smooth curve between the two data points bounding the point for which an interpolated value is to be determine and to provide a smooth transition from the third-degree polynomial between the previous two points. The interpolating equation is yi = a1 (xi x(k))3 + a2 (xi x(k))2 + a3 (xi x(k)) + a4 for which x(k) xi x(k + 1). The coecients a1 , a2 , a3 and a4 are determined so that the following three conditions are met: 1. The polynomial must pass through the data points at its end points x(k) and x(k + 1). For xi = x(k), this requires that yi = y(k), so that a4 = y(k). 2. The slopes (rst derivatives) of cubic polynomials in adjacent data intervals must be equal at their common data point. 3. The curvatures of adjacent polynomials must be equal at their common data point. The corresponding Matlab function is: yi = spline(x,y,xi) Returns vector yi of the length of xi, containing the interpolated y values corresponding to xi using cubic-spline interpolation. The vector x, which must have values in ascending order, species the points at which the data y is given. Vectors x and y must be of the same length. The values of xi must be within the range of the x values. 218 To illustrate, apply cubic-spline interpolation to the temperature data considered above. >> x = 0:5; >> y = [0,20,60,68,77,110]; >> temp = spline(x,y,[2.6,4.9]) temp = 67.3013 105.2020 To compute and plot linear and cubic-spline interpolation curves over a range of values, an xi vector with the desired resolution of the curve can be generated and used as the third parameter in the interp1 function. For example: % Comparison of linear and cubic spline interpolation x = (0:5); y = [0,20,60,68,77,110]; xi = 0:0.1:5; ylin = interp1(x,y,xi); ycub = spline(x,y,xi); plot(xi,ylin,:,xi,ycub,x,y,o),... legend(Linear,Cubic,Measured,4),... title(Linear and cubic spline interpolation),... xlabel(x),... axis([-1,6,-20,120]),... grid The resulting plot is shown in Figure 10.6. 10.4 Applied Problem Solving: Human Hearing To illustrate one-dimensional interpolation, consider the following example: The threshold of audibility (i.e. the lowest perceptible sound level) of the human ear varies with frequency. Plotting frequency in Hz on a log scale against sound pressure level in dB, normalized so that 0 dB appears at 1000 Hz is done with the following script: f = [20:10:100 200:100:1000 1500 2000:1000:10000]; % frequency in Hz spl = [76 66 59 54 49 46 43 40 38 22 ... % sound pressure level in dB 14 9 6 3.5 2.5 1.4 0.7 0 -1 -3 ... -8 -7 -2 2 7 9 11 12]; semilogx(f,spl,-o),... xlabel(Frequency, Hz),... ylabel(Relative Sound Pressure Level, dB),... title(Threshold of human hearing),grid on 219 Linear and cubic spline interpolation 120 100 80 60 40 20 0 Linear Cubic Measured 0 1 2 x 3 4 5 6 20 1 Figure 10.6: Comparison of linear and cubic spline interpolation Threshold of human hearing 80 70 60 Relative Sound Pressure Level, dB 50 40 30 20 10 0 10 1 10 10 2 10 Frequency, Hz 3 10 4 Figure 10.7: Plot of Threshold of Human Hearing 220 The resulting plot is shown in Figure 10.7, where by default, the data points have been connected with straight lines. Based on this plot, the human ear is most sensitive to tones around 3 kHz. The function interp1 can be used to estimate the sound pressure level in several dierent ways at a frequency of 2.5 kHz. >> slin = interp1(f,spl,2500,linear) slin = -5.5000 >> scub = interp1(f,spl,2500,spline) scub = -5.6641 >> snn = interp1(f,spl,2500,nearest) snn = -8 % linear interpolation % cubic spline interpolation % nearest neighbor interpolation where nearest neighbor is another interpolation method, in which, as implied by its name, interpolates with the nearest sample in the original data. Note the dierences in these results. The rst result returns exactly what is shown in the gure at 2.5 kHz since Matlab linearly interpolates between data points on plots. Cubic spline interpolation ts a cubic polynomial to each data interval, so it returns a slightly dierent result. The poorest interpolation in this case is the nearest neighbor. How is an interpolation method to be chosen for a given problem? In many cases, linear interpolation is sucient, which is why it is the default method. While nearest neighbor produced poor results here, it is often used when speed is important or the data set is large. The most time-consuming method is spline, but it often produces the most desirable results. Now use cubic interpolation to investigate the data at a ner interval near the minimum. fi = linspace(2500,5000); spli = interp1(f,spl,fi,cubic); % interpolate near minumum k = find(f>=2000 & f<=5000); % find indices near minumum semilogx(f(k),spl(k),--o,fi,spli),... % plot orig & cubic data legend(Linear,Cubic),... xlabel(Frequency, Hz),... ylabel(Relative Sound Pressure Level, dB),... title(Threshold of human hearing),grid on The resulting plot is shown in Figure 10.8. By specifying a ner resolution on the frequency axis and using cubic convolution, a smoother estimate of the sound pressure level is generated. Note how the slope of the cubic solution does not change abruptly at the data points. With the cubic spline interpolation, a better estimate can be made of the frequency of greatest sensitivity. >> [splmin,kmin] = min(spli) % minimum and index of minimum 221 Threshold of human hearing 2 Linear Cubic 3 Relative Sound Pressure Level, dB 4 5 6 7 8 9 3 10 Frequency, Hz 10 4 Figure 10.8: Plot of threshold of human hearing near minimum splmin = -8.2683 kmin = 32 >> fmin = fi(kmin) fmin = 3.2828e+003 % frequency at minimum Thus, by this analysis, the human ear is most sensitive to tones near 3.3 kHz. 222 Section 11 Integration and Dierentiation Integration and dierentiation are the key concepts presented in the rst two calculus courses and they are fundamental to solving many engineering and science problems. While many of these problems can be solved analytically, there are also many problems that require numerical integration or numerical dierentiation techniques. 11.1 Numerical Integration The integral of a function f (x) for a x b can be interpreted as the area under the curve of f (x) between x = a and x = b, as shown in Figure 11.1. Denoting this area as A, the integral is written as b A= Denitions: a f (x)dx Integrand: f (x) Lower limit of integration: a Upper limit of integration: b Variable of integration: x Numerical integration, called quadrature, involves methods for estimating the value of a denite integral. In these methods, the function f (x) is estimated or approximated by another function f (x), chosen so that the area under f (x) is easy to compute. The better the estimate of f (x) by (x), the better the estimate of the integral of f (x). Two of the most common numerical integration f techniques estimate f (x) with a set of piecewise linear functions or with a set of piecewise parabolic functions. If the function is estimated with piecewise linear functions, the area of the trapezoids that compose the area under the piecewise linear functions is the approximation to the desired 223 Figure 11.1: Integral of f (x) from a to b integral, and the method is known as the trapezoidal rule. If the function is estimated with piecewise quadratic functions, the technique is called Simpsons rule. Trapezoidal Rule In the trapezoidal rule for integration, the interval [a, b] is divided into n equal subintervals and the curve f (x) is approximated by f (x) on each subinterval as a straight line connecting the values of f (x) at the ends of each subinterval. The integral A is the sum of the approximate integrals on each subinterval. The width of each subinterval is x = ba n The range of values of x on subinterval i is [xi , xi+1 ] = [xi , xi + x] = [a + ix, a + (i + 1)x], i = 0, . . . , n 1 The approximation of f (x) on subinterval i is shown in Figure 11.2. The approximating curve f (x) is represented by the dashed line. The approximate area Ai of f (x) over this subinterval is that of the trapezoid under f (x) Ai (xi+1 xi ) x f (xi ) + f (xi+1 ) = (f (xi ) + f (xi+1 )) 2 2 The full integral A is then approximated by n AT = i=0 x (f (xi ) + f (xi+1 )) 2 = x (f (x0 ) + 2f (x1 ) + 2f (x2 ) + + 2f (xn1 ) + f (xn )) 2 The Matlab function for trapezoidal rule integration is 224 Figure 11.2: Approximation of f (x) from xi to xi+1 z = trapz(x,y) Integral of y with respect to x by the trapezoidal rule. x and y must be vectors of the same length, or x must be a column vector and y an array whose rst non-singleton dimension is length(x). trapz operates along this dimension. Integrates across dimension dim of y. The length of x must be the same as size(y,dim)). z = trapz(x,y,dim) To illustrate integration, consider the function humps(x), a demonstration function provided by Matlab that has strong peaks near x = 0.3 and x = 0.9, shown in Figure 11.3. Trapezoidal integration of humps(x) 100 80 60 humps(x) 40 20 0 20 1 0.5 0 0.5 x 1 1.5 2 Figure 11.3: Trapezoidal