C++ Loops

C++ loops run a block of code repeatedly. Besides while, do while and the classic for loop, modern C++ offers the range-based for loop to go through every element of a container.

while and do while

#include <iostream>

int main() {
    int n = 3;
    while (n > 0) {
        std::cout << n << "... ";
        --n;
    }
    std::cout << "Liftoff!\n";

    int tries = 0;
    do {
        ++tries;
    } while (tries < 3);
    std::cout << "Tried " << tries << " times\n";
}

3... 2... 1... Liftoff!
Tried 3 times

The Classic for Loop

#include <iostream>

int main() {
    for (int i = 0; i < 10; i += 3) {
        std::cout << i << ' ';
    }
    std::cout << '\n';
}

0 3 6 9 

Range-Based for Loop in C++

for (element : container) visits each element. Use const auto& to avoid copying, or auto& to modify elements.

#include <iostream>
#include <vector>

int main() {
    std::vector<int> nums{1, 2, 3, 4};
    for (auto& n : nums) n *= 10;
    for (const auto& n : nums) std::cout << n << ' ';
    std::cout << '\n';
}

10 20 30 40 

break and continue

#include <iostream>

int main() {
    for (int i = 1; i <= 10; ++i) {
        if (i % 3 == 0) continue;
        if (i > 8) break;
        std::cout << i << ' ';
    }
    std::cout << '\n';
}

1 2 4 5 7 8 

Frequently asked questions

What is a range-based for loop in C++?

Introduced in C++11, it loops over every element of a container or array without managing an index, for example for (int x : vec).

Should I use i++ or ++i in loops?

For int they are equally fast. For iterators, ++i can avoid an unnecessary copy, so many C++ programmers use ++i by habit.

How do I loop with an index and a value in C++?

Use a classic for loop with an index, or in C++23 std::views::enumerate. In C++20 a simple counter variable alongside a range-based loop also works.