Loops

Loops repeat a block of code. C++ has four loop forms: for, range-based for, while, and do-while.

for loop

The classic for loop has three parts: initialization, condition, and increment:

#include <iostream>

int main() {
    for (int i = 0; i < 5; i++) {
        std::cout << i << " ";
    }
    std::cout << "\n";   // 0 1 2 3 4
}

Any part can be omitted. for (;;) is an infinite loop equivalent to while (true).

Counting down

for (int i = 10; i > 0; i--) {
    std::cout << i << " ";
}
std::cout << "go!\n";

Range-based for

Iterates over every element of a container or array without dealing with indices:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> nums = {10, 20, 30, 40, 50};

    for (int n : nums) {
        std::cout << n << " ";
    }
    std::cout << "\n";
}

Use const auto& to avoid copying large elements:

std::vector<std::string> words = {"alpha", "beta", "gamma"};

for (const auto& w : words) {
    std::cout << w << "\n";
}

Use auto& (without const) when you need to modify the elements in place:

for (auto& n : nums) n *= 2;

while loop

Checks the condition before each iteration:

#include <iostream>

int main() {
    int n = 1;
    while (n <= 5) {
        std::cout << n << " ";
        n++;
    }
    std::cout << "\n";   // 1 2 3 4 5
}

do-while loop

Checks the condition after each iteration — the body always runs at least once:

#include <iostream>

int main() {
    int input;
    do {
        std::cout << "Enter a positive number: ";
        std::cin >> input;
    } while (input <= 0);

    std::cout << "Got: " << input << "\n";
}

break and continue

break exits the innermost loop immediately. continue skips the rest of the current iteration and jumps to the next:

#include <iostream>

int main() {
    for (int i = 0; i < 10; i++) {
        if (i == 7) break;         // stop at 7
        if (i % 2 == 0) continue;  // skip even numbers
        std::cout << i << " ";
    }
    std::cout << "\n";   // 1 3 5
}

Nested loops and labels

C++ has no loop labels, but you can use a flag variable or a goto to break out of nested loops:

bool found = false;
for (int i = 0; i < 5 && !found; i++) {
    for (int j = 0; j < 5; j++) {
        if (i * j == 6) { found = true; break; }
    }
}

Or restructure the inner loop into a function and use return.

Loop variable lifetime

Variables declared inside a for initializer are scoped to the loop:

for (int i = 0; i < 3; i++) { /* i exists here */ }
// i does not exist here