sum_of_digits.c

/*
 * @author: ashwek
 * @date: 8/1/2019
 */

#include <stdio.h>

int sum(int num) {
    if( num <= 0 ){
        return 0;
    }
    return (num%10) + sum(num/10);
}

void main(){

    int num;

    printf("Enter a number = ");
    scanf("%d", &num);

    printf("Sum of digits = %d\n", sum(num));

}