C++ do-while Loop

In this page:

Basic do-while Loop
Guaranteeing At Least One Run
Simulating User Menu Choices
Data Validation Simulation
Nested do-while Loops

Basic do-while Loop

A do-while loop is a post-test loop: it runs the body first and only checks the condition afterward, at the bottom of the loop, which is the opposite order from a regular while loop's condition-first check. This structural difference is what guarantees the loop body always executes at least once.

Example: Basic do-while Loop

cpp

#include <iostream>

int main() {
	int count = 0;
	do {
		std::cout << count << std::endl;
		count++;
	} while (count < 5);
	return 0;
}

Guaranteeing At Least One Run

Unlike a while loop, which might never run its body if the condition starts false, a do-while loop always executes at least one full pass before the condition is even evaluated. This makes it the right structural choice whenever 'do this at least once, then repeat if needed' is genuinely what the logic calls for.

Example: Guaranteeing At Least One Run

cpp

#include <iostream>

int main() {
	int count = 10;
	do {
		std::cout << "Runs once even though condition is false" << std::endl;
		count++;
	} while (count < 5);
	return 0;
}

Simulating User Menu Choices

Interactive menus are a textbook use case for do-while, since you naturally want to display the menu and get the user's first choice before there's anything to check — the loop then repeats showing the menu until the user picks an exit option. A regular while loop would need an awkward workaround to show the menu before its condition could even be evaluated.

Example: Simulating User Menu Choices

cpp

#include <iostream>

int main() {
	int choice;
	int attempts = 0;
	int hardcodedInputs[] = {5, 2};
	do {
		choice = hardcodedInputs[attempts];
		std::cout << "Menu choice: " << choice << std::endl;
		attempts++;
	} while (choice != 2);
	return 0;
}

Data Validation Simulation

Input validation follows the same pattern: you have to actually prompt for and receive input before you can check whether it's valid, which is exactly the 'act first, check after' order a do-while loop provides. This avoids needing to fake an initial value just to satisfy a while loop's upfront condition check.

Example: Data Validation Simulation

cpp

#include <iostream>

int main() {
	int age;
	int hardcodedInputs[] = {-5, 25};
	int attempts = 0;
	do {
		age = hardcodedInputs[attempts];
		std::cout << "Entered age: " << age << std::endl;
		attempts++;
	} while (age < 0);
	return 0;
}

Nested do-while Loops

Nesting do-while loops inside each other works the same way as nesting any other loop type, and is useful for building grid-based patterns or processing multi-dimensional data where each dimension needs to run at least once regardless of any condition, such as generating a fixed-size table.

Example: Nested do-while Loops

cpp

#include <iostream>

int main() {
	int i = 1;
	do {
		int j = 1;
		do {
			std::cout << i << "," << j << " ";
			j++;
		} while (j <= 2);
		std::cout << std::endl;
		i++;
	} while (i <= 2);
	return 0;
}

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