integration of humps(x) Using computed samples of the humps function, the function trapz approximates the area using the trapezoidal approximation. For n = 18 subintervals, the integral approximation is given by: >> x = linspace(-1,2,18) 225 >> y = humps(x); >> area = trapz(x,y) area = 25.1406 Based on the gure, this is probably not a very accurate estimate of the area. However, if a ner discretization is used by increasing the number of subintervals to 401, better accuracy is achieved: >> x = linspace(-1,2,401); >> y = humps(x); >> area = trapz(x,y) area = 26.3449 This area agrees with the analytic integral to ve signicant digits. Another integral of interest is that for which the upper limit is the variable x: x a f (u)du where the variable of integration has been changed to u to avoid confusion with the upper limit x. The denite integral from the lower limit of integration, a, to any point x is found by evaluating the integral at x. Using the trapezoidal rule, tabulated values of the cumulative integral are computed using the function cumtrapz: z = cumtrapz(x,y) Computes the cumulative integral of y with respect to x using trapezoidal integration. x and y must be vectors of the same length, or x must be a column vector and y an array whose rst non-singleton dimension is length(x). cumtrapz operates across this dimension. Integrates along dimension dim of y. The length of x must be the same as size(y,dim)). z = cumtrapz(x,y,dim) For example, consider integration of humps(x) x = linspace(-1,2,201); y = humps(x); z = cumtrapz(x,y); plot(x,y,x,z,--),... title(Cumulative integral of humps(x)),... xlabel(x),ylabel(humps(x) and integral of humps(x)),... grid,legend(humps(x),integral of humps(x)) The resulting plot is shown in Figure 11.4. Example 11.1 Velocity from an accelerometer 226 Cumulative integral of humps(x) 100 humps(x) integral of humps(x) 80 humps(x) and integral of humps(x) 60 40 20 0 20 1 0.5 0 0.5 x 1 1.5 2 Figure 11.4: Cumulative integral of humps(x) An accelerometer measures acceleration and the resulting measurements can be used to estimate velocity and displacement. The acceleration is integrated to estimate velocity and the velocity in turn is integrated to estimate displacement. Suppose a vehicle starts from rest at time t = 0 and that its measured acceleration is given by the following table: Time (s) 0 1 2 3 4 5 6 7 8 9 10 Accel (m/s2 ) 0 2 4 7 11 17 24 32 41 48 51 A script to estimate the velocity: t = [0:10]; a = [0,2,4,7,11,17,24,32,41,48,51]; v = cumtrapz(t,a); disp([ time accel velocity]) disp([t,a,v]) 227 The displayed results: time 0 1.0000 2.0000 3.0000 4.0000 5.0000 6.0000 7.0000 8.0000 9.0000 10.0000 accel 0 2.0000 4.0000 7.0000 11.0000 17.0000 24.0000 32.0000 41.0000 48.0000 51.0000 velocity 0 1.0000 4.0000 9.5000 18.5000 32.5000 53.0000 81.0000 117.5000 162.0000 211.5000 Simpsons Rule If the area under a curve is approximated by areas under quadratic curve, with the interval [a, b] divided into 2n equal subintervals, then the area can be approximated by Simpsons rule: AS = x (f (x0 ) + 4f (x1 ) + 2f (x2 ) + 4f (x3 ) + + 2f (x2n2 ) + 4f (x2n1 ) + f (x2n )) 3 where the xk values represent the endpoints of the subintervals and where x0 = a, x2n = b, and x = (b a)/(2n). If the piecewise components of the approximating function are higher-degree functions, the integration techniques are referred to as Newton-Cotes integration techniques. The estimate of an integral improves as more components are used to approximate the area under a curve. An integral approximation can be made to meet a desired relative error by increasing the number of components until the error specication is met. If the function to be integrated contains a singularity (a point at which the function or its derivatives are innity or are not dened), a satisfactory result may not be provided by numerical techniques. Matlab provides two quadrature functions for performing numerical function integration. 228 quad(function,a,b) quad8(function,a,b) Returns the approximation of the integral of the function f(x), whose name is contained in the string function, from a to b to within a relative error of 0.001 using an adaptive recursive Simpsons rule. Function f must return a vector of output values if given a vector of input values. Inf is returned if an excessive recursion level is reached, indicating a possibly singular integral. Returns the approximation of the integral of the function between a and b using an adaptive Newton-Cotes 8-panel rule. This function is better than the quad function at handling some functions with singularities. For example, consider computing the integral of the humps function again: >> quad(humps,-1,2) ans = 26.34497558341242 >> quad8(humps,-1,2) ans = 26.34496024631924 Here the quad solution achieves six signicant digit accuracy and the quad8 solution achieves eight signicant digit accuracy. Two-Dimensional Integration Two-dimensional integration is evaluated using the function dblquad, which approximates the integral ymax ymin xmax xmin f (x, y)dxdy q = dblquad(function,xmin,xmax,ymin,ymax): Returns the evaluation of the double integral f(x,y) using the quad quadrature function where x is the inner variable ranging from xmin to xmax, and y is the outer variable ranging from ymin to ymax. The rst argument function is a string representing the integrand function. This function must be a function of two variables of the form z =f(x,y). The function must take a vector x and a scalar y and return a vector z that is the function evaluated at y and each value of x. To use dblquad, one must rst create a function that evaluates f (x, y). Consider, for example, the function func.m: function z = func(x,y) % FUNC(X,Y) computes an example function of two variables z = sin(x).*cos(y) + 1; Using the commands listed below, this function can be plotted as shown in Figure 11.5: 229 x = linspace(0,pi,20); y = linspace(-pi,pi,20); [xx,yy] = meshgrid(x,y); zz = func(xx,yy); mesh(xx,yy,zz),xlabel(x),ylabel(y) 2 1.5 1 0.5 0 4 2 3 0 2 4 0.5 0 1.5 1 2.5 2 3.5 Figure 11.5: Mesh plot of func(x,y) The integral of this function over limits 0 x , y is given by >> dblquad(func,0,pi,-pi,pi) ans = 19.73921476256606 11.2 Numerical Dierentiation The derivative of a function f (x) is dened as a function f (x) that is equal to the rate of change of f (x) with respect to x df (x) dx f (x) = Dierentiation and integration are complementary operations. For example, if f (x) = g(x)dx 230 then g(x) is the derivative of f (x) with respect to x. This relationship is written as g(x) = df (x) = f (x) dx The derivative f (x) can be interpreted geometrically as the slope of the function at (x, f (x)), where the slope of f (x) is the slope of the tangent line to the function at x, as shown in Figure 11.6. Figure 11.6: Derivative of f (x) at x = a. Formally, the derivative of f at x is f (x) = lim f (x + x) f (x) f (x + x) f (x) = lim x0 (x + x) x x x0 Dierence Expressions Numerical dierentiation techniques estimate the derivative of a function at a point xk by approximating the slope of the tangent line at xk using values of the function at points near xk . The approximation of the slope of the tangent line can be done in several ways, as shown in Figure 11.7. The backward dierence approximation of the derivative at xk is the slope of the line between f (xk1 ) and f (xk ), as shown in Figure 11.7a f (xk ) = f (xk ) f (xk1 ) xk xk1 If the samples of x are uniformly spaced so that xk xk1 = x f (xk ) = f (xk ) f (xk1 ) x The forward dierence approximation of the derivative at xk is the slope of the line between f (xk ) and f (xk+1 ), as shown in Figure 11.7b f (xk ) = f (xk+1 ) f (xk ) xk+1 xk 231 For uniform spacing x f (xk ) = f (xk+1 ) f (xk ) x Note that the forward dierence at xk is the same as the backward dierence at xk+1 . Figure 11.7: Techniques for approximating f (xk ). The quality of both of these derivative approximations depends heavily on two factors: the spacing of the samples and the scatter in the data due to measurement error. The greater the spacing, the more dicult it is to estimate the derivative. The approximation improves as the spacing between the two points decreases. The central dierence is the average of the forward and backward dierences. For uniform spacing f (xk ) = 1 2 f (xk+1 ) f (xk ) f (xk ) f (xk1 ) + x x = f (xk+1 ) f (xk1 ) 2x This average tends to cancel out the eects of measurement error. The second derivative of a function f (x) is the derivative of the rst derivative of the function f (xk ) = df (x) dx This function can be approximated using slopes of the rst derivative. Using backward dierences and assuming uniform spacing x f (xk ) = = = f (xk ) f (xk1 ) x 1 f (xk ) f (xk1 ) f (xk1 ) f (xk2 ) x x x f (xk ) 2f (xk1 ) + f (xk2 ) (x)2 232 diff Function The diff function computes dierences between adjacent values in a vector, generating a new vector with one less value. diff(x) For vector