101 ENGR (Sec 100), Fall 2010
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Office hours begin this week
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Introduction to C++
schedule posted on CTools (exceptions TBA)
Due Wednesday 15 Sep at 9pm
Make sure you know how to submit your project
Exam conflicts
Notify Nader Jawad ([e-mail])
ENGR 101, Lecture 2: 13 Sep 10
From Algorithms to Programs
Algorithm: A precise specification of a computational
process.
If the specification is precise enough to execute
automatically, we call it a program, and refer to the text as
code.
Last time: described algorithms using pseudocode.
Machine Language
Good for communicating with people, but does not meet
condition for true code.
Today: real programs.
Computers directly execute
instructions specifying very
low-level operations.
This machine language is
ultimately encoded as strings
of binary digits (bits),
specifying the instructions and
data operated on.
Writing directly in machine
language is taxing for humans
because it is so rudimentary.
NOT
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ENGR 101 (Sec 100), Fall 2010
Programming Language
Textual (usually) representation of programs, designed to be
convenient for humans to express and understand.
Examples: C++, Matlab
Syntax: defines the legal programs (vocabulary and grammar)
Semantics: defines the meaning, how the program corresponds
to an actual computation
heart of
the program
Algorithm:
Read in a number
Multiply by 7
Output result
First line: a declaration
Introduces identifiers x and y
Names for data elements
Declares the type of data
associated with these names
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
return 0;
}
by a program (!) called a compiler or interpreter
A C++ Program: Heart
scaffolding
To execute a program written in a high-level language, it
must first be translated to machine language
#include <iostream>
using namespace std;
Two dimensions to defining a programming language:
A C++ Program
The means to express algorithms in data.
High-level programming language:
C++ Declaration
#include <iostream>
using namespace std;
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
return 0;
}
Simple declarations are of the form:
simple_type identifiers;
double x, y;
simple_type is a symbol specifying a type
identifiers is a comma-separated list of identifiers
In our example, we introduce the identifiers x and y, and
declare that they are of type double.
The type double is one of several numeric data types
supported by C++.
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ENGR 101 (Sec 100), Fall 2010
Numeric Data Types
Much (but not all!) computing is about numbers
Statements
Multiple types of numbers: differ on class of values that
can be expressed
Two basic C++ number types:
int : an integer in restricted range
(typically around 2 billion to +2 billion)
double : number with fractional part, very large range of
magnitudes
Example Expression Statements
The computations in a
program are organized into
statements.
The heart of our example
program includes a
declaration statement and
three expression statements.
Every C++ statement ends in
a semicolon.
All programming languages have special facilities to handle
numeric data
Obtains input from the user
(e.g., keyboard), associates
data with identifier x.
Multiplies x by 7, associates
resulting value with identifier
y.
Presents as output the value
of y to user (e.g., on screen).
;
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
return 0;
}
Back to the Scaffolding
#include <iostream>
using namespace std;
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
#include is a preprocessor
directive.
return 0;
}
#include <iostream>
using namespace std;
not a C++ statement
Tells the compiler to include a
library header file as if it were
part of this program.
<iostream> specifies a
library header file of
declarations related to input/
output streams
#include <iostream>
using namespace std;
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
return 0;
}
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ENGR 101 (Sec 100), Fall 2010
Namespaces
In large programs over multiple
files, it is often difficult to avoid
name conflicts.
C++ provides the concept of
namespace to disambiguate
names defined in different
contexts.
Library identifiers actually
declared using names in std
namespace, e.g., std::cin
The using directive allows us
to refer to identifiers in std
without explicitly writing
namespace prefix.
Comments
#include <iostream>
using namespace std;
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
Every C++ program must
declare a special function
called main which serves as
the main entry point.
The OS calls main to invoke
the program.
The declaration specifies:
input arguments (empty
parentheses means none in this
case), and
the type of return value (int).
return 0;
}
The main Function
You can document programs
by adding comments.
When the C++ compiler
encounters a pair of slash
symbols, //, it ignores
everything from that point to
the end of line.
Comments are for
communicating with humans,
explaining how the algorithm
works and how to use and
maintain it.
#include <iostream>
using namespace std;
// multiply number by 7
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
return 0;
}
Rest of the Scaffolding
#include <iostream>
using namespace std;
// multiply number by 7
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
return 0;
}
Curly braces group a
sequence of statements into
a compound statement.
The return statement
terminates the program and
returns a specified value.
