Digital Root without loops Python

Question

def digit_sum(n):
    \'\'\'(int)->number
    Returns the sum of all the digits in the given integer, n\'\'\'
    if n<10:
        return n
    return n%10 + digit         


        

Answer 1

Wikipedia lists a simple O(1) formula for the digital root:

def digit_root(n): 
    return (n - 1) % 9 + 1

This does not take into account an input less than 1, so you can modify as follows, with the assumption that the input is a whole number:

def digit_root(n): 
    return (n - 1) % 9 + 1 if n else 0

Examples:

>>> digit_root(1)
1
>>> digit_root(11)
2
>>> digit_root(235)
1