C while Loop

Some jobs repeat until something changes, not a fixed number of times. You keep saving until you reach your goal; a phone keeps running until the battery gets low. The C while loop is built for exactly this: it repeats a block of code while a condition stays true. In this lesson, you'll learn how a while loop works, how to write counter loops, how to keep asking for input until it's valid, and how to recognize (and stop) an infinite loop.

What is a while loop in C?

A while loop repeats a statement or block of code as long as a condition is true (non-zero). Its shape is simple:

while (condition) {
    // loop body: runs again and again while condition is true
}

Here's a complete program: a rocket countdown.

#include <stdio.h>

int main(void) {
    int seconds = 5;

    while (seconds > 0) {
        printf("%d...\n", seconds);
        seconds--;
    }

    printf("Liftoff!\n");
    return 0;
}

5...
4...
3...
2...
1...
Liftoff!

Each run of the body is called an iteration. The line seconds--; uses the decrement operator to subtract 1. Without it, seconds would stay 5 forever and the loop would never end.

How does a while loop work step by step?

A while loop always follows the same cycle:

1. Check the condition.
2. If it's true, run the body, then go back to step 1.
3. If it's false, skip the body and continue with the code after the loop.

Because the check happens before the body, a while loop is called an entry-controlled loop (or a pre-test loop).

What happens if the condition is false at the start?

Then the body runs zero times. Picture a shop selling items while there's stock left, but the shelf is already empty:

#include <stdio.h>

int main(void) {
    int stock = 0;

    while (stock > 0) {
        printf("Selling one item...\n");
        stock--;
    }

    printf("Loop finished. Stock: %d\n", stock);
    return 0;
}

Loop finished. Stock: 0

This is usually what you want: if there's nothing to do, do nothing. (If you need the body to run at least once, use the do-while loop from the next lesson.)

Writing a counter loop with while

A counter is a variable that counts how many times something happened. Here's the savings jar from the start of the lesson. We don't know how many weeks it will take, so the loop counts them for us.

#include <stdio.h>

int main(void) {
    double savings = 0.0;
    double weekly_deposit = 45.0;
    double goal = 300.0;
    int weeks = 0;

    while (savings < goal) {
        savings += weekly_deposit;
        weeks++;
    }

    printf("Goal reached after %d weeks.\n", weeks);
    printf("Saved: $%.2f\n", savings);
    return 0;
}

Goal reached after 7 weeks.
Saved: $315.00

After 6 weeks you have $270, which is still less than $300, so the loop runs a 7th time and you end at $315. Every while loop needs these three parts working together:

• Setup before the loop (savings = 0.0, weeks = 0).
• A condition that will eventually become false (savings < goal).
• An update inside the body that moves toward that moment (savings += weekly_deposit).

Loops that stop on a condition, not a count

A while loop shines when the number of repetitions depends on the data. A phone loses 15% of its battery every 30 minutes, and warns you once it drops to 20% or below:

#include <stdio.h>

int main(void) {
    int battery = 100;
    int minutes = 0;

    while (battery > 20) {
        battery -= 15;
        minutes += 30;
        printf("After %d minutes: %d%%\n", minutes, battery);
    }

    printf("Low battery! Plug in your phone.\n");
    return 0;
}

After 30 minutes: 85%
After 60 minutes: 70%
After 90 minutes: 55%
After 120 minutes: 40%
After 150 minutes: 25%
After 180 minutes: 10%
Low battery! Plug in your phone.

In printf, %% prints a single percent sign. Notice that the battery was 25% when the condition was last checked, which is still above 20, so one more drain happened.

Another classic: counting the digits of a number by chopping off its last digit with integer division until nothing is left.

#include <stdio.h>

int main(void) {
    int zip = 90210;
    int remaining = zip;
    int digits = 0;

    while (remaining > 0) {
        remaining = remaining / 10;
        digits++;
    }

    printf("%d has %d digits.\n", zip, digits);
    return 0;
}

90210 has 5 digits.

remaining goes 90210 → 9021 → 902 → 90 → 9 → 0: five divisions, five digits.

How do you validate user input with a while loop?

Users make typos. A while loop can keep asking until the answer makes sense:

#include <stdio.h>

int main(void) {
    int age = 0;

    printf("Enter your age (1-120): ");
    scanf("%d", &age);

    while (age < 1 || age > 120) {
        printf("Invalid age. Try again (1-120): ");
        scanf("%d", &age);
    }

    printf("Thanks! You are %d years old.\n", age);
    return 0;
}

In this run, the user typed 0, then 150, then 34:

Enter your age (1-120): 0
Invalid age. Try again (1-120): 150
Invalid age. Try again (1-120): 34
Thanks! You are 34 years old.

The condition uses the logical OR operator: the age is invalid if it's too small or too large.

What is an infinite loop?

An infinite loop is a loop whose condition never becomes false. Sometimes it's a bug; sometimes it's on purpose. Programs like games, servers and thermostats run "forever" until something tells them to stop:

while (1) {
    // read the temperature
    // turn the heating on or off
}

In C, any non-zero value counts as true, so while (1) never stops by itself. In the break and continue lesson you'll learn how to exit such a loop from the inside.

while vs for: which loop should you use?

Anything a for loop does, a while loop can do too. These two fragments print 1, 2 and 3:

for (int i = 1; i <= 3; i++) {
    printf("%d\n", i);
}

int i = 1;
while (i <= 3) {
    printf("%d\n", i);
    i++;
}

The for loop puts setup, condition and update on one line; the while loop spreads them out. Choose the one that reads best:

Common mistakes with while loops

1. Forgetting the update

int count = 1;
while (count <= 5) {
    printf("%d\n", count);
}

count never changes, so this prints 1 forever. Always make sure something in the body moves the condition toward false.

2. A semicolon right after the condition

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

The lone ; is an empty loop body. The loop repeats "nothing" forever, and the block below it never gets a chance to run. The program just hangs. Never put a semicolon after while (...) when a body follows.

3. Using = instead of ==

while (answer = 1) assigns 1 to answer every time, and 1 is true, so the loop never ends. To compare, use ==. GCC warns about this with -Wall ("suggest parentheses around assignment used as truth value").

4. Off-by-one conditions

while (i < 10) stops before 10; while (i <= 10) includes it. When a loop runs one time too many or too few, check the comparison operator first.

Practice exercise

#include <stdio.h>

int main(void) {
    int views = 100;
    int day = 1;

    while (views <= 10000) {
        views *= 2;
        day++;
    }

    printf("Day %d: %d views\n", day, views);
    return 0;
}

Day 8: 12800 views

What's next?

A while loop may skip its body entirely. When you need the body to run at least once, like showing a menu before asking for a choice, use the C do-while loop.