#include <iostream>
using namespace std;
// multiply number by 7
int main ( )
{
double x, y;
cin >> x;
y = x * 7;
cout << y;
return 0;
}
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ENGR 101 (Sec 100), Fall 2010
An Example with Errors
#include <iostream>
include <cmath>
using namespace std
Find the Errors
scaffolding
{
declaration stmt
double x, z;
cout << "Please enter a number:" << endl;
cin >> x;
expression
cout << "Enter a second number:" << endl;
stmts
cin >> y;
z = x*x;
z = z + y*y
cout << "The answer is:" << sqrt(z) << endl;
{
Find the Errors
1
#include <iostream>
include <cmath>
using namespace std
{
double x, z;
cout << "Please enter a number:" << endl;
cin >> x;
cout << "Enter a second number:" << endl;
cin >> y;
z = x*x;
z = z + y*y
cout << "The answer is:" << sqrt(z) << endl;
{
Example with Errors #2
#include <iostream>
#include <cmath>
;
using namespace std 2
#include <iostream>
#include <cmath>
using namespace std
{
double x, z;
cout << "Please enter a number:" << endl;
cin >> x;
cout << "Enter a second number:" << endl;
cin >> y;
z = x*x;
z = z + y*y
cout << "The answer is:" << sqrt(z) << endl;
{
int main( )
{
double a, b;
cout << "Enter your weight in pounds: " << endl
cin >> a;
cout << "Enter your height in inches: " << endl;
cin >> b;
c = 7.03 * a / pow(b, 2);
cout << "Your BMI is: " << c << endl;
return 0
}
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ENGR 101 (Sec 100), Fall 2010
Running (Executing) a Program
Compilation
A program runs when it is invoked by some caller.
When we issue a command to run a program, it is called
by the computers operating system (OS).
An OS manages machines the resources, including input/output
channels required to exchange data.
I/O channels: files, keyboard, screen, network, microphone,
speaker, camera, printer, mouse,
In this class we use the Linux OS.
Caller
In order to be invoked by the OS, a program must be
expressed in executable form, that is, in machine language.
A compiler translates programs expressed in high-level
languages to executable form.
#include <iostream>
blah blah ;
int main()
{
compiler
executable
}
Data
Program A
Data
In our computing environment, the C++ compiler is
called g++.
Data
Creating and Compiling a C++ Program
programmer
1.
2.
OS (Linux)
3.
Text
Editor
Compiler
4.
project1.cpp
project1
Invoke the text editor via
the OS
Create a text file
containing the program:
project1.cpp
Run the compiler with
project1.cpp as input;
output is executable
project1
Invoke project1 (via the
OS)
Running the C++ Compiler
In our CAEN Linux computing environment, the
command to compile a C++ program is:
g++ mycode.cpp -o myexec
Here mycode.cpp is the name of the text file
containing your program and myexec is the resulting
(machine language) executable file.
You can then execute the new program by typing:
myexec
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ENGR 101 (Sec 100), Fall 2010
Recap to this Point
C++ Data Items
An algorithm is a precise specification of a
computational process.
A programming language defines a means to express
algorithms with a well-defined syntax and semantics.
C++ is the first high-level programming language we
employ in this course.
C++ programs are composed of declarations and
expression statements, plus scaffolding.
A C++ program must be compiled before it can be
executed.
Compound Expressions
Arithmetic Operators
We can create compound expressions by applying
operators to (atomic) expressions.
New compound
expression
operator
exp1
exp2
For example, 5+x is a compound expression with
operator + and operands 5 and x.
Unary operator: has one operand
Binary operator: has two operands
The basic ways to refer to data objects in a C++ program
use literals or identifiers.
Literal: Direct expression literally describes the data value
For example, some numeric literals:
the integer 5
the real number 2.4
1.0 105 (scientific notation)
Identifier : Name associated with data items
5
2.4
1.0e-5
Must be declared
Comprised of letters (including _) and digits, starting with
letter
Atomic expression: a single literal or identifier
Operator
Meaning
Arity
Placement
Precedence
sign change
unary
prefix
15
multiplication
binary
infix
13
*
/
/
%
+
real division
binary
infix
13
integer division
binary
infix
13
remainder
binary
infix
13
addition
binary
infix
12
subtraction
binary
infix
12
Holloway Table 2.1
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ENGR 101 (Sec 100), Fall 2010
The Minus Operators
The Division Operators /
Two versions of operator
Distinguished by whether the sign is in front of a
single expression, or between two.
int
x =
y =
z=
x, y, z;
prefix
5;
x;
// unary minus (sign change)
x 3; // binary minus (subtraction)
Another case of two operator versions
Integer division applies if both operands are integers
Real division applies if either is not an integer
To determine which operator is meant, resolve the
types of operands.
cout << y / 5;
Division operator depends on
type of identifier y.
infix
Integer Division
Real Division
Divides first (left) operand by the second (right).
