Nested loops

Learn to print matrices and tables with nested for loops.

Nested loops

Putting a loop inside another loop creates a nested loop. For each iteration of the outer loop, the inner loop runs completely.

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

Output: 1-1 1-2 1-3 / 2-1 2-2 2-3 / 3-1 3-2 3-3 (each row on a separate line). The outer loop produces rows, the inner produces columns; useful for matrices and tables.

Right triangle

With nested loops you can draw shapes from stars. When the inner loop's limit depends on the outer counter, you get a right triangle.

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= i; j++) {
        printf("*");
    }
    printf("\n");
}

Output: * ** *** ****. The inner loop's bound is j <= i; each row has one more star. This shows the power of nested loops.

Exercise Star right triangle

Write a C program. Write an outer for (int i = 1; i <= 4; i++) and inner for (int j = 1; j <= i; j++) loop. In the inner loop print printf("*");, at the end of the outer print printf("\n");. Put #include <stdio.h> at the top.

Show example solution

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 4; i++) {
        for (int j = 1; j <= i; j++) {
            printf("*");
        }
        printf("\n");
    }
    return 0;
}

Test yourself

Inner loop condition

for (int i = 0; i < 3; i++) {
    for (int j = 0; ____; j++) {
        printf("%d", j);
    }
    printf("\n");
}

Show answer

for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        printf("%d", j);
    }
    printf("\n");
}

Nested loop concepts

outer loop

Produces rows
inner loop

Produces columns
i

Outer counter
j

Inner counter
How many iterations does a 3x3 nested loop do?
```
for (i=0; i<3; i++)
  for (j=0; j<3; j++)
    printf("*");
```
- 3 iterations
- 6 iterations
- 9 iterations
- 12 iterations
Show answer

9 iterations — 3 outer × 3 inner = 9 total iterations.

Next lesson: Summary - Loops

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