C Program to Reverse a Number

How to Write a C Program to Reverse a Number using While Loop, Functions, and Recursion? For this, first, find the remainder by dividing the number by 10.

Here, our task is simple: if the user enters 1234, the program must return 4321. To reverse a number, divide the original value by 10 to extract the last digit of the number. Here, the modulo operator (%) extracts the last digit. Next, the division operator (/) performs integer division and removes the last digit.

C program to reverse a number using a while loop

The below program allows the user to enter any positive integer. Next, this program use the while loop to traverse the digit sin a number and will reverse a number.

#include <stdio.h>

int main(void)
{
    int n, rd, rs = 0;
    
    printf("Please Enter any Number to Reverse = ");
    scanf("%d", & n);
    
    while (n > 0)
    {
        rd = n %10;
        rs = rs * 10+ rd;
        n = n /10;
    }
    
    printf(“Reverse of entered Number is = %d\n", rs);
    
    return 0;
}

This program allows the user to enter any positive integer, and then that number is assigned to a variable. Next, Condition in the C While Loop will make sure that the given number is greater than 0.

From the above Program to Reverse a Number example, User Entered value: n = 1456 and rs = 0. Refer to Arithmetic Operators in C.

First Iteration
rd = n %10 = 1456%10 = 6

rs = rs *10 + rd
rs = 0 * 10 + 6 => 0 + 6 => 6

n = n /10 = 1456 /10 => 145

Second Iteration: From the first Iteration of this C Program, the values of both have changed as Number =145 and Reverse = 6

rd = 145 % 10 = 5

rs = 6 * 10 + 5 => 60 + 5  => 65

n = 145 /10 => 14

Third Iteration: From the Second Iteration of this C program, the values of both changed to 14 and 65

rd = 14 % 10 = 4

rs = 65 * 10 + 4 => 650 + 4 => 654

n = 14 / 10 = 1

Fourth Iteration: From the third Iteration, Number = 1 and Reverse = 654

rd = 1 %10 = 1

rs = 654 * 10 + 1 => 6540 + 1 => 6541

n = 1/ 10 => 0

Here, For the next iteration, n = 0. So, the C Programming while loop condition will fail

C program to Reverse a Number using For loop

This example uses the for loop to iterate the individual digits in a number and reverse them using the logic we used in the while loop example. Refer to the C for loop article.

#include <stdio.h>
int main(void)  {
    int n, rd, rs = 0;
    printf("Please Enter any Number = ");
    scanf("%d", &n);
    
    for(;n > 0; n = n /10){
        rd = n % 10;
        rs = rs * 10+ rd;
    }
    printf("Reverse of entered = %d\n", rs);
    return 0;
}

C Program to Reverse a Number Using Functions

This program allows the user to enter any positive integer. Then, this program reverses a number using Functions. Check out Functions in C.

#include <stdio.h>

int rev_Integer (int);

int main(void)
{
    int Number, rev = 0;
    
    printf("Please Enter any number = ");
    scanf("%d", &Number);
    
    rev = rev_Integer (Number);
    
    printf("Reverse = %d\n", rev);
    
    return 0;
}

int rev_Integer (int Number)
{
    int Reminder, rev = 0;
    
    while (Number > 0)
    {
        Reminder = Number %10;
        rev = rev *10+ Reminder;
        Number = Number /10;
    }
    
    return rev;
}

Within this, When the compiler reaches the rev_Integer (Number) line in the main() program, the compiler will immediately jump to the below function:

int rev_Integer (int Number)

We already explained the code LOGIC in the above example. Please refer to the While Loop Analysis. The last line ends with a return Reverse statement.

C Program to Reverse a Number Using Recursion

This c program allows us to enter any positive integer. Then, it will reverse a number using Recursion. Refer to the Recursion in C.

#include <stdio.h>

int Rev_Integer (int );

int main(void)
{
    int n, rev = 0;
    
    printf("\nPlease Enter any value\n");
    scanf("%d", &n);
    
    rev = Rev_Integer (n);
    
    printf("Reverse= %d\n", rev);
    
    return 0;
}

int Rev_Integer (int n)
{
    static int Rem, rev = 0;
    
    if (n > 0)
    {
        Rem = n %10;
        rev = rev *10+ Rem;
        Rev_Integer (n /10);
    }
    return rev;
}

In this program, When the compiler reaches the Rev_Integer (n) line in the main() program, the compiler will immediately jump to the below function:

int Rev_Integer (int n)

In this function, the Rev_Integer (n / 10); statement will help to call the function Recursively with the updated value. If you miss this statement, it will terminate after completing the first line. For example, if Number = 459, then the output will be 9

Let’s see the If condition.

if (n > 0), the code will check whether the number is greater than 0. For Recursive functions, placing a condition before using the function recursively is crucial. Otherwise, we end up in infinite execution (Same as Infinite Loop).

Reverse number program in C using string

In the following example, we used sprintf () to convert the given integer to a string. Next, the built-in strrev() function reverses the string number. If you convert an integer to a string, there are multiple options. Please refer to the C string reverse article and the C strrev function.

#include <stdio.h>
#include <string.h>
int main()
{
    int n;
    printf("Enter any Number = ");
    scanf("%d", &n);

    char str[100];
    sprintf(str, "%d", n);
    printf("Reversed number = %s", strrev(str));
}

Enter any Number = 1897
Reversed number = 7981

Also Read

• C example to Reverse a Number using Recursive Function
• C Program to Swap Two Numbers