x, returns a vector of dierences between adjacent values in x: [x(2)-x(1) x(3)-x(2) ... x(n)-x(n-1)], where n is length(x). For a matrix x, returns the matrix of column dierences, [x(2:n,:) x(1:n-1,:)]. Example: >> >> >> dx x = [0,1,2,3,4,5]; y = [2,3,1,5,8,10]; dx = diff(x) = 1 1 1 1 >> dy = diff(y) dy = 1 -2 4 3 1 2 An approximate derivative f (x) is computed using diff by dividing a change in y = f (x) by a change in x. >> df = diff(y)./diff(x) df = 1 -2 4 3 2 Obviously, in this example, since samples of x are spaced by 1, dy and df are the same. Note that the values of df are correct for both the forward-dierence and the backward-dierence approximation to the derivative, as explained above. The distinction between the two methods of approximating the derivative is determined by the values of x that correspond to the derivative dy. Since dy has length one less than that of x, it must be related to a vector xd that is the vector x truncated by one sample at either the beginning or the end. The backward-dierence approximation of the derivative is related to x truncated at the beginning: >> xd = x(2:end) xd = 1 2 3 4 5 The forward-dierence approximation of the derivative is related to x truncated at the end: >> xd = x(1:end-1) xd = 0 1 2 3 4 233 The backward dierence and central dierence methods can be compared by considering a sinusoidal signal that is measured 51 times during one half-period. The measurements are in error by a Gaussian distributed error with standard deviation of 0.025. Figure 11.2 shows the measured data and the underlying sine curve. The following script implements the two methods. The results are shown in Figure 11.2. Clearly the central dierence method provides better results in this example. % Comparison of numerical derivative algorithms x = [0:pi/50:pi]; n = length(x); % Sine signal with Gaussian random error yn = sin(x)+0.025*randn(1,n); % Derivative of noiseless sine signal td = cos(x); % Backward difference estimate noisy sine signal dynb = diff(yn)./diff(x); subplot(2,1,1),plot(x(2:n),td(2:n),x(2:n),dynb,o),xlabel(x),... ylabel(Derivative),axis([0 pi -2 2]),... legend(True derivative,Backward difference) % Central difference dync = (yn(3:n)-yn(1:n-2))./(x(3:n)-x(1:n-2)); subplot(2,1,2),plot(x(2:n-1),td(2:n-1),x(2:n-1),dync,o),xlabel(x),... ylabel(Derivative),axis([0 pi -2 2]),... legend(True derivative,Central difference) 1 0.8 0.6 y 0.4 0.2 0 0 0.5 1 1.5 x 2 2.5 3 Figure 11.8: Measurements of a sine signal containing Gaussian distributed random errors Example 11.2 Numerical dierentiation of a polynomial function 234 2 True derivative Backward difference 1 Derivative 0 1 2 0 0.5 1 1.5 x 2 2.5 3 2 True derivative Central difference 1 Derivative 0 1 2 0 0.5 1 1.5 x 2 2.5 3 Figure 11.9: Comparison of backward dierence and central dierence methods Determine the linear acceleration of an object whose speed is dened by s(t) = t3 2t2 + 2 m/s, where t is in seconds, over the interval 0 to 5. Determine the specic acceleration at t = 2.5s. Consider computing the derivative function at three dierent resolutions to determine the eect of increasing the resolution. Begin with a time resolution of t = 0.1 second, compute the speed at each of the time values and compute the acceleration by approximating the derivative of the speed with respect to time: >> t = 0:0.1:5; >> s = t.^3 - 2*t.^2 + 2; >> ds = diff(s)./diff(t); To determine the acceleration at t = 2.5s, it is rst necessary to nd the time index corresponding to this time. Relating t to nd the index k and the time interval t: t = t (k 1) Thus k= or k= 2.5 + 1 = 26 0.1 235 t +1 t and >> ds(26) ans = 9.3100 Thus, the acceleration is 9.31 meters/second2 . Repeating the computations for t = 0.01 second: >> t = 0:0.01:5; >> s = t.^3 - 2*t.^2 + 2; >> ds = diff(s)./diff(t); The time index k for t = 2.5s: k= 2.5 + 1 = 251 0.01 The computed acceleration: >> ds(251) ans = 8.8051 Repeating again for a resolution t = 0.001s, where we are interested in the acceleration at index k= 2.5 + 1 = 2501 0.001 >> t = 0:0.001:5; >> s = t.^3 - 2*t.^2 + 2; >> ds = diff(s)./diff(t); >> ds (2501) ans = 8.7555 Note that the result decreases with each decrease in the time interval. To check the answer, analytically dierentiate s(t) to obtain: s (t) = 3t2 4t At t = 2.5s: s (2.5) = 3(2.5)2 4 2.4 = 8.75m/s2 236 Thus, we observe that the approximation is converging to the correct result. Since the Matlab diff function returns only an approximate derivative, it is necessary to use the resolution required to achieve the accuracy desired. Dierentiation Error Sensitivity Dierentiation is sensitive to minor changes in the shape of a function, as any small change in the function can easily create large changes in its slope in the neighborhood of the change. Because of this inherent sensitivity in dierentiation, numerical dierentiation is avoided whenever possible, especially if the data to be dierentiated are obtained experimentally. In this case, it is best to perform a least squares curve t to the data and then dierentiate the resulting polynomial. For example, reconsider the example from the section on curve tting (Section 10). In the script below, the x and y data values determined by assignment statements and a second degree curve is t to the data and plotted as the rst subplot. The derivative is approximated from the data and then from the second degree curve t and these curves are plotted as the second subplot. Since diff computes the dierence between elements of a vector, the resulting vector contains one less element than the original vector. Thus, to plot the derivative, one element of the x vector is discarded to form the vector xd. The last element was discarded, so yd is a forward dierence approximation. % Generate noisy data x = 0:0.1:1; y = [-0.447 1.978 3.28 6.16 7.08 7.34 7.66 9.56 9.48 9.30 11.2]; % 2nd degree curve fit a = polyfit(x,y,2); xi = linspace(0,1,101); yi = polyval(a,xi); subplot(2,1,1),plot(x,y,--o,xi,yi),... xlabel(x), ylabel(y),... title(Noisy data and 2nd degree curve fit),... legend(Noisy data,2nd Degree Fit,4) % Differentiate noise data and fitted curve yd = diff(y)./diff(x); ad = polyder(a); yid = polyval(ad,xi); xd = x(1:end-1); subplot(2,1,2),plot(xd,yd,--o,xi,yid),... xlabel(x), ylabel(dy/dx),... title(Derivative approximations),... legend(Noisy data,2nd Degree Fit,3) The resulting plot is shown in Figure 11.10. Observing the approximation to the derivative shown in 237 dashed lines in the bottom subplot, it is overwhelmingly apparent that approximating the derivative by nite dierences can lead to poor results. Note that the derivative of the second order curve t shown in the solid curve in the bottom subplot does not show the large uctuations of the approximation. Noisy data and 2nd degree curve fit 12 10 8 6 y 4 2 0 2 0 0.1 0.2 0.3 0.4 0.5 x 0.6 0.7 0.8 0.9 1 Derivative approximations 30 25 20 dy/dx 15 10 5 0 5 0 0.1 0.2 0.3 0.4 0.5 x 0.6 0.7 0.8 0.9 1 Figure 11.10: Curve tting and derivative approximation 238 Section 12 Strings, Time, Base Conversion and Bit Operations 12.1 Character Strings While Matlab is primarily intended for number crunching, there are times when it is desirable to manipulate text, as is needed in placing labels and titles on plots. In Matlab, text is referred to as character strings or simply strings. String Construction Character strings in Matlab are special numerical arrays of ASCII values that are displayed as their character string representation. For example: >> text = This is a character string text = This is a character string >> size(text) ans = 1 26 >> whos Name Size Bytes Class ans text 1x2 1x26 16 52 double array char array Grand total is 28 elements using 68 bytes 239 ASCII Codes Each character in a string is one element in an array that requires two bytes per character for storage, using the ASCII code. This diers from the eight bytes per element required for numerical or double arrays. The ASCII codes for the letters A to Z are the consecutive integers from 65 to 90, while the codes for a to z are 97 to 122. The function abs returns the ASCII codes for a string. >> text = This is a character string text = This is a character string >> d = abs(text) d= Columns 1 through 12 84 104 105 115 32 105 115 Columns 13 through 24 97 114 97 99 116 101 114 Columns 25 through 26 110 103 32 32 97 115 32 116 99 114 104 105 The function char performs the inverse transformation, from ASCII codes to a string: >> char(d) ans = This is a character string The relationship between strings and their ASCII codes allow you to do the following: >> alpha = abs(a):abs(z); >> disp(char(alpha)) abcdefghijklmnopqrstuvwxyz Strings are Arrays Since strings are arrays, they can be manipulated with array manipulation tools: >> text = This is a character string; >> u = text(11:19) u= character As with matrices, character strings can have multiple rows, but each row must have an equal number of columns. Therefore, blanks are