While Loop

Imagine you’re asked to write a program that prints “Hello!” 1000 times. Surely you wouldn’t write std::cout << "Hello!" a thousand lines? Of course not! This is where loops come to the rescue.

Analogy: Keep Eating Until You’re Full

Try to imagine this situation at the school cafeteria:

While stomach is not full:
    Take a bite of rice
    Chew and swallow

You don’t know exactly how many bites you’ll need — it could be 10, it could be 20. The point is, you keep eating as long as you’re not full. Once you’re full, you stop. This is exactly how a while loop works!

Other examples you often experience:

• While homework isn’t done, keep working on it
• While phone isn’t fully charged, keep charging
• While you haven’t reached school, keep walking

While Loop Syntax

while (condition) {
    // code that repeats as long as condition is true
}

The flow:

1. Check the condition inside the parentheses ()
2. If true — execute the code inside {}
3. Go back to step 1 (check the condition again)
4. If false — exit the loop, continue to the line after }

         +----------------+
         | Check condition |<------+
         +--------+-------+       |
           true/    \false        |
               /      \          |
      +-------+    (exit)        |
      | Code  |                  |
      | block |------------------+
      +-------+

Example 1: Countdown from 10

Like a rocket launch countdown!

#include <iostream>

int main() {
    int count = 10;

    std::cout << "Countdown started!" << std::endl;

    while (count > 0) {
        std::cout << count << "..." << std::endl;
        count = count - 1;  // subtract 1 each time
    }

    std::cout << "LIFTOFF! Rocket launched!" << std::endl;

    return 0;
}

Output:

Countdown started!
10...
9...
8...
7...
6...
5...
4...
3...
2...
1...
LIFTOFF! Rocket launched!

Notice: each iteration (cycle), the variable count decreases by 1. When count becomes 0, the condition count > 0 evaluates to false, and the loop stops.

Example 2: Printing Numbers 1 to 10

#include <iostream>

int main() {
    int i = 1;

    while (i <= 10) {
        std::cout << i << " ";
        i++;  // same as i = i + 1
    }

    std::cout << std::endl;
    std::cout << "Done! Variable i is now: " << i << std::endl;

    return 0;
}

Output:

1 2 3 4 5 6 7 8 9 10
Done! Variable i is now: 11

Why does i end up being 11? Because when i becomes 11, the condition i <= 10 evaluates to false, the loop stops, but the value of i remains 11.

Example 3: Sum of Numbers 1 to 100

Want to add up 1 + 2 + 3 + … + 100? Without a loop, you’d have to write an incredibly long addition. With a while loop:

#include <iostream>

int main() {
    int sum = 0;
    int number = 1;

    while (number <= 100) {
        sum = sum + number;  // or: sum += number;
        number++;
    }

    std::cout << "Sum of numbers 1 to 100 = " << sum << std::endl;

    return 0;
}

Output:

Sum of numbers 1 to 100 = 5050

Updating Variables: The Key to Stopping a Loop

The most important part of a while loop is changing something inside the loop so that the condition eventually becomes false. This usually involves:

• Increment: i++ or i += 1
• Decrement: i-- or i -= 1
• Other operations: n = n / 10, remaining = remaining - payment, etc.

Without updating a variable, the loop will never stop!

Danger: Infinite Loop (Never-Ending Loop)

If you forget to update the variable, or the condition never becomes false, you get an infinite loop — the program runs forever without stopping!

// DON'T TRY THIS without being ready to press Ctrl+C!
int i = 1;

while (i <= 10) {
    std::cout << i << " ";
    // Forgot i++! Variable i never changes
    // i is always 1, condition is always true
    // Loop NEVER STOPS!
}

How to avoid infinite loops:

1. Make sure there’s a variable update inside the loop
2. Make sure the update causes the condition to eventually become false
3. Check: does the variable in the condition change inside the loop?

Example 4: Counting the Number of Digits

How many digits does the number 12345 have? We can count by dividing the number by 10 repeatedly until it’s gone:

#include <iostream>

int main() {
    int number = 12345;
    int originalNumber = number;  // save the original value
    int digitCount = 0;

    while (number > 0) {
        number = number / 10;  // remove the last digit
        digitCount++;
    }

    std::cout << "The number " << originalNumber << " has "
              << digitCount << " digits." << std::endl;

    return 0;
}

Output:

The number 12345 has 5 digits.

How it works:

When number becomes 0, the condition number > 0 evaluates to false and the loop stops.

Example 5: Simple Input Validation

While loops are very useful for making sure the user enters valid data:

#include <iostream>

int main() {
    int score;

    std::cout << "Enter a score (0-100): ";
    std::cin >> score;

    while (score < 0 || score > 100) {
        std::cout << "Invalid score! Must be between 0-100." << std::endl;
        std::cout << "Enter a score again: ";
        std::cin >> score;
    }

    std::cout << "Your score: " << score << ". Thank you!" << std::endl;

    return 0;
}

Output (example interaction):

Enter a score (0-100): 150
Invalid score! Must be between 0-100.
Enter a score again: -5
Invalid score! Must be between 0-100.
Enter a score again: 85
Your score: 85. Thank you!

The program keeps asking for input as long as the entered value is invalid. Once it’s valid, the loop stops.

Example 6: Printing Even Numbers

#include <iostream>

int main() {
    int i = 2;

    std::cout << "Even numbers from 2 to 20:" << std::endl;

    while (i <= 20) {
        std::cout << i << " ";
        i += 2;  // jump by 2 each time
    }

    std::cout << std::endl;

    return 0;
}

Output:

Even numbers from 2 to 20:
2 4 6 8 10 12 14 16 18 20

Common Mistakes

1. Forgetting to update the variable (infinite loop)

// WRONG — i never changes
int i = 1;
while (i <= 5) {
    std::cout << i << " ";
    // Missing: i++;
}

// CORRECT
int i = 1;
while (i <= 5) {
    std::cout << i << " ";
    i++;
}

2. Initial condition is already false (loop never runs)

int i = 10;
while (i < 5) {
    std::cout << i << " ";  // Never executed!
    i++;
}
// Because 10 < 5 is false, the loop is skipped entirely

3. Off-by-one error (one number off)

// Want to print 1 to 5, but get 1 to 4
int i = 1;
while (i < 5) {     // Should be i <= 5
    std::cout << i << " ";
    i++;
}
// Output: 1 2 3 4 (missing 5!)

4. Semicolon after while

// WRONG — the semicolon makes the while loop "empty"
int i = 1;
while (i <= 5); {   // This empty loop = infinite loop!
    std::cout << i << " ";
    i++;
}

// CORRECT — no semicolon
int i = 1;
while (i <= 5) {
    std::cout << i << " ";
    i++;
}

Exercises

Exercise 1: Create a program that prints all odd numbers from 1 to 50 using a while loop. Hint: start from i = 1 and add 2 each iteration.

Exercise 2: Create a program that takes a positive integer, then prints that number in reverse. Example: input 12345, output 54321. Hint: use % 10 to get the last digit, and / 10 to remove it.

Exercise 3: Create a program that calculates the factorial of a number. The factorial of n (written n!) is n x (n-1) x (n-2) x … x 1. Example: 5! = 5 x 4 x 3 x 2 x 1 = 120.

Summary

Now you can make programs that repeat actions! But the while loop isn’t the only way. In the next lesson, we’ll learn about for loops — a more concise and tidy way to loop when you already know how many times to repeat.