The C++ `while` loop, explained

Short answer: a while loop repeats its body as long as a condition is true, checking before every pass. It's the loop for "I don't know how many times yet" — everything else is a for in disguise.

A while loop has three moving parts, and only the first is built into the syntax:

Condition — checked before each pass; false ends the loop.
Body — the work.
Update — something inside the body must move the world toward the condition becoming false. Forget this and you have an infinite loop.

Countdown: the minimal example

#include <iostream>
using namespace std;

int main() {
    int n = 5;
    while (n > 0) {
        cout << n << " ";
        n--;               // the update — remove it and loop forever
    }
    cout << endl;
    return 0;
}

Where while shines #1: chewing through digits

When you don't know how many digits a number has, a for is awkward and a while is natural. This one sums the digits of 12345 (the pattern behind our Sum of Digits practice problem):

#include <iostream>
using namespace std;

int main() {
    int n = 12345;
    int sum = 0;
    while (n > 0) {
        sum += n % 10;   // last digit
        n /= 10;         // chop it off — see why 12345/10 is 1234 in our
                         // integer division guide
    }
    cout << sum << endl; // 15
    return 0;
}

Where while shines #2: reading until a sentinel

Read numbers until the user types 0. The count is unknown — that's while territory (numbers go in the input box):

#include <iostream>
using namespace std;

int main() {
    int x, sum = 0;
    cin >> x;
    while (x != 0) {
        sum += x;
        cin >> x;        // read again INSIDE the loop
    }
    cout << "sum = " << sum << endl;
    return 0;
}

A slicker variant works directly on the read itself — cin >> x evaluates to the stream, which converts to true while reads keep succeeding, so the loop ends at end-of-input or a broken read:

#include <iostream>
using namespace std;

int main() {
    int x, sum = 0;
    while (cin >> x) {   // loop while reading succeeds
        sum += x;
    }
    cout << "sum = " << sum << endl;
    return 0;
}

Anatomy of an infinite loop

int n = 5;
while (n > 0) {
    cout << n << " ";
    // forgot n--;   → condition never becomes false
}

Three classic causes: no update, an update that skips over the exit (n-- from 5 with condition n == 0), and a read that happens outside the loop. On this site, a runaway loop freezes the browser tab (the code runs on your device) — so double-check the update line before hitting Run. A break; statement is an emergency exit, but a loop you can only leave by breaking is usually a design smell.

Common beginner bugs

Semicolon after the condition: while (n > 0); — that stray ; is an empty body; the loop either spins forever or does nothing.
Off-by-one in the condition: > vs >=. Trace one iteration by hand with a tiny input.
Updating the wrong variable — chopping copy while testing n, or vice versa.

Interview questions on while

"When do you choose while over for?" — when the iteration count is unknown in advance: sentinel reads, digit processing, waiting for a condition.
"What does while (cin >> x) mean?" — the stream converts to true while reads succeed; the loop ends at end-of-input or invalid input.
"Difference between while and do-while?" — do-while checks the condition after the body, so it always runs at least once (menus, input validation).
"How would you debug an infinite loop?" — print the loop variables each pass; the variable that never moves is your missing update.

Practice it

These problems in our practice section are while-loop territory: Count the Digits, Sum of Digits, Prime Check. And the companion piece: for vs while, side by side.

The C++ while loop, explained