C++ While and Do-While Loops

A loop repeats a block of code. while and do-while loops repeat as long as a condition is true, which suits work where the number of rounds is not known in advance.

while

while checks its condition before each round:

#include <iostream>

int main() {
    int n = 5;
    while (n > 0) {
        std::cout << n << "... ";
        n--;
    }
    std::cout << "Lift off!\n";
    return 0;
}

5... 4... 3... 2... 1... Lift off!

The loop needs three things: a starting value (n = 5), a condition that uses it (n > 0), and a change in the body (n--). Without the change the loop never ends; press Stop in the editor to end such a program.

Reading until a stop value

#include <iostream>

int main() {
    int number, total = 0, count = 0;
    std::cout << "Numbers, 0 to stop:\n";
    while (std::cin >> number && number != 0) {
        total += number;
        count++;
    }
    std::cout << count << " numbers, total " << total << "\n";
    return 0;
}

4
10
7
0

Numbers, 0 to stop:
3 numbers, total 21

std::cin >> number is part of the condition: the loop also stops when the input ends or is not a number.

do-while

do-while checks its condition after each round, so the body always runs at least once. That fits menus, which must be shown before the user can choose:

#include <iostream>

int main() {
    int choice;
    do {
        std::cout << "1) Say hello  2) Quit: ";
        std::cin >> choice;
        if (choice == 1) std::cout << "Hello!\n";
    } while (choice != 2);
    std::cout << "Bye\n";
    return 0;
}

1
1
2

1) Say hello  2) Quit: Hello!
1) Say hello  2) Quit: Hello!
1) Say hello  2) Quit: Bye

Note the semicolon after while (...) at the end of a do-while.

Which loop to choose

• for (next topic): you know how many rounds, or you visit every element of a collection.
• while: repeat until something happens, such as reading until the user stops.
• do-while: the body must run at least once.

All of them can express the same work; picking the one that matches the intention makes code easier to read.

Practice exercises

1. Print the even numbers from 2 to 20 with a while loop.
2. Read numbers until a negative one and print the largest.
3. Write a number guessing game with do-while that ends when the guess is right.