Loops: for, while and do-while

Introduction

A loop repeats a block. C has three kinds, and each fits a different situation.

Repetition is what computers are best at. C's three loops give you precise control over exactly when a block repeats and when it stops, which matters greatly when you are working through memory one element at a time.

How it works

while: until something changes

#include <stdio.h>

int main(void) {
    int n = 4096;
    int count = 0, sum = 0;
    while (n > 0) {
        sum += n % 10;
        n /= 10;
        count++;
    }
    printf("%d digits, digit sum %d\n", count, sum);
    return 0;
}

while checks its condition before each round. This loop takes the last digit with % 10 and removes it with / 10, until nothing is left.

do-while: at least once

A do { ... } while (condition); loop checks at the end, so its body always runs at least once. It suits menus that ask "again?".

break and continue

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 10; i++) {
        if (i % 3 == 0) continue;
        if (i > 7) break;
        printf("%d ", i);
    }
    printf("\n");
    return 0;
}

continue jumps straight to the next round; break leaves the loop completely.

Syntax

for (int i = 0; i < n; i++) { ... }
while (n > 0) { ... }
do { ... } while (again);

Example

#include <stdio.h>

int main(void) {
    int sum = 0;
    for (int i = 1; i <= 5; i++) {
        sum += i;
    }
    printf("1 + 2 + ... + 5 = %d\n", sum);
    return 0;
}

for (int i = 1; i <= 5; i++) has three parts:

1. int i = 1 runs once, before the loop starts.
2. i <= 5 is checked before every round. When it is false, the loop ends.
3. i++ runs after every round.

Step through and watch i and sum change. On the last check, the calculation box shows i <= 5 → 6 <= 5 → false, and the loop ends.

Line by line

1. sum starts at 0, outside the loop.
2. int i = 1 runs once, before the first round.
3. i <= 5 is checked before each round, and it is true while i is at most 5.
4. sum += i adds the current value.
5. i++ runs after each round, and the loop ends when i becomes 6.

Expected result

1 + 2 + ... + 5 = 15

In the real world

A loop reads each byte of a file, updates each pixel of a screen and checks each packet that reaches a router. In embedded systems, the whole program is often one loop that never ends on purpose.

Common mistakes

• An endless loop: the condition never becomes false (for example, forgetting n /= 10). StepVioz stops it after a number of steps.
• for (int i = 0; i <= n; i++) when you meant i < n: one round too many. This is how arrays get overrun.
• A ; right after for (...): the loop then repeats nothing, and the block below runs once.

Best practices

• Declare the loop variable inside the for.
• Write the stop condition as i < n when you count from 0.
• Make sure that something changes the condition on every round.

Summary

for counts, while repeats until its condition fails, and do-while always runs once. break leaves a loop, and continue jumps to the next round.

Mark lesson complete