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

Syntax

cpp

for (int outer = start; outer < limit; outer++) {
  for (int inner = start; inner < limit; inner++) {
    // inner body
  }
}

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?

cpp

// Include std::cout and std::cin
#include <iostream>

// Program execution starts in main()
int main() {
	// Loop: repeat while the condition holds
	for (int i = 1; i <= 2; i++) {
		// Loop: repeat while the condition holds
		for (int j = 1; j <= 3; j++) {
			// Print to the console with cout
			std::cout << i << "-" << j << " ";
		}
	}
	// Print to the console with cout
	std::cout << std::endl;
	// Return 0 to signal that the program finished successfully
	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

cpp

// Include std::cout and std::cin
#include <iostream>

// Program execution starts in main()
int main() {
	// Loop: repeat while the condition holds
	for (int row = 1; row <= 3; row++) {
		// Loop: repeat while the condition holds
		for (int col = 1; col <= 3; col++) {
			// Print to the console with cout
			std::cout << "* ";
		}
		// Print to the console with cout
		std::cout << std::endl;
	}
	// Return 0 to signal that the program finished successfully
	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

cpp

// Include std::cout and std::cin
#include <iostream>

// Program execution starts in main()
int main() {
	int grid[2][3] = {{1, 2, 3}, {4, 5, 6}};
	// Loop: repeat while the condition holds
	for (int row = 0; row < 2; row++) {
		// Loop: repeat while the condition holds
		for (int col = 0; col < 3; col++) {
			// Print to the console with cout
			std::cout << grid[row][col] << " ";
		}
	}
	// Print to the console with cout
	std::cout << std::endl;
	// Return 0 to signal that the program finished successfully
	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

cpp

// Include std::cout and std::cin
#include <iostream>

// Program execution starts in main()
int main() {
	// Loop: repeat while the condition holds
	for (int i = 1; i <= 3; i++) {
		// Loop: repeat while the condition holds
		for (int j = 1; j <= i; j++) {
			// Print to the console with cout
			std::cout << "* ";
		}
		// Print to the console with cout
		std::cout << std::endl;
	}
	// Return 0 to signal that the program finished successfully
	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

cpp

// Include std::cout and std::cin
#include <iostream>

// Program execution starts in main()
int main() {
	// Declare outerCount and set it to 100
	int outerCount = 100;
	// Declare innerCount and set it to 100
	int innerCount = 100;
	long totalIterations = (long)outerCount * innerCount;
	// Print to the console with cout
	std::cout << "Total iterations: " << totalIterations << std::endl;
	// Return 0 to signal that the program finished successfully
	return 0;
}

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