C-If Statement with Example

We can see like most other languages; C uses the keyword if to implement the decision control instruction. The general form of if statement looks like below example.

if (this condition is true)
execute this statement;

The keyword C if statement tells the compiler that what follows is a decision control instruction. The condition following the keyword if is always enclosed within a pair of parentheses. If the condition, whatever it is, is true, then the statement is executed. If the condition is not true then the statement is not executed; instead the program skips past it.

C If Statement

As a general rule, we express condition using C’s ‘relational’ operators. The relational operators allow us to compare two values to see whether they are equal to each other, unequal, or whether one is greater than the other. Here’s how they look and how they are evaluated in C.

Here is simple program, which demonstrates the use of if and the relational operators.

/* Demonstration of if statement */
main( )
{
int num ;
printf ( "Enter a number less than 10 " ) ;
scanf ( "%d", &num ) ;
if ( num <= 10 )
printf ( "What an person you are !" ) ;
}

On execution of this program, if you type a number less than or equal to 10, you get a message on the screen through printf( ). If you type some other number, the program doesn’t do anything. The following would help you understand the flow of control in the program.

Example, 

/* Calculation of total expenses */

main( )
{
int qty, dis = 0 ;
float rate, tot ;
printf ( "Enter quantity and rate " ) ;
scanf ( "%d %f", &qty, &rate) ;
if ( qty > 1000 )
dis = 10 ;
tot = ( qty * rate ) - ( qty * rate * dis / 100 ) ;
printf ( "Total expenses = Rs. %f", tot ) ;
}

Here is some sample interaction with the program.

Enter quantity and rate 1200 15.50
Total expenses = Rs. 16740.000000
Enter quantity and rate 200 15.50
Total expenses = Rs. 3100.000000

In the first run of the program, the condition evaluates to true, as 1200 (value of qty) is greater than 1000. Therefore, the variable dis, which was earlier set to 0, now gets new value 10. Using this new value total expenses are calculated and printed.

In the second run the condition evaluates to false, as 200 (the value of qty) isn’t greater than 1000. Thus, dis, which is earlier set to 0, remains 0, and hence the expression after the minus sign evaluates to zero, thereby offering no discount.