The while Loop

Why loops?

Imagine printing the numbers 1 to 100. Writing 100 cout lines would be silly. A loop repeats a block of code for you.

while

A while loop keeps repeating its block as long as its condition is true:

#include <iostream>
using namespace std;

int main() {
    int count = 1;
    while (count <= 5) {
        cout << "Count: " << count << "\n";
        count++;
    }
    cout << "Done!\n";
    return 0;
}

Output

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Done!

Step by step:

• count starts at 1. Is 1 <= 5? Yes, so the block runs: it prints and adds 1.
• Back to the top: is 2 <= 5? Yes... and so on.
• When count becomes 6, 6 <= 5 is false, so the loop stops and the program moves on.

Think of it like this: A while loop is like eating from a bowl of chips: while there are chips left, take one. When the bowl is empty, you stop.

Common mistake: Forgetting to change the loop variable. Without count++, count stays 1 forever and the loop never ends. This is called an infinite loop.

Adding things up

Loops are often used to build up a total:

#include <iostream>
using namespace std;

int main() {
    int number = 1;
    int total = 0;
    while (number <= 100) {
        total += number;
        number++;
    }
    cout << "1 + 2 + ... + 100 = " << total << "\n";
    return 0;
}

Output

1 + 2 + ... + 100 = 5050

Reading until a stop value

A while loop is perfect when you don't know in advance how many times to repeat. Here we read numbers until the user types 0:

#include <iostream>
using namespace std;

int main() {
    int n;
    int sum = 0;
    cin >> n;
    while (n != 0) {
        sum += n;
        cin >> n;
    }
    cout << "Sum: " << sum << "\n";
    return 0;
}

Input (what the user types)

4 10 6 0

Output

Sum: 20