C++ While Loop

Welcome to The Coding College! In this tutorial, we’ll dive into the while loop in C++. Loops are essential in programming for handling repetitive tasks, and the while loop is one of the simplest yet powerful looping constructs.

What is a While Loop?

A while loop is used to repeatedly execute a block of code as long as a specified condition is true. It’s useful when you don’t know the number of iterations in advance and need to rely on a condition to control the loop.

Syntax

while (condition) {
    // Code to execute while the condition is true
}

• condition: A Boolean expression that determines whether the loop continues. If the condition is false, the loop exits.
• The code inside the loop is called the loop body.

Example: Simple While Loop

Let’s print numbers from 1 to 5 using a while loop:

#include <iostream>
using namespace std;

int main() {
    int i = 1;

    while (i <= 5) {
        cout << i << endl;
        i++;
    }

    return 0;
}

Output:

1  
2  
3  
4  
5  

Example: Sum of First N Natural Numbers

#include <iostream>
using namespace std;

int main() {
    int n, sum = 0, i = 1;

    cout << "Enter a positive integer: ";
    cin >> n;

    while (i <= n) {
        sum += i;
        i++;
    }

    cout << "The sum of the first " << n << " numbers is " << sum << "." << endl;

    return 0;
}

Output Example:

Enter a positive integer: 5  
The sum of the first 5 numbers is 15.

Example: Input Validation

A while loop is often used to validate user input.

#include <iostream>
using namespace std;

int main() {
    int number;

    cout << "Enter a number greater than 0: ";
    cin >> number;

    while (number <= 0) {
        cout << "Invalid input. Try again: ";
        cin >> number;
    }

    cout << "You entered: " << number << endl;

    return 0;
}

Output Example:

Enter a number greater than 0: -3  
Invalid input. Try again: 5  
You entered: 5

Infinite Loop

A while loop can run infinitely if the condition never becomes false.

#include <iostream>
using namespace std;

int main() {
    while (true) {
        cout << "This is an infinite loop!" << endl;
    }

    return 0;
}

Note: Infinite loops should generally be avoided unless they are intentionally used (e.g., for servers or event listeners).

Breaking Out of a While Loop

You can use the break statement to exit a while loop when needed.

Example: Break Condition

#include <iostream>
using namespace std;

int main() {
    int i = 1;

    while (true) {
        if (i > 5) {
            break;
        }

        cout << i << endl;
        i++;
    }

    return 0;
}

Output:

1  
2  
3  
4  
5  

Example: Find the Smallest Divisor

#include <iostream>
using namespace std;

int main() {
    int n, divisor = 2;

    cout << "Enter a number greater than 1: ";
    cin >> n;

    while (divisor <= n) {
        if (n % divisor == 0) {
            cout << "The smallest divisor of " << n << " is " << divisor << "." << endl;
            break;
        }
        divisor++;
    }

    return 0;
}

Output Example:

Enter a number greater than 1: 15  
The smallest divisor of 15 is 3.

Common Mistakes with While Loops

• Infinite Loops: Forgetting to update the condition inside the loop.

int i = 1;
while (i <= 5) {
    cout << i << endl; // Infinite loop: i is never incremented
}

• Off-by-One Errors: Ensure the condition correctly defines the loop bounds.
• Unreachable Code: Placing statements after an infinite loop without a break.

Summary

• While loops repeat a block of code as long as the condition is true.
• Use them when you don’t know the exact number of iterations in advance.
• Combine them with conditions and the break statement for more control.

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What’s Next?

• Learn about the do…while loop, which guarantees at least one iteration.
• Explore the for loop for fixed iterations.
• Master nested loops for advanced scenarios.