Do-While

The do-while statement also repeats instructions, but it has a very important difference with respect to while.

In this lesson you will learn:

• how do-while works
• how it differs from while
• why it executes at least once
• when it’s worth using it

Syntax

do {
    instructions;
} while (condition);

Fundamental difference with while

In while, the condition is evaluated before entering the block.

In do-while, the condition is evaluated after executing the block.

That means the block executes at least once.

Example

#include <stdio.h>

int main() {
    int i = 1;

    do {
        printf("%d\n", i);
        i = i + 1;
    } while (i <= 5);

    return 0;
}

Example where the difference matters

#include <stdio.h>

int main() {
    int number = 10;

    do {
        printf("The number is %d\n", number);
    } while (number < 5);

    return 0;
}

Although number < 5 is false, the block executes once.

When is do-while worth using?

It’s worth using when you want the block to execute at least once before checking if it should repeat.

Common mistakes

1. Believing while and do-while are the same

They’re not. The difference is when the condition is evaluated.

2. Forgetting the final ;

} while (condition);

That semicolon is part of the syntax.

Summary

• do-while evaluates the condition at the end
• the block executes at least once
• it’s useful when you need to guarantee a first execution

Final idea

do-while shows that in programming it matters not just what condition is evaluated, but also at what moment it’s evaluated.