Question

import sys

def keepsumming(number):
    numberlist = []
    for digit in str(number):
        numberlist.append(int(digit))
    total = reduce(add, numberlist)
    if total > 9:
        keepsumming(total)
    if total <= 9:
        return total

def add(x,y):
    return x+y

keepsumming(sys.argv[1])

I want to create a function that adds the individual digits of any number, and to keep summing digits until the result is only one digit. (e.g. 1048576 = 1+0+4+8+5+7+6 = 31 = 3+1 = 4). The function seems to work in some laces but not in others. for example:

$python csp39.py 29

returns None, but:

$python csp39.py 30

returns 3, as it should…

Any help would be appreciated!

Answer 1

As others have mentioned, the problem seems to be with the part

if total > 9:
    keepsumming(total)  # you need return here!

Just for completeness, I want to present you some examples how this task could be solved a bit more elegantly (if you are interested). The first also uses strings:

while number >= 10:
  number = sum(int(c) for c in str(number))

The second uses modulo so that no string operations are needed at all (which should be quite a lot faster):

while number >= 10:
  total = 0
  while number:
    number, digit = divmod(number, 10)
    total += digit
  number = total

You can also use an iterator if you want to do different things with the digits:

def digits(number, base = 10):
  while number:
    yield number % base
    number //= base

number = 12345

# sum digits
print sum(digits(number))
# multiply digits
from operator import mul
print reduce(mul, digits(number), 1)

This last one is very nice and idiomatic Python, IMHO. You can use it to implement your original function:

def keepsumming(number, base = 10):
  if number < base:
    return number
  return keepsumming(sum(digits(number, base)), base)

Or iteratively:

def keepsumming(number, base = 10):
  while number >= base:
    number = sum(digits(number, base))

UPDATE: Thanks to Karl Knechtel for the hint that this actually is a very trivial problem. It can be solved in one line if the underlying mathematics are exploited properly:

def keepsumming(number, base = 10):
  return 1 + (number - 1) % (b - 1)