Difference between while & do-while in C Programming

• Tech Area
• May 11, 2024

In this tutorial, We will see the difference between while & do-while in C programming language.

while loop

The syntax of the while loop is:

while (TestExpression) {
  // the body of the loop 
}

How while loop works?

The while loop evaluates the TestExpression inside the parentheses ().

If TestExpression is true, statements inside the body of while loop are executed. Then, TestExpression is evaluated again.

The process goes on until TestExpression is evaluated to false. If TestExpression is false, the loop terminates.

Program to print numbers using while loop

#include<stdio.h>
#include<conio.h>
void main()
{
int a=1;
clrscr();
while(a<=5)
{
printf("%d\n", a);
a++;
}
getch();
}

Output

1
2
3
4
5

Here, we have initialized a to 1.

When a = 1, the test expression a <= 5 is true. Hence, the body of the while loop is executed. This prints 1 on the screen and the value of a is increased to 2.

Now, a = 2, the test expression a <= 5 is again true. The body of the while loop is executed again. This prints 2 on the screen and the value of a is increased to 3.

This process goes on until a becomes 6. Then, the test expression a <= 5 will be false and the loop terminates.

do while loop

The do while loop is similar to the while loop with one important difference. The body of do while loop is executed at least once. Only then, the test expression is evaluated.

The syntax of the do while loop is:

do {
  // the body of the loop
}
while (TestExpression);

How do while loop works?

The body of do while loop is executed once. Only then, the TestExpression is evaluated.

If TestExpression is true, the body of the loop is executed again and TestExpression is evaluated once more.

This process goes on until TestExpression becomes false. If TestExpression is false, the loop terminates.

Program to print numbers using do while loop

#include<stdio.h>
#include<conio.h>
void main()
{
int a=1;
clrscr();
do
{
printf("%d\n", a);
a++;
}
while(a<=0);
getch();
}

Output

1

Here, we have initialized a to 1.

This prints 1 on the screen and the value of a is increased to 2 and then condition is checked. The test expression a <= 0 will be false and the loop terminates.

The do while loop executes at least once i.e. the first iteration runs without checking the condition. The condition is checked only after the first iteration has been executed.