C Introduction

© 2000 Prentice Hall, Inc. All rights reserved.
1
Chapter 2 - Introduction to C
Programming
Outline
2.1 Introduction
2.2 A Simple C Program: Printing a Line of Text
2.3 Another Simple C Program: Adding Two Integers
2.4 Memory Concepts
2.5 Arithmetic in C
2.6 Decision Making: Equality and Relational Operators

2
2.1 Introduction
• C programming language
– Structured and disciplined approach to program design
• Structured programming
– Introduced in chapters 3 and 4
– Used throughout the remainder of the book

3
2.2 A Simple C Program:
Printing a Line of Text
• Comments
– Text surrounded by /* and */ is ignored by computer
– Used to describe program
• #include <stdio.h>
– Preprocessor directive
• Tells computer to load contents of a certain file
– <stdio.h> allows standard input/output operations
1 /* Fig. 2.1: fig02_01.c
2 A first program in C */
3 #include <stdio.h>
4
5 int main()
6 {
7 printf( "Welcome to C!n" );
8
9 return 0;
10 }
Welcome to C!

4
• int main()
– C++ programs contain one or more functions, exactly one of
which must be main
– Parenthesis used to indicate a function
– int means that main "returns" an integer value
– Braces ({ and }) indicate a block
• The bodies of all functions must be contained in braces

5
• printf( "Welcome to C!n" );
– Instructs computer to perform an action
• Specifically, prints the string of characters within quotes (“ ”)
– Entire line called a statement
• All statements must end with a semicolon (;)
– Escape character ()
• Indicates that printf should do something out of the ordinary
• n is the newline character

6
• return 0;
– A way to exit a function
– return 0, in this case, means that the program terminated
normally
• Right brace }
– Indicates end of main has been reached
• Linker
– When a function is called, linker locates it in the library
– Inserts it into object program
– If function name is misspelled, the linker will produce an
error because it will not be able to find function in the
library

OutlineOutline
7
1. Initialize variables
2. Input
2.1 Sum
3. Print
Program Output
1 /* Fig. 2.5: fig02_05.c
2 Addition program */
4
5 int main()
6 {
7 int integer1, integer2, sum; /* declaration */
8
9 printf( "Enter first integern" ); /* prompt */
10 scanf( "%d", &integer1 ); /* read an integer */
11 printf( "Enter second integern" ); /* prompt */
12 scanf( "%d", &integer2 ); /* read an integer */
13 sum = integer1 + integer2; /* assignment of sum */
14 printf( "Sum is %dn", sum ); /* print sum */
15
16 return 0; /* indicate that program ended successfully */
17 }
Enter first integer
45
Enter second integer
72
Sum is 117

8
2.3 Another Simple C Program:
Adding Two Integers
• As before
– Comments, #include <stdio.h> and main
• int integer1, integer2, sum;
– Declaration of variables
• Variables: locations in memory where a value can be stored
– int means the variables can hold integers (-1, 3, 0, 47)
– Variable names (identifiers)
• integer1, integer2, sum
• Identifiers: consist of letters, digits (cannot begin with a digit)
and underscores( _ )
– Case sensitive
– Declarations appear before executable statements
• If an executable statement references and undeclared variable
it will produce a syntax (compiler) error

9
Adding Two Integers
• scanf( "%d", &integer1 );
– Obtains a value from the user
• scanf uses standard input (usually keyboard)
– This scanf statement has two arguments
• %d - indicates data should be a decimal integer
• &integer1 - location in memory to store variable
• & is confusing in beginning – for now, just remember to
include it with the variable name in scanf statements
– When executing the program the user responds to the
scanf statement by typing in a number, then pressing the
enter (return) key

10
Adding Two Integers
• = (assignment operator)
– Assigns a value to a variable
– Is a binary operator (has two operands)
sum = variable1 + variable2;
sum gets variable1 + variable2;
– Variable receiving value on left
• printf( "Sum is %dn", sum );
– Similar to scanf
• %d means decimal integer will be printed
• sum specifies what integer will be printed
– Calculations can be performed inside printf statements
printf( "Sum is %dn", integer1 + integer2 );

