The do-while Loop in C: Run First, Check After

How C’s do-while loop works: the body always runs at least once before the condition is tested. Two PIN-entry examples, plus do-while vs while and common pitfalls.
Reading 5 min

By the end of this lesson you will

  • Explain why a do-while body runs at least once
  • Choose between while and do-while

Resources

Flowchart: the loop body runs, then the condition PIN != 2471 is tested. If true, control returns to the body; if false, the loop exits.

A do-while loop runs its body first, then tests the condition

Most loops check their condition before each pass. A do-while loop does the opposite: it runs the body first and checks the condition afterwards, so the body always runs at least once.

That makes it the natural choice whenever we need to do something before we can know whether to repeat it: asking for input, showing a menu, or retrying an action. Asking for a PIN is a perfect example, because we can’t validate a PIN we haven’t read yet.

The general form is:

do {
    // body: runs at least once
} while (condition);   // note the semicolon

Example 1: keep asking until the PIN is right

#include <stdio.h>

int main(void) {
    int PIN;

    do {
        printf("Enter PIN\n");
        scanf("%d", &PIN);
    } while (PIN != 2471);

    printf("Correct PIN!\n");
    return 0;
}

A sample run:

Enter PIN
1111
Enter PIN
2471
Correct PIN!

The prompt appears once no matter what, and then again for as long as PIN != 2471 is true. With an ordinary while loop we would have to give PIN a dummy starting value first just to get inside the loop. do-while avoids that.

Example 2: limit the attempts

Real systems don’t let anyone guess forever. Here we count attempts, leave early with break on success, and lock the account after 10 failures:

#include <stdio.h>

int main(void) {
    int PIN;
    int attempts = 0;

    do {
        printf("Enter PIN: ");
        scanf("%d", &PIN);
        attempts++;

        if (PIN == 2471) {
            break;   // correct PIN: exit the loop early
        }

        if (attempts >= 10) {
            printf("Too many wrong attempts. Account locked.\n");
            return 1;   // exit the program with a failure code
        }

        printf("Wrong PIN. %d attempts left.\n", 10 - attempts);
    } while (1);   // loop "forever"; we leave via break or return

    printf("Correct PIN!\n");
    return 0;
}

A sample run (entering 1, 2, then the correct PIN):

Enter PIN: 1
Wrong PIN. 9 attempts left.
Enter PIN: 2
Wrong PIN. 8 attempts left.
Enter PIN: 2471
Correct PIN!

Here the condition is while (1), which is always true, so the loop only ends through break (success) or return (locked out). We use break to leave just this loop and return 1 to end the whole program with a non-zero exit code, the usual way to signal failure.

do-while vs while

The only difference is when the test happens. Compare what happens when the condition is false from the start:

int n = 10;

do {
    printf("do-while body, n = %d\n", n);   // runs once
} while (n < 5);

while (n < 5) {
    printf("while body\n");                 // never runs
}

Output:

do-while body, n = 10
while do-while
Condition checked Before each pass After each pass
Minimum runs 0 1
Typical use Loop while there is work to do Ask/act first, then decide to repeat

Common pitfalls

  • Missing semicolon. do { ... } while (cond); needs the ; after the condition. Forgetting it is a compile error.
  • Bad input can loop forever. If the user types a letter, scanf("%d", ...) fails and leaves the text in the input buffer, so the loop above may repeat endlessly. Real programs check scanf’s return value (it is 1 on success) and clear the buffer.
  • A condition that never becomes false. Make sure something inside the body changes what the condition tests, or exit with break.
  • Using do-while when zero passes is valid. If the loop should be skipped when there is nothing to do, use while or for instead.

Key takeaway

do-while = do the body, then decide whether to repeat. Use it when the body must run at least once, such as prompts, menus and retry logic. Otherwise, while or for is usually clearer.

  • The body of a do-while always runs at least once.
  • The condition is tested after the body.
  • Note the semicolon after while (...).

Sentinel. A special input value, such as 0, that tells a loop to stop.

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