While Loops — Repeat Until Done

Real-world analogy

What is it?

A while loop is a control structure that repeats a block of code as long as a given condition remains true. Before each repetition, C++ checks the condition. If true, it runs the body. If false, it skips past the loop and continues with the rest of the program.

Real-world relevance

While loops are everywhere in competitive programming. Reading input until end-of-file, searching for a value, simulating a process step by step, or repeatedly halving a number (like in binary search) all use while loops. They are your go-to tool when you do not know in advance how many times you need to repeat.

Key points

• The While Loop Syntax — A while loop looks like: while(condition) { body }. First C++ checks the condition. If it is true, it runs the body. Then it checks again. This repeats until the condition becomes false.
• Counting with While — You can count from 1 to N using a while loop: set i = 1, then while(i <= N), print i, then do i++. Each time through the loop, i grows by 1 until it passes N.
• Summing Numbers — To add numbers from 1 to N: start sum = 0 and i = 1. While i <= N, do sum += i and i++. When the loop ends, sum holds the total. For N = 5, you get 1+2+3+4+5 = 15.
• The Danger of Infinite Loops — If you forget to change the variable that the condition checks, the condition stays true forever and your program never stops. This is called an infinite loop. Always make sure something inside the loop moves you toward the condition becoming false.
• Break — Emergency Exit — The break statement immediately exits the loop, no matter what the condition says. It is like an emergency stop button. Use it when you find what you need and want to stop looping early.
• Continue — Skip This Round — The continue statement skips the rest of the current iteration and jumps back to the condition check. It is like saying 'skip this one, move to the next'. Useful for ignoring certain values.
• While vs If — An if statement checks a condition once. A while loop checks the same condition over and over. If you need to repeat something, use while. If you only need to decide once, use if.
• Reading Until a Special Value — A common CP pattern: read numbers until the user enters 0 or -1. Use while(true) with cin >> x, and break when x equals the sentinel value. This lets you process an unknown number of inputs.

Code example

#include <bits/stdc++.h>
using namespace std;

int main() {
    // Count from 1 to 5
    int i = 1;
    while (i <= 5) {
        cout << i << " ";
        i++;
    }
    cout << endl;
    // Output: 1 2 3 4 5

    // Sum from 1 to 10
    int sum = 0;
    int j = 1;
    while (j <= 10) {
        sum += j;
        j++;
    }
    cout << "Sum = " << sum << endl;
    // Output: Sum = 55

    // Read numbers until 0, print their sum
    int x, total = 0;
    cout << "Enter numbers (0 to stop): ";
    cin >> x;
    while (x != 0) {
        total += x;
        cin >> x;
    }
    cout << "Total = " << total << endl;

    return 0;
}

Line-by-line walkthrough

1. 1. We include bits/stdc++.h for all standard tools.
2. 2. int i = 1; — We start our counter at 1.
3. 3. while (i <= 5) — Before each repetition, check: is i still 5 or less?
4. 4. cout << i << " "; — Print the current value of i with a space.
5. 5. i++; — Increase i by 1. Without this line, we would loop forever!
6. 6. After i becomes 6, the condition i <= 5 is false, so the loop stops.
7. 7. For the sum example, sum starts at 0 and we keep adding j to it.
8. 8. Each time through the loop, j increases, so we add 1, then 2, then 3... up to 10.
9. 9. The sentinel example reads x first, then enters the loop only if x is not 0.
10. 10. Inside the loop, we add x to total and read the next x. When x is 0, the loop ends.

Spot the bug

#include <bits/stdc++.h>
using namespace std;

int main() {
    int i = 1;
    int sum = 0;
    while (i <= 100) {
        sum += i;
    }
    cout << "Sum = " << sum << endl;
    return 0;
}

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• C++ While Loops Explained (YouTube)
• While Loop — cppreference (cppreference.com)
• Loop Practice Problems (Codeforces)