explicitly required to make all rows the same length. For example: 240 >> v = [Character strings having more than one row must have the same number of columns - just like matrices ] v= Character strings having more than one row must have the same number of columns - just like matrices >> size(v) ans = 3 34 Concatenation of Strings Because strings are arrays, they may be concatenated (joined) with square brackets. For example: >> today = May; >> today = [today, 18] today = May 18 String Conversions char(x) Converts the array x that contains positive integers representing character codes into a character array (the rst 127 codes are ASCII). The result for any elements of x outside the range from 0 to 65535 is not dened. Rounds the elements of the matrix x to integers and converts the result into a string matrix. Converts the matrix x into a string representation with about 4 digits and an exponent if required. This is useful for labeling plots with the title, xlabel, ylabel, and text commands. Converts the string s, which should be an ASCII character representation of a numeric value, to numeric representation. The string may contain digits, a decimal point, a leading + or - sign, an e preceeding a power of 10 scale factor, and an i for a complex unit. int2str(x) num2str(x) str2num(s) The num2str function can be used to convert numerical results into strings for use in formating displayed results with disp. For example, consider the following portion of a script: tg = 2.2774; xg = 144.6364; disp([time of flight: num2str(tg) s]) disp([distance traveled : num2str(xg) ft]) The arguments for each of the disp commands are vectors of strings, with the rst element being a label, the second being a number converted to a string, and the third being the units of the 241 quantity. The label, the numerical results, and the units are displayed on one line, which was not possible with other forms of the use of disp: time of flight: 2.2774 s distance traveled: 144.6364 ft 242 String Functions blanks(n) Returns a string of n blanks. Used with disp, eg. disp ([xxx blanks(20) yyy]). disp(blanks(n)) moves the cursor down n lines. Removes trailing blanks from string s. Execute the string s as a Matlab expression or statement. Provides the ability to catch errors. Executes string s1 and returns if the operation was successful. If the operation generates an error, string s2 is evaluated before returning. This can be thought of as eval(try,catch). Find one string within another. Returns the starting indices of any occurrences of the shorter of the two strings in the longer. Returns 1 if s is a character array and 0 otherwise. Returns 1 for each element of character array s containing letters of the alphabet and 0 otherwise. Returns 1 for each element of character s containing white space characters and 0 otherwise. White space characters are spaces, newlines, carriage returns, tabs, vertical tabs, and formfeeds. Returns a string containing the last error message issued. lasterr is usually used in conjunction with the two argument form of eval: eval(try,catch). The catch action can examine the lasterr string to determine the cause of the error and take appropriate action. Converts any uppercase characters in string s to the corresponding lowercase character and leaves all other characters unchanged. Horizontally concatenates corresponding rows of the character arrays s1, s2, s3 etc. The trailing padding is ignored. All the inputs must have the same number of rows (or any can be a single string). When the inputs are all character arrays, the output is also a character array. Returns 1 if strings s1 and s2 are identical and 0 otherwise. Returns a right justied version of the character array s. Searches through the rows of the character array of strings strs to nd strings that begin with string str, returning the matching row indices. Returns 1 if the rst n characters of the strings s1 and s2 are identical and 0 otherwise. Replaces all occurrences of the string s2 in string s1 with the string s3. The new string is returned. Converts any lower case characters in s to the corresponding upper case character and leaves all other characters unchanged. deblank(s) eval(s) eval(s1,s2) findstr(s1,s2) ischar(s) isletter(s) isspace(s) lasterr lower(s) strcat(s1,s2,s3,...) strcmp(s1,s2) strjust(s) strmatch(str,strs) strncmp(s1,s2,n) strrep(s1,s2,s3) upper(s) 243 12.2 Time Computations Current Date and Time Three formats are supported for dates: clock Returns a six element date vector vector containing the current time and date in decimal form: [year month day hour minute seconds]. The rst ve elements are integers. The seconds element is accurate to several digits beyond the decimal point. Returns a string containing the date in dd-mmm-yyyy format. Returns the current date and time as a serial date number. date now Examples: >> t = clock t= 1998 6 >> date ans = 10-Jun-1998 >> format long >> d = now d= 7.299164376449074e+005 10 10 18 59.57 The date number can be converted to a string with the datestr function: datestr(d,dateform): Converts a data number d (such as that returned by now) into a date string. The string is formatted according to the format number dateform (see table below). By default, dateform is 1, 16, or 0 depending on whether d contains dates, times or both. 244 dateform 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Examples: >> ds = datestr(d) ds = 10-Jun-1998 10:30:13 >> datestr(d,14) ans = 10:30:13 AM Date format dd-mmm-yyyy HH:MM:SS mm/dd/yy mmm m mm mm/dd dd ddd d yyyy yy mmmyy HH:MM:SS HH:MM:SS PM HH:MM HH:MM PM QQ-YY QQ Example 01-Mar-1995 15:45:17 03/01/95 Mar M 3 03/01 1 Wed W 1995 95 Mar95 15:45:17 3:45:17 PM 15:45 3:45 PM Q1-96 Q1 The function datenum is used to compute a date number. It has three forms: datenum(s): Returns the date number corresponding to the date string s. datenum(year,month,day): Returns the date number corresponding to the specied year, month, and day. datenum(year,month,day,hour minute,second): Returns the date number corresponding to the specied year, month, day, hour, minute, and second. Examples: >> datenum(ds) ans = 7.299164376504630e+005 >> datenum(1998,6,13) ans = 729919 245 >> datenum(1998,6,16,10,30,00) ans = 7.299224375000000e+005 Date Functions The day of the week may be found from a date string or a date number using weekday, using the convention that Sunday = 1 and Saturday = 7. >> [d s] = weekday(9/9/90) d= 1 s= Sun A calendar can be generated for a desired month, for display in the Command window or to be placed in a 6-by-7 array. >> calendar(9/9/90) Sep 1990 S M Tu W 0 0 0 0 2 3 4 5 9 10 11 12 16 17 18 19 23 24 25 26 30 0 0 0 >> a = calendar(1978,6) a= 0 0 0 0 4 5 6 7 11 12 13 14 18 19 20 21 25 26 27 28 0 0 0 0 Th 0 6 13 20 27 0 F 0 7 14 21 28 0 S 1 8 15 22 29 0 1 8 15 22 29 0 2 9 16 23 30 0 3 10 17 24 0 0 Timing Functions The functions tic and toc are used to time a sequence of commands. tic starts the timer; toc stops the timer and displays the elapsed time in seconds. Example: tic 246 for t=1:5000 y(t)=sin(2*pi*t/10); end toc Executing: elapsed_time = 4.5100 cputime returns the amount of central processing unit (CPU) time in seconds since the current session was started. Computing processing times at various points in a script and taking dierences can be used to determine the CPU time required for segments of the script, possibly locating portions of the code that could be rewritten to decrease the total computation time. 12.3 Base Conversions and Bit Operations Base Conversion Matlab provides functions for converting decimal numbers to other bases in the form of character strings. These conversion functions include: dec2bin(d) dec2bin(d,N) bin2dec(b) dec2hex(d) hex2dec(h) Returns the binary representation of d as a string. d must be a non-negative integer smaller than 252 . Produces a binary representation with at least N bits. Interprets the binary string b and returns the equivalent decimal number. Returns the hexadecimal representation of decimal integer d as a string. d must be a non-negative integer smaller than 252 . Interprets the hexadecimal string h and returns the equivalent decimal number. If h is a character array, each row is interpreted as a hexadecimal string. Returns the representation of d as a string in base b. d must be a non-negative integer smaller than 252 and b must be an integer between 2 and 36. Produces a representation with at least N digits. dec2base(d,b) dec2base(d,b,N) Examples: >> a = dec2bin(18) a= 10010 >> bin2dec(a) ans = 18 % find binary representation of 18 % convert a back to decimal 247 >> b = dec2hex(30) b= 1E >> hex2dec(b) ans = 30 >> c = dec2base(30,4) c= 132 >> base2dec(c,4) ans = 30 % hex representation of 30 % convert b back to decimal % 30 in base 4 % convert c back to decimal Bit Operations Matlab provides functions to implement logical operations on the individual bits of oating-point integers. The Matlab bitwise functions