11
2.4 Memory Concepts
• Variables
– Variable names correspond to locations in the computer's
memory
– Every variable has a name, a type, a size and a value
– Whenever a new value is placed into a variable (through
scanf, for example), it replaces (and destroys) the previous
value
– Reading variables from memory does not change them
• A visual representation
integer1 45

12
2.5 Arithmetic
• Arithmetic calculations
– Use * for multiplication and / for division
– Integer division truncates remainder
• 7 / 5 evaluates to 1
– Modulus operator(%) returns the remainder
• 7 % 5 evaluates to 2
• Operator precedence
– Some arithmetic operators act before others (i.e.,
multiplication before addition)
• Use parenthesis when needed
– Example: Find the average of three variables a, b and c
• Do not use: a + b + c / 3
• Use: (a + b + c ) / 3

13
2.5 Arithmetic
• Arithmetic operators:
• Rules of operator precedence:
C operation Arithmetic
operator
Algebraic
expression
C expression
Addition + f + 7 f + 7
Subtraction - p – c p - c
Multiplication * bm b * m
Division / x / y x / y
Modulus % r mod s r % s
Operator(s) Operation(s) Order of evaluation (precedence)
() Parentheses Evaluated first. If the parentheses are nested, the
expression in the innermost pair is evaluated first. If there
are several pairs of parentheses “on the same level” (i.e.,
not nested), they are evaluated left to right.
*, /, or % Multiplication,Divi
sion, Modulus
Evaluated second. If there are several, they are
evaluated left to right.
+ or - Addition
Subtraction
Evaluated last. If there are several, they are
evaluated left to right.

14
2.6 Decision Making: Equality and
Relational Operators
• Executable statements
– Perform actions (calculations, input/output of data)
– Perform decisions
• May want to print "pass" or "fail" given the value of a
test grade
• if control structure
– Simple version in this section, more detail later
– If a condition is true, then the body of the if statement
executed
• 0 is false, non-zero is true
– Control always resumes after the if structure
• Keywords
– Special words reserved for C
– Cannot be used as identifiers or variable names

15
Standard algebraic
equality operator or
relational operator
C equality or
relational
operator
Example of C
condition
Meaning of C
condition
Equality Operators
= == x == y x is equal to y
not = != x != y x is not equal to y
> > x > y x is greater than y
< < x < y x is less than y
>= >= x >= y x is greater than or
equal to y
<= <= x <= y x is less than or
equal to y

16
Keywords
auto double int struct
break else long switch
case enum register typedef
char extern return union
const float short unsigned
continue for signed void
default goto sizeof volatile
do if static while

OutlineOutline
17
1. Declare variables
2. Input
2.1 if statements
3. Print
1 /* Fig. 2.13: fig02_13.c
2 Using if statements, relational
3 operators, and equality operators */
5
6 int main()
7 {
8 int num1, num2;
9
10 printf( "Enter two integers, and I will tell youn" );
11 printf( "the relationships they satisfy: " );
12 scanf( "%d%d", &num1, &num2 ); /* read two integers */
13
14 if ( num1 == num2 )
15 printf( "%d is equal to %dn", num1, num2 );
16
17 if ( num1 != num2 )
18 printf( "%d is not equal to %dn", num1, num2 );
19
20 if ( num1 < num2 )
21 printf( "%d is less than %dn", num1, num2 );
22
23 if ( num1 > num2 )
24 printf( "%d is greater than %dn", num1, num2 );
25
26 if ( num1 <= num2 )
27 printf( "%d is less than or equal to %dn",
28 num1, num2 );

OutlineOutline
18
3.1 Exit main
Program Output
29
30 if ( num1 >= num2 )
31 printf( "%d is greater than or equal to %dn",
32 num1, num2 );
33
34 return 0; /* indicate program ended successfully */
35 }
Enter two integers, and I will tell you
the relationships they satisfy: 3 7
3 is not equal to 7
3 is less than 7
3 is less than or equal to 7
Enter two integers, and I will tell you
the relationships they satisfy: 22 12
22 is not equal to 12
22 is greater than 12
22 is greater than or equal to 12

C Introduction

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