C program to find sum of first and last digit of any number

Write a C program to input a number and find sum of first and last digit of the number using for loop. How to find sum of first and last digit of a number in C programming using loop. Logic to find sum of first and last digit of a number without using loop in C program.

Input number: 12345

Sum of first and last digit: 6

Required knowledge

Basic C programming, While loop, For loop

Logic to find sum of first and last digit using loop

Step by step descriptive logic to find sum of first and last digit using loop.

1. Input a number from user. Store it in some variable say num.
2. To find last digit of given number we modulo divide the given number by 10. Which is lastDigit = num % 10.
3. To find first digit we divide the given number by 10 till num is greater than 0.
4. Finally calculate sum of first and last digit i.e. sum = firstDigit + lastDigit.

Program to find sum of first and last digit using loop

/**
 * C program to find sum of first and last digit of a number using loop
 */

#include <stdio.h>

int main()
{
    int num, sum=0, firstDigit, lastDigit;

    /* Input a number from user */
    printf("Enter any number to find sum of first and last digit: ");
    scanf("%d", &num);
    
    /* Find last digit to sum */
    lastDigit = num % 10;
    

    /* Copy num to first digit */
    firstDigit = num;

    /* Find the first digit by dividing num by 10 until first digit is left */
    while(num >= 10)
    {
        num = num / 10;
    }
    firstDigit = num;


    /* Find sum of first and last digit*/
    sum = firstDigit + lastDigit; 

    printf("Sum of first and last digit = %d", sum);

    return 0;
}

Program to find sum of first and last digit without using loop

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

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

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

    sum = 0;

    /* Input a number from user */
    printf("Enter any number to find sum of first and last digit: ");
    scanf("%d", &num);
    
    /* Find last digit */
    lastDigit  = num % 10;


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

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

    /* Calculate the sum */
    sum = firstDigit + lastDigit; 

    printf("Sum of first and last digit = %d", sum);

    return 0;
}

In the above program I have used two mathematical function pow() and log10(). Both the function are present in math.h header file.

• pow() function is used to find power of two number.
• log10() function is used find log base 10 value of a number. We can use the result of log10() to find total digits count of a number – 1.

Enter any number to find sum of first and last digit: 12345
Sum of first and last digit = 6

Happy coding 😉