C program to find sum of digits using recursion

Write a recursive function in C programming to calculate sum of digits of a number. How to calculate sum of digits of a given number using recursion in C program. Logic to find sum of digits using recursion in C programming.

Input number: 1234

Sum of digits: 10

Required knowledge

Basic C programming, If statement, Functions, Recursion

Learn more – Progrma to find sum of digits using loop.

Declare recursive function to find sum of digits of a number

1. First give a meaningful name to the function, say sumOfDigits().
2. Next the function takes an integer as input, hence change the function declaration to sumOfDigits(int num);.
3. The function returns an integer i.e. sum of digits. Therefore return type of function should be int.

Recursive function declaration to find sum of digits of a number is – int sumOfDigits(int num);

Logic to find sum of digits using recursion

Finding sum of digits includes three basic steps:

1. Find last digit of number using modular division by 10.
2. Add the last digit found above to sum variable.
3. Remove last digit from given number by dividing it by 10.

Repeat the above three steps till the number becomes 0. Below are the conditions used to convert above iterative approach to recursive approach:
sum(0) = 0 {Base condition}
sum(num) = num%10 + sum(num/10)

Program to find sum of digits using recursion

/**
 * C program to calculate sum of digits using recursion
 */
 
#include <stdio.h>

/* Function declaration */
int sumOfDigits(int num);


int main()
{
    int num, sum;
    
    printf("Enter any number to find sum of digits: ");
    scanf("%d", &num);
    
    sum = sumOfDigits(num);
    
    printf("Sum of digits of %d = %d", num, sum);
    
    return 0;
}


/**
 * Recursive function to find sum of digits of a number
 */
int sumOfDigits(int num)
{
    // Base condition
    if(num == 0)
        return 0;
        
    return ((num % 10) + sumOfDigits(num / 10));
}

Enter any number to find sum of digits: 1234
Sum of digits of 1234 = 10

Happy coding 😉