The while Loop

Big idea

A while loop repeats a block as long as its condition stays true.

The idea

while (condition) {
    // repeats while condition is true
}

Before every round, C checks the condition. True → run the block, then check again. False → skip past the loop and carry on.

Almost every loop has three parts:

1. Start — set up a variable before the loop: int i = 1;
2. Check — the condition: while (i <= 3)
3. Change — move towards the end inside the loop: i = i + 1;

Watch it run

Step through line by line and watch i change:

When you don't know how many rounds

while shines when the number of rounds depends on the data. How many digits does a number have? Keep dividing by 10 until nothing's left:

#include <stdio.h>

int main(void) {
    int n;
    scanf("%d", &n);
    int digits = 0;
    while (n > 0) {
        n = n / 10;
        digits = digits + 1;
    }
    printf("%d digits\n", digits);
    return 0;
}

input
90210

output
5 digits

Watch it

Think of it like

Stirring soup: "while it's not boiling, keep stirring." You don't know how many stirs it'll take — you just keep checking.

🕷️ Spidey-sense: while (city_in_danger) { patrol(); } — the day I forget to make the danger go down inside that loop is the day I never get home for dinner.

Common mistake

Forgetting the change step. If nothing inside the loop moves the condition towards false, it never ends — an infinite loop:

#include <stdio.h>

int main(void) {
    int i = 1;
    while (i <= 3) {
        printf("%d\n", i);
    }
    return 0;
}

output
1
2
3

(Running it stops after the time limit — that's the app protecting you from a loop that never ends.)

Key terms

• Loop — Code that repeats.
• Iteration — One round of a loop.
• Infinite loop — A loop whose condition never becomes false.