operate on integers between 0 and bitmax, which is 253 1 = 9.0072 1015 : c c c c = = = = bitand(a,b) bitor(a,b) bitxor(a,b) bitcmp(a,N) Returns the bit-wise AND of the two arguments a and b. Returns the bit-wise OR of the two arguments a and b. Returns the bit-wise exclusive OR of the two arguments a and b. Returns the bit-wise complement of a as an N-bit non-negative integer. Sets the bit at position bit to the value v, where v must be either 0 or 1. Returns the value of the bit at position bit in a. a must contain non-negative integers and bit must be a number between 1 and the number of bits in a oating point integer (52 for IEEE machines). Returns the value of a shifted by N bits. If N > 0, this is same as a multiplication by 2N (left shift). If N < 0, this is the same as a division by 2^(-N) (right shift). c = bitset(a,bit,v) c = bitget(a,bit) c = bitshift(a,N) Examples of the use of these functions are best understood by displaying the results of each operation by dec2bin: >> a = 6; >> b = 10; >> dec2bin(a,4) ans = 0110 >> dec2bin(b,4) ans = 1010 >> dec2bin(bitand(a,b),4) ans = 0010 248 >> dec2bin(bitor(a,b),4) ans = 1110 >> dec2bin(bitxor(a,b),4) ans = 1100 >> dec2bin(bitcmp(a,4),4) ans = 1001 >> dec2bin(bitset(a,4,1),4) ans = 1110 >> bitget(a,2) ans = 1 >> dec2bin(bitshift(a,1)) ans = 1100 249 Section 13 Symbolic Processing We have focused on the use of Matlab to perform numerical operations, involving numerical data represented by double precision oating point numbers. We also given some consideration to the manipulation of text, represented by character strings. In this section, we introduce the use of Matlab to perform symbolic processing to manipulate mathematical expressions, in much the way that we do with pencil and paper. The objective of symbolic processing is to obtain what are known as closed form solutions, expressions that dont need to be iterated or updated in order to obtain answers. An understanding of these solutions often provides better physical and mathematical insight into the problem under investigation. For more information, type help symbolic in Matlab. A tutorial demonstration is available with the command symintro. The following notes represent a short introduction to the symbolic processing capabilities of Matlab. 13.1 Symbolic Expressions and Algebra To introduce symbolic processing, rst consider the handling of symbolic expressions and the manipulation of algebra. Declaring Symbolic Variables and Constants To enable symbolic processing, the variables and constants involved must rst be declared as symbolic objects. For example, to create the symbolic variables with names x and y: >> syms x y 250 If x and y are to be assumed to be real variables, they are created with the command: >> syms x y real To declare symbolic constants, the sym function is used. Its argument is a string containing the name of a special variable, a numeric expression, or a function evaluation. It is used in an assignment statement which serves as a declaration of a symbolic variable for the assigned variable. Examples include: >> pi = sym(pi); >> delta = sym(1/10); >> sqroot2 = sym(sqrt(2)); If the symbolic constant pi is created this way, it replaces the special variable pi in the workspace. The advantage of using symbolic constants is that they maintain full accuracy until a numeric evaluation is required. Symbolic variables and constants are represented by the data type symbolic object. For example, as displayed by the function whos for the symbolic variables and constants declared in the commands above: >> whos Name delta pi sqroot2 x y Size 1x1 1x1 1x1 1x1 1x1 Bytes 132 128 138 126 126 Class sym sym sym sym sym object object object object object Grand total is 20 elements using 650 bytes Symbolic Expressions Symbolic variables can be used in expressions and as arguments of functions in much the same way as numeric variables have been used. The operators + - * / ^ and the built-in functions can also be used in the same way as they have been used in numeric calculations. For example: >> syms s t A >> f = s^2 + 4*s + 5 f= s^2+4*s+5 >> g = s + 2 g= s+2 251 >> h = f*g h= (s^2+4*s+5)*(s+2) >> z = exp(-s*t) z= exp(-s*t) >> y = A*exp(-s*t) y= A*exp(-s*t) The symbolic variables s, t, and A are rst declared, then used in symbolic expressions to create the symbolic variables f, g, h, z, and y. The displayed results show that the created variables remain as symbolic objects, rather than being evaluated numerically. Symbolic processing doesnt seem to obey the format compact command, as the displayed output is always double-spaced. These blank lines have been removed from these notes to conserve paper. The variable x is the default independent variable, but as can be seen with the expressions above, other variables can be specied to be the independent variable. It is important to know which variable is the independent variable in an expression. The command to nd the independent variable is: findsym(S) Finds the symbolic variables in a symbolic expression or matrix S by returning a string containing all of the symbolic variables appearing in S. The variables are returned in alphabetical order and are separated by commas. If no symbolic variables are found, findsym returns the empty string. Examples based on the declarations and expressions in the examples above are: >> findsym(f) ans = s >> findsym(z) Register to View Answers, t The vector and matrix notation used in Matlab also applies to symbolic variables. For example, the symbolic matrix B can be created with the commands: >> n = 3; >> syms x >> B = x.^((0:n)*(0:n)) B= [ 1, 1, 1, 1] [ 1, x, x^2, x^3] [ 1, x^2, x^4, x^6] [ 1, x^3, x^6, x^9] 252 Manipulating Polynomial Expressions In the examples above, symbolic variables were declared and were used in symbolic expressions to create polynomials. We now wish to manipulate these polynomial expressions algebraically. The Matlab commands for this purpose include: expand(S) Expands each element of a symbolic expression S as a product of its factors. expand is most often used on polynomials, but also expands trigonometric, exponential and logarithmic functions. Factors each element of the symbolic matrix S. Simplies each element of the symbolic matrix S. Returns two symbolic expressions that represent the numerator expression num and the denominator expression den for the rational representation of the symbolic expression S. Symbolic substitution, replacing symbolic variable old with symbolic variable new in the symbolic expression S. factor(S) simplify(S) [n,d] = numden(S) subs(S,old,new) These commands can be used to implement symbolic polynomial operations that were previously considered as numeric operations in Section 7.2 of these notes. Addition: >> syms s >> A = s^4 -3*s^3 -s +2; >> B = 4*s^3 -2*s^2 +5*s -16; >> C = A + B C= s^4+s^3+4*s-14-2*s^2 Note that the result is correct, although it is not in form the we prefer, as the terms are not ordered in decreasing powers of s. Scalar multiple: >> syms s >> A = s^4 -3*s^3 -s +2; >> C = 3*A C= 3*s^4-9*s^3-3*s+6 Multiplication: >> >> >> >> syms s A = s+2; B = s+3; C = A*B 253 C= (s+2)*(s+3) >> C = expand(C) C= s^2+5*s+6 Factoring: >> syms s >> D = s^2 + 6*s + 9; >> D = factor(D) D= (s+3)^2 >> P = s^3 - 2*s^2 -3*s + 10; >> P = factor(P) P= (s+2)*(s^2-4*s+5) Common denominator: Consider the expression H(s) = 1/2 2/3 1/6 + s+3 s+1 s This can be expressed as a ratio of polynomials by nding the common denominator for the three terms, as follows: >> syms s >> H = -(1/6)/(s+3) -(1/2)/(s+1) +(2/3)/s; >> [N,D] = numden(H) N= s+2 D= (s+3)*(s+1)*s >> D = expand(D) D= s^3+4*s^2+3*s Thus, H(s) can be expressed in the form H(s) = s3 s+2 + 4s2 + 3s As a second example, consider G(s) = s + 4 + 3 2 + s+4 s+2 Manipulating with Matlab: 254 >> syms s >> G = s+4 + 2/(s+4) + 3/(s+2); >> [N,D] = numden(G) N= s^3+10*s^2+37*s+48 D= (s+4)*(s+2) >> D = expand(D) D= s^2+6*s+8 Thus, G(s) can also be expressed in the form: G(s) = s3 + 10s2 + 37s + 48 s2 + 6s + 8 In this example, G(s) is an improper rational function. Cancellation of terms: For a ratio of polynomials, Matlab can be applied to see if any terms cancel. For example H(s) = Applying Matlab: >> syms s >> H = (s^3 +2*s^2 +5*s +10)/(s^2 + 5); >> H = simplify(H) H= s+2 Factoring the denominator shows why the cancellation occurs: >> factor(s^3 +2*s^2 +5*s +10) ans = (s+2)*(s^2+5) Thus, H(s) = s + 2 Variable substitution: Consider the ratio of polynomials H(s) = s+3 s2 + 6s + 8 s3 + 2s2 + 5s + 10 s2 + 5 Dene a second expression G(s) = H(s)|s=s+2 Evaluating G(s) with Matlab: 255 >> syms s >> H = (s+3)/(s^2 +6*s + 8); >> G = subs(H,s,s+2) G= (s+5)/((s+2)^2+6*s+20) >> G = collect(G) G= (s+5)/(s^2+10*s+24) Thus G(s) = s2 s+5 + 10s + 24 Commands are also provided to convert between the numeric representation of a polynomial as the vector of its coecients and the symbolic representation. sym2poly(P) poly2sym(p) Converts from a symbolic polynomial P to a row vector containing the polynomial coecients. Converts from a polynomial coecient vector p to a symbolic polynomial in the variable x. poly2sym(p,v) uses the symbolic the variable v. For example, consider the polynomial A(s) = s3 + 4s2 7s 10 In Matlab: >> a = [1 4 -7 -10]; >> A = poly2sym(a,s) A= s^3+4*s^2-7*s-10 For the polynomial B(s) = 4s3 2s2 + 5s 16 >> syms s >> B = 4*s^3 -2*s^2 +5*s -16; >> b = sym2poly(B) b= 4 -2 5 -16 Forms of Expressions As we have seen in some of the examples above, Matlab does not always arrange expressions in the form that we would prefer. For example, Matlab expresses results in the form 1/a*b, while we would prefer b/a. For example: 256 >> syms s >> H = s^2 +6*s + 8; >> G = -H/3 G= -1/3*s^2-2*s-8/3 The result is G = (1/3)s2 2s 8/3, while we would prefer G = (s2 + 6s + 8)/3. 