Value of the expression is the whole number part of
the result.
Examples
200 / 20
201 / 20
20 / 200
50 / 20
-50 / 20
10
10
0
2
-2
Divides first (left) operand by the second (right).
Value of the expression is the (non-integer) result.
also an integer
Examples
200.0 / 20
201 / 20.0
20.0 / 200
50.0 / 20.0
-50 / 20.0
10.0
10.05
0.1
2.5
-2.5
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ENGR 101 (Sec 100), Fall 2010
Remainder Operator (%)
Mathematical Operator Precedence
Divides first (left) operand by the second (right).
Value of the expression is the remainder.
In evaluating a compound expression with multiple operators, the
order of operations can make a difference.
In C++, every operator has a precedence level, and expression
evaluation respects this order.
For arithmetic operators:
Operands and results are integers
Examples
200 % 20
201 % 20
20 % 200
50 % 20
-50 % 20
0
1
20
10
-10
First sign changes (unary minus),
then multiplication, division, and remainder,
then addition and subtraction.
7 + 20 * 3
67
To override precedence order, or just avoid confusion, use
parentheses:
Evaluate:
For example: 7 + 20 * 3
(7 + 20) * 3
7 + (20 * 3)
81
67
Evaluate:
10.0 + 3.0 / 4.0
10 + 3 / 4
A- 3
A- 3
B- 3.25
B- 3.25
C- 10
C- 10
D- 10.75
D- 10.75
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ENGR 101 (Sec 100), Fall 2010
Evaluate:
Evaluate:
(10 + 3) / 4
(7 % 2 + 1) / 2
A- 3
A- 0
B- 3.25
B- 1
C- 10
C- 2
D- 10.75
D- 3
Insertion Operator <<
More Operators
Operator
<<
>>
=
Arity
Placement
Precedence
insertion
binary
infix
binary
infix
11
assignment
binary
infix
Expects an ostream (output stream) as left operand
11
extraction
2
Operators are not just for arithmetic
Meaning
(and not all operands are numbers)
cout : standard output stream (usually the screen)
an output file
Sends the right operand value to the ostream
For example, result of evaluating an arithmetic expression
Can also operate on text, e.g., a string literal
<< has lower precedence than any of the math operators
series of characters delimited by double-quote symbol (")
cout << "Enter a number: ";
<< sends output to an external stream (e.g., screen)
>> gets input from external stream (e.g., keyboard)
= associates an identifier with a value
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ENGR 101 (Sec 100), Fall 2010
Series of Insertion Operators <<
Result of evaluating the << expression is the ostream
A series of << operations are evaluated left to right.
T racing an Example
cout << 3.0+2.5 << 2 << endl;
left associative
Examples:
cout << "The answer is: " << 42 << endl;
evaluates to
cout << 5.5 << 2 << endl;
(cout after inserting string)
evaluates to
(cout after inserting string, then number)
evaluates to
cout << endl;
evaluates to
cout
cout << "The first answer is " << 3 + 17 / 3
<< endl << "And the second answer is "
<< 14 % 3 + 5 * 8 << endl;
A The first answer is 8 And the second answer is 44
B The first answer is 8
And the second answer is 42
// outputs 5.5
cout << 2 << endl;
(cout after inserting string, then number, then endl)
What Prints Out?
5.52!
!
^
^! ^! !
^!
// outputs 2
// skips another line
Extraction Operator >>
Expects an istream (input stream) as left operand
cin : standard input stream (usually the keyboard)
an input file
Expects an identifier as right operand
Obtains data from the istream and associates the value
with the identifier
C The first answer is 6 And the second answer is 42
D The first answer is 6
And the second answer is 44
E The first answer is 8 And the second answer is 42
F
The first answer is 6
And the second answer is 44
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ENGR 101 (Sec 100), Fall 2010
Assignment Operator =
Expects an identifier as left operand
Assigns the value of right operand to the identifier
Next Time
Functions and Procedures
identifier = expression;
expression is evaluated and the value is associated with
the identifier.
Associating the value: storing it in memory location
corresponding to identifier.
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