🔄 Mastering While Loops: From Simple to Moderate Problems (C Examples)

 

At first glance, it may look similar with other loops, but the while loop shines when:

1. You don’t know in advance how many times a loop should run.

2. You want to keep looping until a condition becomes false.

3. You’re handling input, interactive menus, or problems where the end depends on data (not just a fixed count).

Let’s start from simple problems and gradually move to moderate ones, all in C, with code and walkthroughs.

🔹 1. Basics of While Loop

The structure of a while loop looks like this:

while(condition) {
    // code block
}

👉 The loop keeps executing as long as the condition is true.

👉 Be careful: if the condition never becomes false, you’ll create an infinite loop.

🔹 2. Print Numbers from 1 to 5

Code:

#include <stdio.h>
int main() {
    int i = 1;
    while(i <= 5) {
        printf("%d ", i);
        i++;
    }
    return 0;
}

Walkthrough:

• Start with i = 1.

• Condition: i <= 5 → true → print i.

• Increment i.

• Loop stops when i = 6.

👉 Output: 1 2 3 4 5

🔹 3. Sum of First N Natural Numbers

Code:

#include <stdio.h>
int main() {
    int n = 5, sum = 0, i = 1;
    while(i <= n) {
        sum += i;  // add i to sum
        i++;
    }
    printf("Sum = %d", sum);
    return 0;
}

👉 For n = 5, calculation is 1+2+3+4+5 = 15.

Output: Sum = 15

🔹 4. Reverse Digits of a Number

This is a classic use of while loop because we don’t know the number of digits beforehand.

Code:

#include <stdio.h>
int main() {
    int num = 1234, rev = 0;
    while(num > 0) {
        int digit = num % 10;      // extract last digit
        rev = rev * 10 + digit;    // build reverse number
        num = num / 10;            // remove last digit
    }
    printf("Reversed = %d", rev);
    return 0;
}

Walkthrough:

• Start: num = 1234.

• Iteration 1 → digit = 4 → rev = 4 → num = 123.

• Iteration 2 → digit = 3 → rev = 43 → num = 12.

• Iteration 3 → digit = 2 → rev = 432 → num = 1.

• Iteration 4 → digit = 1 → rev = 4321 → num = 0 → stop.

👉 Output: Reversed = 4321

🔹 5. Factorial of a Number

Code:

#include <stdio.h>
int main() {
    int n = 5, fact = 1;
    while(n > 0) {
        fact *= n;
        n--;
    }
    printf("Factorial = %d", fact);
    return 0;
}

👉 Calculation: 5*4*3*2*1 = 120.

Output:Factorial = 120

🔹 6. Check for Palindrome Number (Moderate)

A palindrome number reads the same backward and forward (like 121, 1331).

Code:

#include <stdio.h>
int main() {
    int num = 1331, temp = num, rev = 0;
    while(num > 0) {
        int digit = num % 10;
        rev = rev * 10 + digit;
        num /= 10;
    }
    if(temp == rev)
        printf("Palindrome Number");
    else
        printf("Not a Palindrome");
    return 0;
}

👉 Output: Palindrome Number

🔹 7. Menu-Driven Program (Moderate)

While loops are great for interactive programs like menus.

Code:

#include <stdio.h>
int main() {
    int choice;
    while(1) {
        printf("\n--- MENU ---\n");
        printf("1. Print Hello\n");
        printf("2. Print Bye\n");
        printf("3. Exit\n");
        printf("Enter choice: ");
        scanf("%d", &choice);

        if(choice == 1)
            printf("Hello!\n");
        else if(choice == 2)
            printf("Bye!\n");
        else if(choice == 3)
            break;   // exit the loop
        else
            printf("Invalid choice!\n");
    }
    return 0;
}

👉 Example Run:

--- MENU ---
1. Print Hello
2. Print Bye
3. Exit
Enter choice: 1
Hello!

Enter choice: 2
Bye!

Enter choice: 3
(exit program)

🎯 Key Takeaways

• While loop keeps running until condition becomes false.

• Useful when you don’t know how many times to loop in advance.

• Great for digit problems, condition-based tasks, and menus.

• Common pitfalls: forgetting to update variable → infinite loop.

⚡ Challenge for You!

Try to solve this using a while loop:

👉 Write a program to find the sum of digits of a given number.

Example: Input: 987

Output: 24   (9+8+7)

If you solve this, you’ll have nailed the essentials and moderate use-cases of while loops in C 🎉