Loops Inside Loops

A loop can contain another loop. The inner loop runs completely for every round of the outer loop:

#include <stdio.h>

int main() {
    for (int row = 1; row <= 3; row++) {
        for (int col = 1; col <= 4; col++) {
            printf("* ");
        }
        printf("\n");
    }
    return 0;
}

Output

* * * * 
* * * * 
* * * * 

Think of it like this: Think of a clock. The minute hand (inner loop) goes all the way round 60 times for every one step of the hour hand (outer loop).

Drawing a triangle

Make the inner loop depend on the outer counter, and you get a triangle:

#include <stdio.h>

int main() {
    int n = 5;
    for (int row = 1; row <= n; row++) {
        for (int i = 1; i <= row; i++) {
            printf("*");
        }
        printf("\n");
    }
    return 0;
}

Output

*
**
***
****
*****

A multiplication table

#include <stdio.h>

int main() {
    for (int a = 1; a <= 5; a++) {
        for (int b = 1; b <= 5; b++) {
            printf("%4d", a * b);
        }
        printf("\n");
    }
    return 0;
}

Output

   1   2   3   4   5
   2   4   6   8  10
   3   6   9  12  15
   4   8  12  16  20
   5  10  15  20  25

Tip: %4d gives every number the same width, so the columns line up nicely.

Remember: Use different counter names for each loop (row/col or i/j) so they don't get mixed up.