C program to swap first and last digit of a number

Write a C program to input a number from user and swap first and last digit of the given number. How to swap first and last digits of a number in C programming. Logic to swap first and last digit of a number in C program.

Input any number: 12345

Number after swapping first and last digit: 52341

Required knowledge

Basic C programming, Basic Mathematics

Must know – Program to find first and last digit

Logic to swap first and last digit of a number

Logic to swap first and last digit of a number
Begin:
    read(num)
    lastDigit ← num % 10;
    digits ← log10(num);
    firstDigit ← num / pow(10, digits);
    
    swappedNum ← lastDigit * pow(10, digits);
    swappedNum ← swappedNum + num % pow(10, digits);
    swappedNum ← swappedNum - lastDigit;
    swappedNum ← swappedNum + firstDigit;
End

Let us do a dry run of the algorithm to get grip on the logic.

Suppose num = 12345
--------------------
lastDigit  = 12345 % 10 => 5
digits     = log10(12345) => 4
firstDigit = 12345 / pow (10, 4) => 12345 / 10000 => 1

swappedNum = 5 * pow(10, 4) => 5 * 10000 => 50000
swappedNum = 50000 + (12345 % 10000) => 50000 + 2345 => 52345
swappedNum = 52345 - 5 => 52340
swappedNum = 52340 + 1 => 52341

Program to swap first and last digit of a number

/**
 * C program to swap first and last digit of a number
 */

#include <stdio.h>
#include <math.h>

int main()
{
    int num, swappedNum;
    int firstDigit, lastDigit, digits;

    /* Input number from user */
    printf("Enter any number: ");
    scanf("%d", &num);

    /* Find last digit */
    lastDigit  = num % 10;

    /* Find total number of digit - 1 */
    digits     = (int)log10(num); 

    /* Find first digit */
    firstDigit = (int)(num / pow(10, digits)); 

    swappedNum  = lastDigit;
    swappedNum *= (int) pow(10, digits);
    swappedNum += num % ((int) pow(10, digits));
    swappedNum -= lastDigit;
    swappedNum += firstDigit;

    printf("Original number = %d", num);
    printf("Number after swapping first and last digit: %d", swappedNum);

    return 0;
}

Note: In some compiler the above program may not produce valid results for some inputs. You may use the below program with similar logic with a little change in the code.

Program to swap first and last digit of a number

/**
 * C program to swap first and last digit of a number
 */

#include <stdio.h>
#include <math.h>

int main()
{
    int num, swappedNum;
    int firstDigit, lastDigit, digits;

    /* Input a number from user */
    printf("Enter any number: ");
    scanf("%d", &num);

    /* Find last digit */
    lastDigit  = num % 10; 

    /* Total number of digit - 1 */
    digits     = (int) log10(num); 

    /* Find first digit */
    firstDigit = (int) (num / pow(10, digits));

    swappedNum  = lastDigit;
    swappedNum *= (int) round(pow(10, digits));
    swappedNum += num % ((int)round(pow(10, digits)));
    swappedNum -= lastDigit;
    swappedNum += firstDigit;

    printf("Original number = %d", num);
    printf("Number after swapping first and last digit: %d", swappedNum);

    return 0;
}

Note: In the above program I have used three mathematical function pow(), log10() and round().

• pow() is used to find power of a number.
• log10() is used to find log base 10 value of the passed parameter.
• round() function is used to round a number to nearest integer.

Enter any number: 1234
Original number = 1234 
Number after swapping first and last digit: 4231

Happy coding 😉