C Program to Reverse a Number with Example

Reversing a number might seem easy, but it’s pretty handy in programming. Think about needing to display a phone number backward for a fun app feature. You can totally do this once you know how to reverse digits. Here's how to reverse a number using C.

Prerequisites for the Reverse a Number Program in C

Make sure you know these basics before diving in:

• How to use a while loop in C
• Understanding of the % and / operators
• Basic integer operations
• Include the <stdio.h> library
• Any C compiler such as GCC 9 or newer — or just run it in the EduSeekho online compiler

Source Code of C Program to Reverse a Number

Below is the complete source code to reverse a number in C.

#include <stdio.h>

int main() {
    int n, reverse = 0, rem;
    printf("Enter a number: ");
    scanf("%d", &n);
    while (n != 0) {
        rem = n % 10;
        reverse = reverse * 10 + rem;
        n /= 10;
    }
    printf("Reversed Number: %d\n", reverse);
    return 0;
}

Output of the Reverse a Number Program

When you run this code and enter '1234', you'll see:

Enter a number: 1234
Reversed Number: 4321

Algorithm to Reverse a Number in C

Reversing a number means processing its digits from last to first. Here is the exact logic the program uses, step by step.

1. Read the integer n from the user.
2. Initialize reverse to 0.
3. Repeat while n is not zero:
4. Extract the last digit of n by n % 10.
5. Multiply reverse by 10 and add the digit.
6. Remove the last digit from n by dividing by 10.
7. Continue until the number n becomes zero.
8. Print the reversed number.

flowchart TD
  A([Start]) --> B[/Read n/]
  B --> C[reverse = 0]
  C --> D{"n != 0?"}
  D -->|Yes| E["rem = n %% 10"]
  E --> F["reverse = reverse * 10 + rem"]
  F --> G[n = n / 10]
  G --> D
  D -->|No| H([End])

How the C Program to Reverse a Number Works

Let's break down the code to see how it reverses a number.

• The program starts by including the <stdio.h> library to use the printf and scanf functions.
• The main() function begins execution. It declares integer variables n, reverse, and rem.
• The program prompts you to enter a number and reads it using scanf.
• The program enters a while loop that continues until n becomes zero.
• Inside the loop, the remainder rem is calculated using n % 10, which gets the last digit of n.
• The variable reverse is updated to store the digits in reversed order.
• The variable n is divided by 10 to drop the last digit.
• The loop repeats this process until n is zero, reversing the number.
• Finally, the reversed number is printed using printf.
• The function returns 0, indicating successful execution.

Time and Space Complexity of C Program to Reverse a Number

Let's analyze how the program performs in terms of time and space.

Time Complexity: O(d). Here, d is the number of digits in the input number n — like 4 for 1234. This is because the loop iterates once per digit, seen in n /= 10. So, 10 digits mean 10 iterations.

Space Complexity: O(1). Only three integer variables n, reverse, and rem are used, and they occupy the same space, regardless of number size.

Key takeaway:Reversing a number is just a simple loop that peels off digits from the end and puts them back in reverse order.

FAQs About Reversing a Number in C

Here are some common questions about reversing numbers in C.

Related Programs

If you found this program helpful, check out these related programs:

• C Program to Count Number of Digits in an Integer
• C Program to Check Whether a Number is Palindrome or Not
• C Program to Find Factorial of a Number
• C Program to Display Fibonacci Sequence with Example
• C Program to Add Two Integers with Explanation and Output

Conclusion

Reversing a number is a useful trick you'll often need in programming. Once you get the basic logic with loops, you’ll see it pop up in coding challenges and real-world scenarios. Dive deeper into more complex number manipulations to enhance your skills.

Check out more number-based coding challenges in C on our platform. Test yourself with our C language quiz and use the C online compiler to play around with your code.