16 - Matrices in MATLAB

# 16 - Matrices in MATLAB - Engineering 101 Quote of the Day...

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Unformatted text preview: Engineering 101 Quote of the Day An eye for an eye makes the whole world blind. - Mahatma Gandhi Matrices in MATLAB Accessing Individual Vector Elements You can access specific locations in a vector just like C++: a = [0 1 2 3 4 5 6 7 8 9] a(1) [0] a(8) [7] a(1:5) [0 1 2 3 4] a(4:10) [3 4 5 6 7 8 9] a(3:3:10) [2 5 8] Shortcut Expressions for Matrix Initialization If you want a matrix to have all zeros or all ones you can use a built-in function. builtzeros(n) zeros(n, m) ones(n) ones(n, m) eye(n) eye(n, m) creates an n x n matrix of zeros creates an n x m matrix of zeros creates an n x n matrix of ones creates an n x m matrix of ones creates an n x n identity matrix creates an n x m identity matrix Shortcut Expressions for Matrix Initialization You can use the size of a previous matrix as a template for a new matrix length(arr) returns the length of a vector or the longest dimension of a matrix size(arr) returns the rows and columns of an array x = [1 2 3; 4 5 6]; y = zeros(size(x)); Getting Data From the User You can use the input function to prompt the user for input. val = input(`Enter a number:'); The user can enter a number or an array in brackets If you need to get a string then val = input(`Enter a string:', `s'); 1 Exercise After these commands b=? a = [7:2:11]' b = [a a a] 127 7 9 11 7 9 11 9 9 9 9 11 7 9 11 11 11 11 7 9 11 7 9 11 Exercise After these commands b=? b = 3 * eye(4); b(1, 4) = 3; b(4, 1) = 5; 4- 7 9 11 7 9 11 7 9 11 1- 3 0 0 5 0 3 5 0 0 3 3 0 3 0 0 3 2- 3 0 0 3 0 3 3 0 0 5 3 0 5 0 0 3 3- 3 0 0 3 0 3 0 0 0 0 3 0 5 0 0 3 4- 3 0 0 5 0 3 0 0 0 0 3 0 3 0 0 3 3- 7 7 7 Scalar Operations The following operations are defined between two scalars: Addition Subtraction Multiplication Division Exponentiation a+b ab a*b a/b a^b Scalar / Array Operations The same operations may be performed between an array and a scalar. In this case the operation is performed to every element of the array. Array vs. Matrix operations When two arrays are being operated on MATLAB makes a distinction between array an matrix operations. Matrix operations are the standard Linear Algebra operations Array operations are done on an element-byelement-byelement basis Array vs. Matrix operations Array or Matrix operations (no difference) Addition Subtraction Multiplication Right Division Left Division Exponentiation a+b ab a .* b a ./ b a .\ b .\ a .^ b Array (Element-by-element) operations (Element-by- 2 Exercise Array vs. Matrix operations Matrix operations Multiplication Right Division Left Division a*b a/b b\a Match each expression to the matrix that corresponds to the result of the operation. a = [1 2 ; 5 1 ] b = [0 1; 1 2 ] Multiplication does standard matrix multiplication. The columns of a must equal the rows of b. Division inverts the denominator matrix and multiplies by the numerator. 1- a+b A1 4 1 -3 2- a-b B0 5 2 -2 3- a.*b C2 -1 4- a*b 5 3 D1 6 3 1 Matrix operations Recall that we often want to solve problems where: [A] x = b That is where [A] is a matrix and b is a set of known quantities and we want to know x. We can solve this using MATLAB by the command: x=b/A M-Files: MATLAB Programs We can solve our statics problem by creating an M-file. Once we create an M-file we can execute it by Msimply typing the name of the M-file leaving off Mthe ".m" Comments in M-files are preceded by a % sign. M- Recall our General Statics Problem W1 T1 T5 W2 y x T4 W/2 Convert to a Matrix We were able to convert these equations into a matrix which could be solved by computer. -1 1/2 1/ 0 -1/2 1/ 0 0 0 0 1 0 0 0 0 -1/2 1/ 0 1/2 1/ 0 0 0 0 0 0 0 0 0 0 -1 0 1 0 0 -1/2 1/ 0 -1/2 1/ 1 1/2 1/ 0 1/2 1/ 0 0 0 0 0 0 0 0 -1 0 0 0 0 -1 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 T1 0 T2 0 T3 0 T4 0 T5 = 0 T6 0 W1x 0 W1y W/2 W2x 0 W2y W/2 T1 + W1x = 0 - T1 - T5 / 2 + T2 / 2 = 0 W1y = 0 - T5 / 2 - T6 - T2 / 2 = 0 W2 x + T4 + T5 / 2 = 0 T6 T3 W/2 OPEN 24 HOURS T2 W2 y + T5 / 2 = 0 T3 - T4 = 0 - W / 2 + T6 = 0 - T3 - T2 / 2 = 0 - W / 2 + T2 / 2 = 0 3 cs = 1/sqrt(2.0); labels= ['T1 ';'T2 ';'T3 ';'T4 ';'T5 ';'T6 ';'W1x';'W1y';'W2x';'W2y`]; matrix = [ -1 0 0 0 1 0 0 0 0 0 cs -cs 0 0 0 0 -cs cs 0 0 0 0 0 0 0 0 -1 0 1 0 0 0 1 0 0 0 0 0 -1 0 -cs -cs cs cs 0 0 0 0 0 0 0 -1 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 ; ; ; ; ; ; ; ; ; ]; Script Files A script file is created by typing your command line inputs into a file and saving it with a .m suffix (multi_plots.m) (multi_plots.m) Variables in a script file have global scope Variables in a function file have local scope W = 1.0; b = [ 0 0 0 0 0 0 0 W/2.0 0 W/2.0 ]'; sol = matrix\b; strcat(labels, ' = ', num2str(sol)) Input Define the variable in the command window prior to running the script file: >> score1 = 50; >> score2 = 75; >> score3 = 100; >> calculateAverage average = 75 % This script file calculates the average % of three values that are defined in the % command window average=(score1+score2+score3) / 3 Input Define the variable inside the script file. It will remain "active" after the script file is finished. >> calculateAverage average = 75 % This script file calculates the average % of three values that are defined in the % script file score1 = 50; score2 = 75; score3 = 100; average=(score1+score2+score3) / 3 display_test.m Input Ask for a value in the script using: >> calculateAverage Gimme score1! Gimme score2! Gimme score3! average = 75 50 75 100 % This script file calculates the average % of three values that are input by the user score1 = input(`Gimme score1! '); score2 = input(`Gimme score2! '); score3 = input(`Gimme score3! '); average=(score1+score2+score3) / 3 Output display( ) is called when you don't include a semicolon after a MATLAB line where an assignment occurs disp( data ) will output only the data to the screen. Unlike display( ), disp( ) does not put a ), "data =" message before the output. 4 Next Lecture Function Files and Managing Data 5 ...
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