Write a Program to Reverse a Given Number in C

October 16, 2024

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Write a Program to Reverse a Given Number

Reversing a number is a common problem in programming and is often used as a building block for more complex algorithms. Whether you're preparing for a coding interview or sharpening your C programming skills, this exercise will help you improve your understanding of number manipulation and loops. In this blog post, we’ll show you how to write a C program to reverse a given number, explain the logic behind it, and break down the code in detail.

Why Reverse a Number?

Reversing a number has many practical applications, especially in competitive programming and algorithms. Here are a few scenarios where reversing a number might be useful:

Palindromes: Checking whether a number is a palindrome involves reversing the number and comparing it to the original.
Number Manipulation: Reversing digits can be useful in problems where you need to transform numbers.
String-like Operations: In some cases, you may need to treat numbers as sequences of digits, similar to string manipulations.

1. Logic Behind Reversing a Number

Reversing a number is straightforward. You extract the digits one by one from the given number and rebuild a new number by appending those digits in reverse order. The steps to reverse a number are as follows:

Initialize a variable to store the reversed number (starting with 0).
Extract the last digit of the original number using the modulus operator (%).
Append the extracted digit to the reversed number by multiplying the reversed number by 10 and adding the digit.
Remove the last digit from the original number by dividing it by 10.
Repeat steps 2 to 4 until the original number becomes 0.

2. Writing the Program to Reverse a Number

Let’s now implement the logic in C. Below is the C program to reverse a given number.

Code Example: Reverse a Number in C

#include <stdio.h>

int main() {
    int num, reversed = 0, remainder;

    // Ask the user to enter a number
    printf("Enter an integer: ");
    scanf("%d", &num);

    // Reverse the given number
    while (num != 0) {
        remainder = num % 10;  // Get the last digit
        reversed = reversed * 10 + remainder;  // Append the digit to the reversed number
        num /= 10;  // Remove the last digit from the original number
    }

    // Output the reversed number
    printf("Reversed number: %d\n", reversed);

    return 0;
}

Explanation of the Program

In this program, we follow the basic algorithm described above to reverse a number:

Variables: We declare three variables:
- num: the original number entered by the user.
- reversed: stores the reversed number, initialized to 0.
- remainder: holds the last digit of the number during each iteration.
Loop: The while loop runs as long as num is not 0. For each iteration:
- The last digit of num is extracted using the modulus operator (%).
- The extracted digit is added to reversed by multiplying reversed by 10 and adding the digit.
- num is divided by 10 to remove the last digit and move to the next one.

3. Example Output

Here’s an example of how the program works:

Enter an integer: 12345
Reversed number: 54321

In this example, the user inputs the number 12345, and the program successfully reverses it to 54321.

4. Handling Negative Numbers

The program above works well for positive integers. But what if the user inputs a negative number? In that case, the program will reverse the digits but ignore the negative sign. Let’s modify the program to handle negative numbers by checking if the input is negative and applying the negative sign after reversing the digits.

Code Example: Reverse a Negative Number

#include <stdio.h>

int main() {
    int num, reversed = 0, remainder;
    int isNegative = 0;

    // Ask the user to enter a number
    printf("Enter an integer: ");
    scanf("%d", &num);

    // Check if the number is negative
    if (num < 0) {
        isNegative = 1;  // Flag for negative number
        num = -num;  // Convert to positive for reversal
    }

    // Reverse the number
    while (num != 0) {
        remainder = num % 10;
        reversed = reversed * 10 + remainder;
        num /= 10;
    }

    // Restore the negative sign if necessary
    if (isNegative) {
        reversed = -reversed;
    }

    // Output the reversed number
    printf("Reversed number: %d\n", reversed);

    return 0;
}

Explanation of the Modified Program

The updated version of the program includes a check for negative numbers:

If the input is negative, we set a flag (isNegative) and convert the number to positive before reversing the digits.
After reversing, we restore the negative sign if the original number was negative.

5. Example Output for Negative Numbers

Here’s how the program works with a negative number:

Enter an integer: -6789
Reversed number: -9876

In this example, the user inputs the negative number -6789, and the program correctly reverses it to -9876.

6. Common Mistakes to Avoid

Handling 0: The reversal of 0 is still 0. Make sure the program works correctly for edge cases like 0.
Modifying the Original Number: Be careful when manipulating the original number inside the loop. Once the digits are reversed, the original number is reduced to 0.
Negative Sign Handling: When reversing negative numbers, make sure to restore the negative sign at the end.

7. Expanding the Program

After mastering the basics of reversing a number, here are some ideas to further enhance the program:

Modify the program to reverse a floating-point number.
Extend the program to handle large numbers using arrays or strings.
Combine the reversal logic with other operations, such as checking for palindromes.

8. Conclusion

In this post, we’ve walked through how to write a C program to reverse a given number. We explored both positive and negative numbers, explained the logic behind the reversal process, and discussed common pitfalls to avoid. Reversing a number is a useful skill that can help you build more complex algorithms in the future.

We encourage you to try writing your own number-reversal program, experiment with edge cases, and explore more advanced techniques. Happy coding!