C Nested for Loops

A nested for loop places one loop inside another, with the inner loop running fully for each iteration of the outer loop, commonly used for grids, patterns, and 2D array traversal.

In this page:

What is a Nested for Loop?
Printing a Grid Pattern
Nested Loops over a 2D Array
Nested Loops with Different Bounds
Performance of Nested Loops

What is a Nested for Loop?

A nested for loop places one for loop entirely inside the body of another, and the inner loop runs through its full range once for every single iteration of the outer loop.

Example: What is a Nested for Loop?

#include <stdio.h>
int main() {
	for (int i = 0; i < 2; i++) {
		for (int j = 0; j < 2; j++) {
			printf("(%d,%d) ", i, j);
		}
	}
	return 0;
}

Printing a Grid Pattern

Nested loops are the standard technique for printing two-dimensional patterns like grids or triangles, with the outer loop typically controlling rows and the inner loop controlling columns within each row.

Example: Printing a Grid Pattern

#include <stdio.h>
int main() {
	for (int row = 0; row < 3; row++) {
		for (int col = 0; col < 3; col++) {
			printf("* ");
		}
		printf("\n");
	}
	return 0;
}

Nested Loops over a 2D Array

Iterating over a two-dimensional array almost always uses nested loops, with the outer loop's index selecting a row and the inner loop's index selecting a column within that row.

Example: Nested Loops over a 2D Array

#include <stdio.h>
int main() {
	int grid[2][2] = {{1, 2}, {3, 4}};
	for (int row = 0; row < 2; row++) {
		for (int col = 0; col < 2; col++) {
			printf("%d ", grid[row][col]);
		}
	}
	return 0;
}

Nested Loops with Different Bounds

The outer and inner loops in a nested pair don't need matching bounds, and the inner loop's bound can even depend on the outer loop's current variable, which is how growing or shrinking patterns are produced.

Example: Nested Loops with Different Bounds

#include <stdio.h>
int main() {
	for (int i = 1; i <= 3; i++) {
		for (int j = 0; j < i; j++) {
			printf("*");
		}
		printf("\n");
	}
	return 0;
}

Performance of Nested Loops

The total number of iterations in a nested loop is the product of the outer and inner loop counts, so nested loops over large ranges can quickly become expensive, an important consideration when reasoning about performance.

Example: Performance of Nested Loops

#include <stdio.h>
int main() {
	int count = 0;
	for (int i = 0; i < 3; i++) {
		for (int j = 0; j < 4; j++) {
			count++;
		}
	}
	printf("%d", count);
	return 0;
}

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