C++ while and do-while Loops: Conditions, Input Validation and Sentinels

A for loop is ideal for counting. A while loop is ideal when you repeat until something happens: a balance reaches a target, a number runs out of digits, or the user types a valid answer.

while: check first, then run#

#include <iostream>

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

3...
2...
1...
Lift off!

The condition is tested before each repetition. If it is false at the start, the body runs zero times.

Processing digits#

Many number problems peel off the last digit with % 10 and remove it with / 10, until nothing is left. You do not know in advance how many digits there are, so while fits:

#include <iostream>

int main() {
    int n = 50721;
    int digit_sum = 0;
    int reversed = 0;

    while (n > 0) {
        int digit = n % 10;
        digit_sum += digit;
        reversed = reversed * 10 + digit;
        n /= 10;
    }

    std::cout << "Digit sum: " << digit_sum << '\n';
    std::cout << "Reversed: " << reversed << '\n';
}

Digit sum: 15
Reversed: 12705

Sentinel values#

A sentinel is a special value that means "stop". For example, read prices until the user types 0:

#include <iostream>

int main() {
    double total = 0;
    double price = 0;

    std::cout << "Enter prices (0 to finish):\n";
    std::cin >> price;
    while (price != 0) {
        total += price;
        std::cin >> price;
    }
    std::cout << "Total: " << total << '\n';
}

Enter prices (0 to finish):
120
80
45
0
Total: 245

Notice the pattern: read once before the loop, then read again at the end of each repetition, so the sentinel is never added to the total.

do-while: run first, then check#

The body of a do ... while always runs at least once, which is exactly what menus need: show the menu, then decide whether to show it again.

#include <iostream>

int main() {
    int choice = 0;
    int counter = 0;

    do {
        std::cout << "1) Add one  2) Quit: ";
        std::cin >> choice;
        if (choice == 1) {
            ++counter;
            std::cout << "Counter is " << counter << '\n';
        }
    } while (choice != 2);

    std::cout << "Final value: " << counter << '\n';
}

1) Add one  2) Quit: 1
Counter is 1
1) Add one  2) Quit: 1
Counter is 2
1) Add one  2) Quit: 2
Final value: 2

Robust input validation#

When a user types text where a number is expected, std::cin enters a fail state and stops reading. To recover and ask again you must:

1. std::cin.clear() to reset the fail state;
2. std::cin.ignore(...) to throw away the bad input on that line.

#include <iostream>
#include <limits>

int main() {
    int age = 0;

    while (true) {
        std::cout << "Age: ";
        if (std::cin >> age && age > 0 && age < 130) {
            break;   // valid: leave the loop
        }
        std::cout << "Please enter a whole number between 1 and 129.\n";
        std::cin.clear();
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }
    std::cout << "Thank you. You are " << age << ".\n";
}

Age: abc
Please enter a whole number between 1 and 129.
Age: -5
Please enter a whole number between 1 and 129.
Age: 30
Thank you. You are 30.

while (true) with a break on success is a clear way to write "keep asking until it is right". See break and continue.

Avoiding infinite loops#

Every while loop needs something inside it that eventually makes the condition false. Check these three things when a loop never ends:

1. Is the loop variable updated inside the body?
2. Is it updated in the right direction (++ vs --)?
3. Can the condition actually become false? while (x != 10) with x += 3 starting from 0 skips over 10 and never stops; while (x < 10) is safer.

Common mistakes#

Practice#

#include <iostream>

int main() {
    int n = 9040017;
    int digits = 0;

    do {
        ++digits;
        n /= 10;
    } while (n != 0);

    std::cout << digits << '\n';
}

7

#include <iostream>

int main() {
    int n = 6;
    int steps = 0;

    std::cout << n;
    while (n != 1) {
        if (n % 2 == 0) {
            n /= 2;
        } else {
            n = 3 * n + 1;
        }
        std::cout << ' ' << n;
        ++steps;
    }
    std::cout << "\nSteps: " << steps << '\n';
}

6 3 10 5 16 8 4 2 1
Steps: 8

Key takeaway#