Loops inside loops

The inner loop runs fully each time

With a loop inside a loop, the inner one runs from start to finish for every turn of the outer one. The outer loop usually picks the row, the inner one fills it.

A small triangle

#include <stdio.h>

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

1. Line 4: Row 1.
2. Line 6: The inner loop runs once (col 1 to 1).
3. Line 8: End the row.
4. Line 6: Row 2: the inner loop runs twice.
5. Line 8: End the row.
6. Line 6: Row 3: three stars.
7. Line 8: Done.

Exercise: Number triangle

Read n and print n rows. Row r holds the numbers 1 to r separated by spaces:

1

1 2

1 2 3

Exercise: Centred pyramid

Read n and print a pyramid of n rows. Row r has n − r spaces, then 2r − 1 stars. For n = 3:

*

***

*****