13.2 Manipulating Trigonometric Expressions Trigonometric expressions can also be manipulated symbolically in Matlab, primarily with the use of the expand function. For example: >> syms theta phi >> A = sin(theta + phi) A= sin(theta+phi) >> A = expand(A) A= sin(theta)*cos(phi)+cos(theta)*sin(phi) >> B = cos(2*theta) B= cos(2*theta) >> B = expand(B) B= 2*cos(theta)^2-1 >> C = 6*((sin(theta))^2 + (cos(theta))^2) C= 6*sin(theta)^2+6*cos(theta)^2 >> C = expand(C) C= 6*sin(theta)^2+6*cos(theta)^2 Thus, Matlab was able to apply trigonometric identities to expressions A and B, but it was not successful with C, as we know C = 6(sin2 () + cos2 ()) = 6 Matlab can also manipulate expressions involving complex exponential functions. For example: >> syms theta real >> A = real(exp(j*theta)) A= 1/2*exp(i*theta)+1/2*exp(-i*theta) 257 >> A = simplify(A) A= cos(theta) 13.3 Evaluating and Plotting Symbolic Expressions In many applications, we eventually want to obtain numerical results or a plot from a symbolic expression. The function double produces numerical results: double(S) Converts the symbolic matrix expression S to a matrix of double precision oating point numbers. S must not contain any symbolic variables, except possibly eps. Since the symbolic expression cannot contain any symbolic variables, it is necessary to use subs to substitute numerical values for the symbolic variables prior to applying double. For example: >> E = s^3 -14*s^2 +65*s -100; >> F = subs(E,s,7.1) F= 13671/1000 >> G = double(F) G= 13.6710 The symbolic form is F and the numeric quantity is G, as conrmed by the display from whos: >> whos Name E F G s Size 1x1 1x1 1x1 1x1 Bytes 162 144 8 126 Class sym object sym object double array sym object Grand total is 34 elements using 440 bytes Symbolic expressions can be plotted with the Matlab function ezplot: ezplot(f) Plots a graph of f(x) where f is a string or a symbolic expression representing a mathematical expression involving a single symbolic variable, say x. The default range of the x-axis is [2, 2] Plots the graph using the specied x-range instead of the default range. ezplot(f,xmin,xmax) 258 For example consider plotting the polynomial function A(s) = s3 + 4s2 7s 10 over the range [1, 3]: syms s a = [1 4 -7 -10]; A = poly2sym(a,s) ezplot(A,-1,3), ylabel(A(s)) The resulting plot is shown in Figure 13.3. Note that the expression plotted is automatically placed at the top of the plot and that the axis label for the independent variable is automatically placed. A ylabel command was used to label the dependent variable. s^3+4*s^27*s10 35 30 25 20 15 A(s) 10 5 0 5 10 15 1 0.5 0 0.5 1 s 1.5 2 2.5 3 Figure 13.1: Plot of a polynomial function using ezplot 13.4 Solving Algebraic and Transcendental Equations The symbolic math toolbox can be used to solve algebraic and transcendental equations, as well as systems of such equations. A transcendental equation is one that contains one or more transcendental functions, such as cos x, ex , or ln x. The function used in solving these equations is solve. There are several forms of solve, but only the following forms will be presented in these notes: 259 solve(E1, E2,...,EN) solve(E1, E2,...,EN, var1, var2,...,varN) where E1, E2,...,EN are the names of symbolic expressions and var1, var2,..., varN are variables in the expressions that have been declared to be symbolic. The solutions obtained are the roots of the expressions; that is, symbolic expressions for the variables under the conditions E1 = 0, E2 = 0, . . . EN = 0. For one equation and one variable, the resulting output solution is returned as a single symbolic variable. For example: >> syms s >> E = s+2; >> s = solve(E) s= -2 For N equations, the N solutions are returned as a symbolic vector. For example: >> syms s >> D = s^2 +6*s +9; >> s = solve(D) s= [ -3] [ -3] Thus, the solution is the symbolic representation of the repeated real roots of the quadratic polynomial, providing the same results as those obtained earlier in numeric representation using the function roots. Complex roots can also be obtained, as shown in the following example: >> syms s >> P = s^3 -2*s^2 -3*s + 10; >> s = solve(P) s= [ -2] [ 2+i] [ 2-i] Similar results can be obtained in the solution of transcendental equations. An example in trigonometry: >> syms theta x z 260 >> E = z*cos(theta) - x; >> theta = solve(E,theta) theta = acos(x/z) For an example involving ex , consider the solution to e2x + 4ex = 32: >> syms x >> E = exp(2*x) + 4*exp(x) -32; >> x = solve(E) x= [ log(-8)] [ log(4)] >> log(-8) ans = 2.0794+ 3.1416i >> log(4) ans = 1.3863 Note that two solutions are provided, with the numeric results showing that the rst solution log(8) = 2.0794 + 3.1416i is complex, while the second solution log(4) = 1.3863 is real. The issue as to whether both of these solutions are meaningful would depend on the application that led to the original equation. Equations containing periodic functions can have an innite number of solutions. In such cases, solve restricts the search for solutions to the region near 0. For example, to solve the equation cos(2) sin() = 0: >> E = cos(2*theta)-sin(theta); >> solve(E) ans = [ -1/2*pi] [ 1/6*pi] [ 5/6*pi] Example 13.1 Positioning a robot arm Consider again the application to robot motion that was presented in Section 10.4. The robot arm has two joints and two links. The (x, y) coordinates of the hand are given by x1 = L1 cos 1 + L2 cos(1 + 2 ) x2 = L1 sin 1 + L2 sin(1 + 2 ) 261 where 1 and 2 are the joint angles and L1 = 4 feet and L2 = 3 feet are the link lengths. A part of the previous solution that was not determined was the joint angles needed to position the hand at a given set of coordinates. For the initial hand position of (x, y) = (6.5, 0), the following commands determine the required angles: >> syms theta1 theta2 >> E1 = 4*cos(theta1)+3*cos(theta1+theta2)-6.5; >> E2 = 4*sin(theta1)+3*sin(theta1+theta2); >> [theta1, theta2] = solve(E1,E2) theta1 = [ atan(9/197*55^(1/2))] [ atan(-9/197*55^(1/2))] theta2 = [ -atan(3/23*55^(1/2))] [ -atan(-3/23*55^(1/2))] >> theta1 = double(theta1*(180/pi)) theta1 = 18.7170 -18.7170 >> theta2 = double(theta2*(180/pi)) theta2 = -44.0486 44.0486 There are two solutions, given rst in symbolic form, then converted into numeric form using double. The rst is 1 = 18.717 , 2 = 44.0486 , which is the elbow up solution. The second is 1 = 18.717 , 2 = 44.0486 , the elbow down solution. 13.5 Calculus Symbolic expressions can be dierentiated and integrated to obtain closed form results. Dierentiation The diff function, when applied to a symbolic expression, provides a symbolic derivative. diff(E) diff(E,v) diff(E,n) diff(S,v,n) Dierentiates a symbolic expression E with respect to its free variable as determined by findsym. Dierentiates E with respect to symbolic variable v. Dierentiates E n times for positive integer n. Dierentiates E n times with respect to symbolic variable v. 262 Examples of derivatives of polynomial functions: >> syms s n >> p = s^3 + 4*s^2 -7*s -10; >> d = diff(p) d= 3*s^2+8*s-7 >> e = diff(p,2) e= 6*s+8 >> f = diff(p,3) f= 6 >> g = s^n; >> h = diff(g) h= s^n*n/s >> h = simplify(h) h= s^(n-1)*n Examples of derivatives of transcendental functions: >> syms x >> f1 = log(x); >> df1 = diff(f1) df1 = 1/x >> f2 = (cos(x))^2; >> df2 = diff(f2) df2 = -2*cos(x)*sin(x) >> f3 = sin(x^2); >> df3 = diff(f3) df3 = 2*cos(x^2)*x >> df3 = simplify(df3) df3 = 2*cos(x^2)*x >> f4 = cos(2*x); >> df4 = diff(f4) df4 = -2*sin(2*x) >> f5 = exp(-(x^2)/2); >> df5 = diff(f5) df5 = -x*exp(-1/2*x^2) 263 Min-Max Problems The derivative can be used to nd the maximum or minimum of a continuous function, say, f (x), over an interval a x b. A local maximum or local minimum (one that does not occur at one of the boundaries x = a or x = b) can occur only at a critical point, which is a point where either df /dx = 0 or df /dx does not exist. Example 13.2 Minimum cost tank design Consider again the tank design problem that was solved numerically in Example 7.1. In this problem, the tank radius is R, the height is H and the tank volume is such that 2 500 = R2 H + R3 3 The cost of the tank, a function of surface area, is C = 300(2RH) + 400(2R2 ) The problem is to solve for R and H providing the minimum cost tank providing the specied volume. The symbolic approach is to solve the volume equation for H as a function of R, express cost C symbolically, then dierentiate C with respect to R and solve the resulting equation for R. >> syms R H >> V = pi*R^2*H + (2/3)*pi*R^3 -500; % Equation for volume >> H = solve(V,H) % Solve volume for height H H= -2/3*(pi*R^3-750)/pi/R^2 >> C = 300*(2*pi*R*H) + 400*(2*pi*R^2); % Equation for cost >> dCdR = diff(C,R) % Derivative of cost wrt R dCdR = 400/R^2*(pi*R^3-750)+400*pi*R >> Rmins = solve(dCdR,R) % Solve dC/dR for R: Rmin Rmins = [ 5/pi*3^(1/3)*(pi^2)^(1/3)] [ -5/2/pi*3^(1/3)*(pi^2)^(1/3)+5/2*i*3^(5/6)/pi*(pi^2)^(1/3)] [ -5/2/pi*3^(1/3)*(pi^2)^(1/3)-5/2*i*3^(5/6)/pi*(pi^2)^(1/3)] >> Rmins = double(Rmins) Rmin = 4.9237 -2.4619+ 4.2641i -2.4619- 4.2641i >> Rmin = Rmins(1) Rmin = 4.9237 264 >> Hmin = double(subs(H,R,Rmin)) Hmin = 3.2825 >> Cmin = double(subs(C,{R,H},{Rmin,Hmin})) Cmin = 9.1394e+004 Note that there are three symbolic solutions for R to provide minimum cost (Rmins). Converting these solutions to numeric quantities with double, we see that the second and third solutions are complex, which are not physically meaningful. Thus, we choose Rmin to be Rmins(1) and we compute Hmin and Cmin from this value. These symbolic results obtained here are more accurate than those determined previously in Example 7.1, as there has been no need to consider samples of R and H at a limited resolution. However, note that the results determined by the two methods are very close. Integration The int function, when applied to a symbolic expression, provides a symbolic integration. int(E) Indenite integral of symbolic expression E with respect to its symbolic variable as dened by findsym. If E is a constant, the integral is with respect to x. Indenite integral of E with respect to scalar symbolic variable v. Denite integral of E with respect to its symbolic variable from a to b, where a and b are each double or symbolic scalars. Denite integral of E with respect to v from a to b. int(E,v) int(E,a,b) int(E,v,a,b) Examples of integrals of polynomials: >> syms x n a b t >> int(x^n) ans = x^(n+1)/(n+1) >> int(x^3 +4*x^2 + 7*x + 10) ans = 1/4*x^4+4/3*x^3+7/2*x^2+10*x >> int(x,1,t) ans = 1/2*t^2-1/2 >> int(x^3,a,b) ans = 1/4*b^4-1/4*a^4 Examples of integrals of transcendental functions: 265 >> syms x >> int(1/x) ans = log(x) >> int(cos(x)) ans = sin(x) >> int(1/(1+x^2)) ans = atan(x) >> int(exp(-x^2)) ans = 1/2*pi^(1/2)*erf(x) The last integral above introduces the error function erf(x) for each element of x, where x is real. The error function is dened as: 2 erf(x) = x 0 et dt 2 13.6 Linear Algebra Operations on symbolic matrices can be performed in much the same way as with numeric matrices. The following are examples of matrix inverse, product, and determinant. >> A = sym([2,1; 4,3]) A= [ 2, 1] [ 4, 3] >> Ainv = inv(A) Ainv = [ 3/2, -1/2] [ -2, 1] >> C = A*Ainv C= [ 1, 0] [ 0, 1] >> B = sym([1 3 0; -1 5 2; 1 2 1]) B= [ 1, 3, 0] [ -1, 5, 2] [ 1, 2, 1] >> detB = det(B) detB = 10 266 Systems of linear equations can also be solved symbolically. Consider the following example that was previously solved numerically: >> syms x >> A = sym([3 2 -1; -1 3 2; 1 -1 -1]) A= [ 3, 2, -1] [ -1, 3, 2] [ 1, -1, -1] >> b = sym([10; 5; -1]) b= [ 10] [ 5] [ -1] >> x = A\b x= [ -2] [ 5] [ -6] The results are the same as those obtained numerically. For this problem, there is little advantage to nding the result symbolically. However, solving a system of equations with respect to a parameter, the symbolic approach provides an advantage. Consider the following set of equations 2x1 3x2 = 3 5x1 + cx2 = 19 To solve for x1 and x2 as functions of the parameter c: >> syms c >> A = sym([2 -3; 5 c]); >> b = sym([3; 19]); >> x = A\b x= [ 3*(19+c)/(2*c+15)] [ 23/(2*c+15)] 267

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George Mason - IT - 441
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IT441 Network Servers &amp; InfrastructureClass 04 27 Sep 2004IT441: Network Servers &amp; InfrastructureCLASS 4 : 27 Sep 200413:30 16:15Ex cept where otherwise noted, these slides are copyright Charles Snow, 2004. Perm ission to copy these slides i
George Mason - IT - 441
IT441 Network Servers &amp; InfrastructureClass 04 27 Sep 2004IT441: Net work Servers &amp; Infrast ructureCLASS 4 : 27 Sep 200413:30 16:15Last Tim e: serversaddresses, of int erfaces socket s and port s, sam ple programm ing basicst ransmissi
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George Mason - IT - 441
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George Mason - IT - 441
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George Mason - IT - 441
IT441: Network Servers &amp; InfrastructureCLASS 12 : 22 Nov 200413:30 16:15Except where otherwise noted, these slides are copyright Charles Snow, 2004. Permission to copy these slides is granted so long as each slide contains the copyright notice
TN Tech - CHEM - 6970
CHEM 6970Fall 2004Project 4: Minimum and Saddle Point Optimizations Each student should investigate the system below using the following a hybrid density functional method (called MPW1K) whose Gaussian 03 keywords are mPWPW91/6-31+G(d,p) IOp(3/7
TN Tech - CHEM - 1120
CHEM 1120-003Spring 2009CHEM 1120 Syllabus Instructor InformationName: Office: E-mail: Phone: Website: Office hours: Titus V. Albu Foster Hall 303 albu@tntech.edu 931-372-6324 http:/iweb.tntech.edu/albu Monday and Wednesday 2:00 4:00 pm. If you
University of Iowa - CS - 145
22c:145 Articial IntelligenceFall 2005First-Order LogicHantao Zhang The University of IowaCopyright 2001-05 Cesare Tinelli and Hantao Zhang.a aThese notes are copyrighted material and may not be used in other course settings outside of the
University of Iowa - CS - 196
University of Iowa - CS - 196
University of Iowa - CS - 181
22c181: Formal Methods in Software EngineeringThe University of IowaSpring 2008From OCL to Propositional and First-order Logic: Part ICopyright 2007-8 Reiner Hhnle and Cesare Tinelli. Notes originally developed by Reiner Hhnle at Chalmers Unive
University of Iowa - CS - 196
Logic as a Query Language: from Frege to XMLVictor Vianu U.C. San DiegoLogic and Databases: a success story FO lies at the core of modern database systems Relational query languages are based on FO: SQL, QBE More powerful query languages (all
Rensselaer Polytechnic Institute - PHYS - 1010
A Passion for PhysicsThe Physics Freshman SeminarFall Semester 2008Course Organizer/Instructor: Jim Napolitano See information on course web pagePhysics Research at RPINo talk summary needs to be handed in for today. . BUT you might want to pr
Rensselaer Polytechnic Institute - PHYS - 1010
Rapid global warming 55 myrs ago: The Paleocene/Eocene Thermal Maximum Mimi Katz Earth &amp; Environmental SciencesStable Isotopes 16O 99.76% 17O 0.04% 18O 0.20% 18O = 1000 x (18/16Osample - 18/16Ostd)/(18/16Ostd)12C 13C 14C98.89% 1.11% not stable
Rensselaer Polytechnic Institute - PHYS - 410
3.1 Linear AlgebraVector spacesInstead of visualizing a complete set of orthonormal basis functions, f(r), consider a set of basis vectors which spans a space, {| } e.g., | x , | y , | z plus a set of scalars, {a}, which can be multipliers basis
Rensselaer Polytechnic Institute - ASTR - 2050
Workforce Development / New York Space Grant Affiliate InternshipNASA/New York Space Grant Undergraduate Summer Research Program SUMMER 2005 Application FormYOU MUST BE A U.S. CITIZEN TO PARTICIPATEDEADLINE FOR RECEIPT OF THIS APPLICATION IS FEBR
Rensselaer Polytechnic Institute - ASTR - 2050
Neutron stars for undergraduatesRichard R. Silbara) and Sanjay Reddyb)Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545Received 16 September 2003; accepted 13 February 2004 The calculation of the structure of whit
Rensselaer Polytechnic Institute - ASTR - 2050
Erratum: Neutron stars for undergraduates Am. J. Phys. 72 7, 892905 2004Richard R. Silbar and Sanjay ReddyTheoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545Received 30 November 2004; accepted 3 December 2004 DOI: 1
Rensselaer Polytechnic Institute - ASTR - 2050
Visualizing curved spacetimeRickard M. Jonssona)Department of Theoretical Physics, Physics and Engineering Physics, Chalmers University of Technology and Goteborg University, 412 96 Gothenburg, SwedenReceived 23 April 2003; accepted 8 October 20
Rensselaer Polytechnic Institute - ASTR - 2050
1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H1995A&amp;A.297.556H
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Introductory Astronomy &amp; Astrophysics Spring 2005Welcome to our introduction to astronomy! We will explore lots of neat things in the sky and try to understand why they look the way they do. INSTRUCTOR: Jim Napolitano SC 1W07 x8019 email: n
