Fastest method for adding/summing the individual digit components of a number

Question

I saw a question on a math forum a while back where a person was discussing adding up the digits in a number over and over again until a single digit is achieved. (i.e. \"362\"

Answer 1

For short code, try this:

int digit_sum(int n){
    if (n<10) return n;
    return n%10 + digit_sum(n/10);
}

Or, in words,

-If the number is less than ten, then the digit sum is the number itself.

-Otherwise, the digit sum is the current last digit (a.k.a. n mod10 or n%10), plus the digit sum of everything to the left of that number (n divided by 10, using integer division).

-This algorithm can also be generalized for any base, substituting the base in for 10.