while & do-while Loops

Where a for loop is built around a counter, while and do-while loops are built around a condition — they keep running as long as that condition stays true, with no fixed number of iterations in mind. They're the natural choice when you don't know in advance how many times you'll need to loop.

while Loop

A while loop checks its condition before every iteration, including the very first one. If the condition is false to begin with, the body never runs at all.

CPP

#include <iostream>
using namespace std;

int main() {
    int count = 0;

    while (count < 5) {
        cout << count << " ";
        count++;
    }
    cout << endl;
    // Output: 0 1 2 3 4

    return 0;
}

do-while Loop

A do-while loop checks its condition after running the body, so the body always executes at least once, even if the condition is false from the start. Notice the required semicolon after the closing while(...).

CPP

#include <iostream>
using namespace std;

int main() {
    int count = 10;

    do {
        cout << count << " ";
        count++;
    } while (count < 5); // condition is false immediately...
    cout << endl;
    // Output: 10   <- body still ran once before checking

    return 0;
}

Loop	Condition checked	Runs body at least once?
while	Before each iteration	No
do-while	After each iteration	Yes

Common Pattern: Input Validation

do-while shines when you need to do something at least once before you can even check the condition — the textbook example is asking a user for input and re-prompting until it's valid.

CPP

#include <iostream>
using namespace std;

int main() {
    int age;

    do {
        cout << "Enter your age (0-120): ";
        cin >> age;
    } while (age < 0 || age > 120);

    cout << "Thanks! You entered " << age << endl;

    return 0;
}

Common Pattern: Sentinel-Controlled Loop

A "sentinel" is a special value that signals the loop to stop. This is a classic use case for while, since you may not want the body to run at all if the very first input is already the sentinel.

CPP

#include <iostream>
using namespace std;

int main() {
    int number;
    int sum = 0;

    cout << "Enter numbers to sum (-1 to stop): " << endl;
    cin >> number;

    while (number != -1) {
        sum += number;
        cin >> number;
    }

    cout << "Total: " << sum << endl;

    return 0;
}

Warning

A while (or do-while) loop whose condition never becomes false is an infinite loop. This almost always happens because the loop body forgets to update the variable the condition depends on. Always double-check that every path through the loop body moves the condition closer to becoming false.

CPP

// Bug: 'i' is never modified, so this never stops.
int i = 0;
while (i < 10) {
    cout << i << endl;
    // forgot: i++;
}

Key Points

while checks its condition before the loop body runs, so the body may run zero times.
do-while checks its condition after the loop body runs, so the body always runs at least once.
do-while requires a trailing semicolon after while(condition);.
Sentinel loops and input validation are classic use cases for while and do-while, respectively.
Always ensure the loop body updates whatever the condition depends on, to avoid infinite loops.