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 18 January 2005Welcome to this course!Instructor: Jim Napolitano Teaching Assistants: Tom Parker and Miranda NordhausWhat makes astronomy so fascinating?My theory.0 ! = AnythingExample: Two Radio SourcesT
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 11am 18 January 2005In this class we will cover: The brightness of stars Electromagnetic radiation The color of stars: Blackbody radiation The Planck Radiation FormulaBrightness and MagnitudeThe ancients looked
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Intro A&amp;A Studio Exercise Worksheet Week#1 January 21, 2005Names: _ _ The Mass of the SunFor any of the planets, the orbit radius and length of a year can be used to estimate the mass of the Sun. In this exercise, youll do this for each
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 25 January 2005Please turn in your homework now! In this class we will cover: Review: Blackbody radiation and Plancks law Measuring the color of a star Parallax and the distance to nearby stars Absolute and Bo
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 11am 25 January 2005In this class we will cover: Spectral lines in the laboratory and in stars Spectral classications The origin of spectral lines in stars The Hertzprung-Russell (HR) diagram1Spectral LinesSo f
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 1 February 2005Please turn in your homework now! In this class we will cover Telescopes: What does a telescope do? Kinds of optical telescopes Photography and light detection Observing outside the visible1T
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 8 February 2005Please turn in your homework now! In this class we will cover Binary Stars:Binary star systems provide most of what we know about the structure and properties of stars!1 Kinds of binary stars B
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 11am 8 February 2005In this class we will . Review what weve learned about stars Study The Sun1What we can measure about stars(1) Temperature More properly Effective Surface Temperature The old way: Spectral Cla
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 15 February 2005Note: No class next Tuesday, but see lab for Friday Also: See updated HW assignment (Ch.9) due in two weeks1 Stellar energy sources: Nuclear Physics Stellar structure and stellar models Outsta
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 1 March 2005Please turn in your homework now! In this class we will cover: The Main Sequence: Brief review Stellar evolution off the Main Sequence Cepheid variable stars: Measuring distance1The Main Sequence
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 11am 1 March 2005In this class we will talk about stars in their dying throes: Planetary nebulae: Low mass stars Supernovae: High mass stars1Recall from last class.Low mass stars (M &lt; 5M ) Relatively long lived H
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 8 March 2005Please turn in your homework now! In this class we will study compact objects: Degeneracy pressure White dwarfs: End state of low mass stars Neutron stars: End state of high mass stars Pulsars: Rot
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 11am 8 March 2005In this class we will cover: Hints: Escape Velocity Relativity and the Equivalence Principle Visualization of Curved Spacetime Black Holes1Escape VelocityConservation of Energy v m M R Set tota
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 11am 22 March 2005In this class we will cover Clusters of Stars: Types of clusters Distances to clusters Cluster dynamics: The Virial Theorem HR diagrams: Ages and populations1Types of ClustersAssumption: All s
Rensselaer Polytechnic Institute - ASTR - 2050
Star FormationAstronomy &amp; Astrophysics April 5, 2005Important Equations3 GM U= 5 R2E = K+ UL = 4 R T23M K = kT 2 m ( )4Star Formation TriggerCloud CollapseGravitational binding: gravitational forces holding the cloud together
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 12 April 2005Please turn in your homework now! In this class we will cover The Milky Way: Overview Galactic rotation Spiral structure The Galactic center1OverviewWe live in the Milky Way, aka GalaxyGalact
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 11am 12 April 2005In this class we will cover Normal Galaxies: Types of galaxies Spiral arms and star formation Rotation curves and dark matter1Types of GalaxiesSpiral Galaxy M31 Elliptical Galaxy M872Proper
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 19 April 2005Please turn in your homework now! In this class we will cover: Hubbles Law: Review of Fridays lab exercise Active galaxies Quasars Very distant objects: Gravitational lensingExam #3 will be in cl
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Spring 2005Lecture 10am 3 May 2005No Homework is Due Today! Final Exam: Wed 11 May 3-6pm Troy 2018In this class we will cover Cosmology and the Big Bang.Today is our last class. I had a good time teaching this material! I hope you had
Rensselaer Polytechnic Institute - PHYS - 2100
PHYS 2100 Introduction to Methods of Theoretical Physics Fall 1998Midterm Exam #1 NAME:Solution KeyYou have two hours to complete this exam. There are a total of four problems and you are to solve all of them. Not all the problems are worth the
Rensselaer Polytechnic Institute - PHYS - 2100
PHYS 2100 Introduction to Methods of Theoretical Physics Fall 1998Midterm Exam #2 NAME:Solution KeyYou have two hours to complete this exam. There are a total of four problems and you are to solve all of them. Not all the problems are worth the
Rensselaer Polytechnic Institute - PHYS - 2100
PHYS 2100 Introduction to Methods of Theoretical Physics Fall 1998Midterm Exam #3 NAME:Solution KeyYou have two hours to complete this exam. There are a total of three problems and you are to solve all of them. Not all the problems are worth th
Rensselaer Polytechnic Institute - PHYS - 2100
PHYS 2100 Introduction to Methods of Theoretical Physics Fall 1998Final Examination NAME:You have three hours to complete this exam. There are a total of seven problems. Solve as much of them as you can, but the point total is greater than 100, s
Rensselaer Polytechnic Institute - ASTR - 2050
ASTR2050 Intro A&amp;ANAMES: _ _ Scale Model of the Solar System Work sheetBuild a scale model of the solar system, including the sizes and orbital radii of the sun and planets. Most of the data you need can be found in Kutner, Appendices B and D, an
Rensselaer Polytechnic Institute - PHYS - 79205
Problem 5 (25 points): An astronomer measures the n=43 transition of atomic hydrogen in some star-like object, at the same wavelength as the n=32 transition in the laboratory. a (10 points) Is the object moving towards her or away from her? Explain y
Rensselaer Polytechnic Institute - PHYS - 79205
Exam #2 79205 Astronomy Fall 1997NAME:Solution KeyYou have two hours to complete this exam. There are a total of ve problems and you are to solve all of them. Not all the problems are worth the same number of points. You may use Introductory As
Rensselaer Polytechnic Institute - PHYS - 79205
Exam #3 79205 Astronomy Fall 1997NAME:SolutionsYou have two hours to complete this exam. There are a total of six problems and you are to solve all of them. Not all the problems are worth the same number of points. You may use Introductory Astr
Rensselaer Polytechnic Institute - PHYS - 79205
Final Exam 79205 Astronomy Fall 1997NAME:Solution keyYou have three hours to complete this exam. Part I has 27 multiple choice questions, each worth 2 points, and one one six-point question. Part II has four short problems, each worth 10 points
Rensselaer Polytechnic Institute - PHYS - 4100
Introductory Quantum Mechanics (Spring 2006)April 14, 2006Day Date Thu 19 Jan Mon 23 Jan Thu 26 Jan Mon 30 Jan Thu 2 Feb Mon 6 Feb Thu 9 Feb Mon 13 Feb Thu 16 Feb Tue 21 Feb Thu 23 Feb Mon 27 Feb Thu 2 Mar Mon 6 Mar Thu 9 Mar 13-17 Mar Mon 20 Mar
Rensselaer Polytechnic Institute - PHYS - 4100
letters to nature.Experimental violation of a Bell's inequality with efcient detectionM. A. Rowe*, D. Kielpinski*, V. Meyer*, C. A. Sackett*, W. M. Itano*, C. Monroe &amp; D. J. Wineland** Time and Frequency Division, National Institute of Standards
Rensselaer Polytechnic Institute - PHYS - 4100
Neutron Spin RotationWerner, et al., Physical Review Letters 35(1975)1053 Also see textbook Figure 4.4(a)Neutron Beam Neutron Detectors 2 cmRotation through 4 Rotation through 0T=2Muon Spin Rotation and Measurement of g-2Brown, et al., Physic
Rensselaer Polytechnic Institute - PHYS - 4100
RAPID COMMUNICATIONSPHYSICAL REVIEW D, VOLUME 62, 091101 RImproved measurement of the positive muon anomalous magnetic momentH. N. Brown,2 G. Bunce,2 R. M. Carey,1 P. Cushman,10 G. T. Danby,2 P. T. Debevec,7 H. Deng,12 W. Deninger,7 S. K. Dhawan
Rensselaer Polytechnic Institute - PHYS - 4100
Rensselaer Polytechnic Institute - PHYS - 4100