Loops

Loops repeat code. C++ has while and do/while for "repeat until", the classic counting for loop, and the range-based for loop — the everyday way to visit every element of a container.

while and do-while

C++ 
#include <iostream>

int main() {
    double balance = 1000.0;
    int years = 0;
    while (balance < 2000.0) {
        balance *= 1.05;
        ++years;
    }
    std::cout << "Doubled after " << years << " years\n";

    int n = 10;
    do {
        std::cout << "runs at least once (n = " << n << ")\n";
    } while (n < 5);
}
Output
Doubled after 15 years
runs at least once (n = 10)

while checks before each pass; do/while checks after, so its body always runs at least once.

The counting for loop

C++ 
#include <iostream>

int main() {
    for (int i = 1; i <= 5; ++i) {
        std::cout << i << " squared is " << i * i << '\n';
    }
    for (int i = 10; i > 0; i -= 3) std::cout << i << ' ';
    std::cout << '\n';
}
Output
1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25
10 7 4 1

The three parts are the start, the condition checked before each pass, and the step after each pass.

Range-based for

To visit every element of a container, the range-based for loop is clearer and can't go out of bounds:

C++ 
#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<double> prices{3.50, 12.00, 7.25};
    double total = 0;
    for (double p : prices) total += p;
    std::cout << "total " << total << '\n';

    for (char c : std::string{"abc"}) std::cout << c << '-';
    std::cout << '\n';

    for (int n : {2, 3, 5, 7}) std::cout << n << ' ';
    std::cout << '\n';
}
Output
total 22.75
a-b-c-
2 3 5 7

Copies, references and const references

The loop variable is a copy of each element by default, so changing it changes nothing in the container. The compiler even notices:

C++ 
#include <vector>

int main() {
    std::vector<int> scores{70, 85, 90};
    for (int s : scores) s += 5;            // changes copies: no effect
    return scores[0];
}
Compiler warnings
main.cpp:5:14: warning: variable 's' set but not used [-Wunused-but-set-variable]

Use a reference (&) to change elements in place, and a const reference to avoid copying large elements you only read:

C++ 
#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<int> scores{70, 85, 90};
    for (int& s : scores) s += 5;           // changes the elements
    for (int s : scores) std::cout << s << ' ';
    std::cout << '\n';

    std::vector<std::string> names{"Ada", "Grace", "Linus"};
    for (const auto& name : names) {        // no copies, read-only
        std::cout << name << " (" << name.size() << ")\n";
    }
}
Output
75 90 95
Ada (3)
Grace (5)
Linus (5)

const auto& is the right default for reading containers of anything bigger than a number.

break and continue

C++ 
#include <iostream>
#include <vector>

int main() {
    std::vector<int> readings{21, -999, 23, 22, -999, 25, 99, 24};
    int sum = 0, used = 0;
    for (int r : readings) {
        if (r == -999) continue;       // skip sensor errors
        if (r > 50) break;             // stop at the first impossible value
        sum += r;
        ++used;
    }
    std::cout << used << " readings, sum " << sum << '\n';
}
Output
4 readings, sum 91

Nested loops

C++ 
#include <iomanip>
#include <iostream>

int main() {
    for (int row = 1; row <= 4; ++row) {
        for (int col = 1; col <= 4; ++col) {
            std::cout << std::setw(4) << row * col;
        }
        std::cout << '\n';
    }
}
Output
   1   2   3   4
   2   4   6   8
   3   6   9  12
   4   8  12  16

To leave several nested loops at once, put them in a function and return.

Looping with an index

When you need the position as well as the element, a counting loop over size() works. Use std::size_t (unsigned) to match the container's size type, which avoids sign-comparison warnings:

C++ 
#include <iostream>
#include <string>
#include <vector>

int main() {
    std::vector<std::string> podium{"Ana", "Ben", "Chloe"};
    for (std::size_t i = 0; i < podium.size(); ++i) {
        std::cout << i + 1 << ". " << podium[i] << '\n';
    }
}
Output
1. Ana
2. Ben
3. Chloe

Try it yourself

Print the prime numbers below 50.

Try it yourself · main.cpp
Edit the code, copy it into main.cpp, then compile and run it with c++ -std=c++20 -Wall main.cpp -o main && ./main. How to set up a C++ compiler
Output
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47
Previousif and switch