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 ends.

Flowchart of do…while Loop

Example 2: do…while loop

// Program to add numbers until the user enters zero

#include <stdio.h>
int main() {
  double number, sum = 0;

  // the body of the loop is executed at least once
  do {
    printf("Enter a number: ");
    scanf("%lf", &number);
    sum += number;
  }
  while(number != 0.0);

  printf("Sum = %.2lf",sum);

  return 0;
}

Output

Enter a number: 1.5
Enter a number: 2.4
Enter a number: -3.4
Enter a number: 4.2
Enter a number: 0
Sum = 4.70

Here, we have used a do...while loop to prompt the user to enter a number. The loop works as long as the input number is not 0.

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.

do {
  printf("Enter a number: ");
  scanf("%lf", &number);
  sum += number;
}
while(number != 0.0);

So, if the first input is a non-zero number, that number is added to the sum variable and the loop continues to the next iteration. This process is repeated until the user enters 0.

But if the first input is 0, there will be no second iteration of the loop and sum becomes 0.0.

Outside the loop, we print the value of sum.