While Loop with Example in C: Computer Programming

A set of statements may have to be repeatedly executed till a certain condition is reached, in every computer programming. In such situations (C language), we do not know exactly how many times a set of statements have to be repeated. So, naturally we need to have a condition controlled loop like in looping statements. In C language, the condition controlling is done generally by a while statement or while loop. It is also called test and do structure.

Syntax
while (exp)
{
  statement1;
  statement2;
  .
  .
  .
statementn;
}
Where

• while -  is a reserve word or keyword
• exp – is the expression which is evaluated to TRUE or FALSE.

Working of the while loop: 

The following sequences are carried out after executing the statements that appear just before the while loop

• If the expression exp is evaluated to FALSE at the time of entry into the loop, the control comes out of the loop without executing the body of the loop. Later, the statements after the while loop are executed.
• If the expression exp is evaluated to TRUE, the body of the loop is executed. After executing the body of the loop, control goes back to the beginning of the while loop and exp is again checked for TRUE or FALSE.
• Thus, the body of the while loop is repeatedly executed as long as exp is evaluated TRUE. Once the expression exp is evaluated FALSE, the control comes out of the while loop and the statements which after the while loops are executed.

As the expression exp is evaluated to TRUE or FALSE in the beginning of the while loop, the loop is also called entry controlled or top testing loop.

If a relational expression is evaluated to TRUE, the result is 1. If a relational expression is evaluated to FALSE, the result is 0. If an arithmetic/logical/bitwise or any other expression is evaluated to non-zero value (for example 10, 999, etc) then the condition is TRUE. After evaluation, if the result is zero (0), then the condition is FALSE.

Some special forms of while loop are shown in table below:

Algorithm: to reverse a given number

Step1:    [Input the number]
    Read : N
Step2:    [Initialize reverse number rev to 0]
    Rev = 0
Step3:    [Separate and reverse the number]
    While N !=0
        digit = N%10
        N = N/10
        Rev = rev*10+digit
    [End of while]
Step4:    [Output the reversed number]
    Write: rev
Step5:    Exit

Let N is 123, which is the give number to be reversed. In step1, N will be 12. In step2, N will be 1 and finally in step3, N is zero after separating a digit each time. So, the above three statements should be executed as long as N is not zero. Also, in step1, note that initial value of reverse is 0 and at the end of final step, reverse is 321.

Step by step Analysis
Step1:   
    Obtain 1st digit     =    123%10     = 3
    Reverse         =    0*10+3      = 3
    Separate 1st digit    =    123/10      = 12
Step2:   
    Obtain 2nd digit     =    12%10         = 2
    Reverse         =    3*10+2      = 32
    Separate 1st digit    =    12/10          = 1
Step3:   
    Obtain 3rd digit     =    1%10         = 0
    Reverse         =    32*10+1      = 321
    Separate 3rd digit    =    1/10          = 0

C Program
main()
{
int n,revnum=0,digit;
printf("Enter any integer number here:\n");
scanf("%d",&n);
while(n!=0)
{
digit=n%10;
revnum=revnum*10+digit;
   n=n/10;
}
printf("The reverse number of %d is %d",n,revnum);
}

The output is exactly the reverse no of input by the user.

Do-While loop in C