C++ for Loop and Range based for

A for loop repeats code a known number of times.

#include <iostream>

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

Steps, counting down and sums

#include <iostream>

int main() {
    for (int i = 0; i <= 20; i += 5) std::cout << i << ' ';
    std::cout << '\n';
    for (int i = 5; i >= 1; i--) std::cout << i << ' ';
    std::cout << '\n';

    int sum = 0;
    for (int i = 1; i <= 100; i++) sum += i;
    std::cout << "Sum 1 to 100 = " << sum << '\n';
}

Nested loops

#include <iostream>

int main() {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 5; j++) {
            std::cout << i * j << '\t';
        }
        std::cout << '\n';
    }
}

Range based for loop

The easy way to go through a container or an array:

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

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

    for (const std::string& fruit : fruits) {      // const reference: no copy
        std::cout << fruit << '\n';
    }

    std::vector<int> marks = {70, 85, 90};
    for (int& m : marks) {                         // reference: we can change the items
        m += 5;
    }
    for (auto m : marks) std::cout << m << ' ';
    std::cout << '\n';
}

Range for with an initializer (C++20)

#include <iostream>
#include <vector>

int main() {
    std::vector<char> letters = {'a', 'b', 'c'};
    for (int index = 0; char c : letters) {
        std::cout << index++ << ':' << c << ' ';
    }
    std::cout << '\n';
}

Common mistakes

• Off by one: i < 5 runs 5 times (0 to 4), i <= 5 runs 6 times.
• Comparing a signed int with size() (unsigned) gives a warning. Use std::size_t or the range for.
• Changing a container (adding items) while looping over it can break the loop.

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