C Nested Loops

Nested Loops

A nested loop is a loop inside another loop. The outer loop runs once, and for each of its passes the inner loop runs all the way through. If the outer loop runs 3 times and the inner loop runs 2 times, the inner body runs a total of 6 times.

#include <stdio.h>

int main() {
  for (int i = 1; i <= 2; i++) {
    printf("Outer: %d\n", i);
    for (int j = 1; j <= 3; j++) {
      printf("  Inner: %d\n", j);
    }
  }
  return 0;
}

For each value of i in the outer loop, the inner loop counts j from 1 to 3. So you see Outer: 1 followed by three inner lines, then Outer: 2 followed by three more inner lines.

#include <stdio.h>

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

The inner loop prints 4 stars on one line, and the outer loop repeats that 3 times. The result is a rectangle of stars, 3 rows tall and 4 columns wide. The printf with a newline sits in the outer loop so each row ends cleanly.

• The outer loop controls the rows.
• The inner loop controls the columns within each row.
• Total passes equal outer count multiplied by inner count.