Do While Loop in C Language

The do while statement allows the creation of loops in which the body of the loop is executed at least once.

The syntax of the do while statement includes two parts: the body of the loop and the control condition. The body is executed before the condition is checked, so a do while loop guarantees that the body is always executed at least once.

In this lesson, we will see how to use a do while loop, what its syntax is, and how it differs from a regular while loop.

Key Takeaways

Using the do while statement makes it possible to create loops where the condition is evaluated at the end of the loop body;
This ensures that the loop body is always executed at least once.

`do` `while` Statement

The do while statement is closely related to the while statement. In fact, one could say that the do statement is essentially a while loop whose control expression is evaluated after each execution of the loop body.

The form of the do while statement is as follows:

do statement while ( expression );

**Picture 1:** Flowchart of the do while statement

The control expression of the do while statement must be enclosed in parentheses. Also, just like with the while statement, the body of the do while statement can be a single statement or a compound statement:

do {
    statement_1;
    statement_2;
    /* ... */
    statement_n;
} while ( expression );

When a do while statement is executed, the body of the loop is executed first, and only then is the control expression evaluated. If the value of the control expression is nonzero, i.e., the expression is true, the loop body is executed again and the expression is checked once more. The execution of a do statement ends when the expression evaluates to zero, i.e., false, after the loop body has been executed.

To better clarify the difference between a do statement and a while statement, it is useful to revisit the example from the lesson on the while statement: printing a countdown.

In the while statement lesson, we implemented a program that prints a countdown starting from 10, like this:

#include <stdio.h>

int main() {
    int n = 10;
    while (n > 0) {
        printf("%d\n", n);
        n--;
    }
    printf("Liftoff!\n");
    return 0;
}

If we want to write the same program using the do while statement, we can implement it like this:

#include <stdio.h>

int main() {
    int n;
    n = 10;
    do {
        printf("%d\n", n);
        --n;
    } while (n > 0);
    printf("Liftoff!\n");
    return 0;
}

By examining the behavior of this program, we can see that:

When the do statement is executed, the loop body is executed first. For this reason, the number is printed first, and then the variable n is decremented. At this point, n is 9;
The condition n > 0 is then tested and evaluates to true, since n is 9;
The body of the loop is therefore executed a second time;
The body of the loop will be executed repeatedly until the message 1 is printed and the variable n is decremented to 0;
At this point, the condition is evaluated and is false, so the loop terminates.

As we can see, a while loop and a do while loop often appear indistinguishable. The real difference lies in the fact that the condition is evaluated after the execution of the loop body. This feature has an important consequence:

Definition

The body of a do while loop is always executed at least once.

Unlike a while loop, the body of a do while loop is executed before the control expression is evaluated. For this reason, even if the control expression is false before the loop starts, the body of a do while loop will always be executed at least once.

This is a crucial difference compared to the while loop, in which the body may not be executed even once.

A good programming practice is to use braces to enclose the body of a do while loop even if they are not strictly necessary. This is because code like the following may mislead an inattentive reader:

do
    n = n / 2;
while (n > 0);

At a quick glance, the while on the third line might be mistaken for the beginning of a while statement.

Hint

Always enclose the body of a do loop in braces.

To improve code readability, it is recommended to always enclose the body of a do loop in braces.

So you should avoid writing code like this:

do
    statement;
while (condition);

Instead, prefer writing it like this:

do {
    statement;
} while (condition);

Summary

In this lesson, we studied the second statement provided by the C language for implementing loops: the do while statement.

Unlike the while statement, with the do while statement the control expression that determines the execution of the next iteration is evaluated after the body of the loop itself. This ensures that the loop body is always executed at least once.

In the next lesson, we will study the for statement, which allows loops to be implemented in a highly flexible manner.

Do While Loop in C Language

do while Statement

Summary

`do` `while` Statement