C while and do...while Loop

In programming, loops are used to repeat a block of code until a specified condition is met.

C programming has three types of loops.

1. for loop
2. while loop
3. do...while loop

In the previous tutorial, we learned about for loop. In this tutorial, we will learn about while and do..while loop.

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 (ends).

To learn more about test expressions (when testExpression is evaluated to true and false), check out relational and logical operators.

Flowchart of while loop

Example 1: while loop

// Print numbers from 1 to 5

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

  return 0;
}

Output

1
2
3
4
5

Here, we have initialized i to 1.

1. When i = 1, the test expression i <= 5 is true. Hence, the body of the while loop is executed. This prints 1 on the screen and the value of i is increased to 2.
2. Now, i = 2, the test expression i <= 5 is again true. The body of the while loop is executed again. This prints 2 on the screen and the value of i is increased to 3.
3. This process goes on until i becomes 6. Then, the test expression i <= 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 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.