while loops

Check, run, check again

while (condition) { … } checks the condition; if it's true it runs the block, then checks again. When the condition becomes false, the program moves on.

Something inside the loop must change the condition, or it runs forever.

Adding 1 to 4

#include <stdio.h>

int main() {
    int i = 1, sum = 0;
    while (i <= 4) {
        sum += i;
        i++;
    }
    printf("%d\n", sum);
    return 0;
}

1. Line 4: Start: i is 1, sum is 0.
2. Line 6: 1 <= 4, so the body runs: sum becomes 1.
3. Line 7: i goes up to 2.
4. Line 6: 2 <= 4: sum becomes 3.
5. Line 6: Then 3 is added: sum is 6.
6. Line 6: Then 4: sum is 10.
7. Line 5: i is now 5, and 5 <= 4 is false, so the loop ends.
8. Line 9: Printed after the loop.

Digits, one at a time

#include <stdio.h>

int main() {
    int n = 507;
    while (n > 0) {
        printf("%d", n % 10);
        n /= 10;
    }
    printf("\n");
    return 0;
}

What does this print?

• 705
• 507
• 75

Exercise: Sum from 1 to n

Read n and print 1 + 2 + … + n, using a loop.

Exercise: Count the digits

Read a positive whole number and print how many digits it has, using a loop.