Learn While Loop In C






The statement(s) enclosed in curly braces get executed until the condition evaluates to false. Here the condition means an expression or it can be any value.

Key Points To Remember:

  1. While loop is control structure. That means we can control the flow of execution with this structure.
  2. This loop will not iterate if the condition is false initially.
  3. It is also called as entry controlled or top tested as the condition is checked at beging, before iterating the statement(s).

Use while loop:

The While loop is used to repeat a section or block of code an unknown number of times until a specific condition is met.

That is, it is used when we do not know how many times we need to iterate the loop?



Write a C program which accepts a number from user and displays how many times that number is divisible by 2.


void main()
 int number,sum=0,original;

 printf("\n Enter any number:- ");
 scanf("%d",&number);  // Accepting a number,storing it into 'number' variable

 original=number; // Storing the original number into 'original' variable

 sum=sum+number%10; // Every time adding the last digit to sum

 printf("\n The sum of the digits of %d is %d \n",original,sum);  // Displaying the original number and sum of its digits



We will see how the while loop works.

Suppose user entered 456, we stored it in 'number' as well as in 'original' variable.

For the first iteration, we have number=456
the condition in while i.e number>0 is checked and yes 456>0 i.e the condition evaluates to true. Now the statements get executed as
  • sum=sum+number%10;
    i.e sum=0+456%10; as sum is initialized to 0 and number=456. i.e sum=0+6; i.e. sum=6;
  • Another statement is
    i.e number=number/10;
    i.e number=456/10; i.e number=45;
So after first iteration, number=45 and sum=6. Now the control moves back to the beginig of loop. The condition in while is checked i.e number>0. Yes, true 45>0. So the statements in loop body get executed for second time.

For second iteration,
  • The first statement ,
    i.e sum=6+45%10; (values of number and sum are taken from previous iteration)
    i.e sum=6+5; i.e sum=11;
  • Next is
    i.e number=45/10;
    i.e number=4;
Now after second iteration is over, we have number=4 and sum=11. Now again the control is moved to begining of th loop, the condition is checked and again the condition is true. So statements will get executed again.

For the third iteration,
  • The first statement ,
    i.e sum=11+4%10; (Values of sum and number are from previous iterations)
    i.e sum=11+4;
    i.e sum=15;
  • Next is
    i.e number=4/10;
    i.e number=0; as number is of integer data type so it will store only integers and not floating point numbers.
Now the third iteration is over and we have number=0 and sum=15 (which is the expected sum of digits of 456). Now the control is again moved to begining of the loop, the condition is checked, as number=0 so the condition number>0 evaluates to false.

After the loop is over we are just displaying the original number along with sum of its digits.

Practice With These Programs:

  1. C program to count the digits of a number
  2. C program to check given number is Armstrong number or not
  3. C program to find the longest line
  4. C program to convert hexadecimal to binary
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