C++ while and do while Loop

Use while when you repeat until a condition changes, and you do not know the count.

#include <iostream>

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

do while: at least one time

#include <iostream>

int main() {
    int i = 10;
    do {
        std::cout << "Runs once, i = " << i << '\n';
        i++;
    } while (i < 5);

    while (i < 5) {
        std::cout << "never printed\n";
    }
    std::cout << "done\n";
}

Digits of a number

#include <iostream>

int main() {
    int number = 9475, sum = 0;
    while (number > 0) {
        sum += number % 10;
        number /= 10;
    }
    std::cout << "Sum of digits: " << sum << '\n';
}

Ask until the input is valid

#include <iostream>
#include <limits>

int main() {
    int age = -1;
    while (age < 0 || age > 120) {
        std::cout << "Age: ";
        if (!(std::cin >> age)) {
            std::cin.clear();                                                  // reset the fail state
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // throw away the bad line
            age = -1;
        }
    }
    std::cout << "\nSaved: " << age << '\n';
}

for or while?

Loop	Use when
for	You count, or you know the number of rounds
while	You repeat until a condition is false
do while	The body must run at least once (menus, input)

Infinite loops

while (true) {
    // runs for ever: use break to leave
}

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