C++ Loops: for, while and do-while Explained

A loop repeats a block of code. Instead of writing the same line ten times, you write it once and let the loop run it as many times as needed. C++ has four kinds of loop; this lesson introduces all of them, and the next lessons go deeper.

The for loop#

Use for when you know how many times to repeat, or you are counting:

#include <iostream>

int main() {
    for (int i = 1; i <= 5; ++i) {
        std::cout << "Lap " << i << '\n';
    }
}

Lap 1
Lap 2
Lap 3
Lap 4
Lap 5

The parentheses hold three parts, separated by semicolons:

1. Initialization int i = 1: runs once, before the loop starts.
2. Condition i <= 5: checked before every repetition; the loop stops when it is false.
3. Update ++i: runs after every repetition.

The while loop#

Use while when you do not know in advance how many repetitions you need, only the condition for continuing:

#include <iostream>

int main() {
    int balance = 1000;
    int years = 0;

    while (balance < 2000) {
        balance = balance * 110 / 100;   // 10% growth per year
        ++years;
    }
    std::cout << "Doubled after " << years << " years: " << balance << '\n';
}

Doubled after 8 years: 2142

The condition is checked before each repetition, so if it is false at the start, the body never runs.

The do-while loop#

do ... while checks the condition after the body, so the body always runs at least once. That suits "ask until the answer is valid":

#include <iostream>

int main() {
    int number = 0;
    do {
        std::cout << "Enter a number from 1 to 10: ";
        std::cin >> number;
    } while (number < 1 || number > 10);

    std::cout << "Thanks, you chose " << number << '\n';
}

Enter a number from 1 to 10: 15
Enter a number from 1 to 10: 7
Thanks, you chose 7

Note the semicolon after while (...) in a do ... while. It is required here, unlike in a normal while.

The range-based for loop#

To visit every element of a collection, the range-based for loop is the simplest and safest choice:

#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<std::string> fruits = {"apple", "banana", "cherry"};

    for (const std::string& fruit : fruits) {
        std::cout << fruit << '\n';
    }
}

apple
banana
cherry

Read it as "for each fruit in fruits". The const ... & avoids copying each string; you will learn why in References. It also works with arrays and strings (one char at a time).

Which loop should I use?#

Any loop can be rewritten as another kind; choose the one that makes the intent clearest.

Infinite loops#

If the condition never becomes false, the loop runs forever:

int i = 0;
while (i < 5) {
    std::cout << i << '\n';
    // forgot ++i, so i stays 0 forever
}

If your program hangs, press Ctrl + C in the terminal to stop it, then check that something inside the loop moves it towards its end. Sometimes an endless loop is intended, such as a game loop or server; it is written while (true) and left with break (see break and continue).

Off-by-one errors#

The most common loop bug is running one time too many or too few. Compare:

#include <iostream>

int main() {
    for (int i = 0; i < 3; ++i) std::cout << i << ' ';
    std::cout << '\n';
    for (int i = 0; i <= 3; ++i) std::cout << i << ' ';
    std::cout << '\n';
}

0 1 2
0 1 2 3

i < 3 runs 3 times (0, 1, 2); i <= 3 runs 4 times. When looping over the positions of an array with n elements, use i = 0; i < n.

Common mistakes#

Practice#

#include <iostream>

int main() {
    for (int i = 1; i <= 10; ++i) {
        std::cout << "7 x " << i << " = " << 7 * i << '\n';
    }
}

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

#include <iostream>

int main() {
    int n = 1000;
    int steps = 0;

    while (n >= 1) {
        n /= 2;
        ++steps;
    }
    std::cout << steps << '\n';
}

10

Key takeaway#