Python: Null Output Instead of Correct Value

Question

I'm trying to write a function to add the digits of a given number repeatedly until I have a single digit.

So addDigits(345) = 12, 1+2 = 3, so this should return 3

Here's my attempt:

class Solution(object):
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        if (num >= 0 and num < 10):
            return num
        else:
            total = 0
            while (num >= 10):
                total += (num % 10)
                num /= 10
            total += num
            self.addDigits(total)

On input 10, I'm getting null back and I have no idea why. I've traced through the code and it looks right to me...

You need to return the result of recursive call. return self.addDigits(total) — thefourtheye
– thefourtheye, Commented Aug 22, 2015 at 14:37

saulspatz · Accepted Answer · 2015-08-22 16:08:56Z

2

The old technique of "casting out nines" gives a quick answer

def sumOfDigits(x):
    if x == 0: return 0 
    y = x % 9
    return 9 if y == 0 else y

An example shows why this works:

433 = 4*(99+1) +3*(9+1) +2 = 4*99 + 3*9 + (4+3+3)

reducing modulo 9 gives (4+3+3) (mod 9)

As Andrea's comment below points out, this gives the wrong answer when the sum of the digits is congruent to 9.

edited Aug 22, 2015 at 16:08

answered Aug 22, 2015 at 15:53

saulspatz

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1 Comment

saulspatz Over a year ago

@Andrea Corbellini My mistake. Let me see if I can correct it.

veggie1 · Accepted Answer · 2015-08-22 16:02:15Z

1

Here is one approach that splits your number into individual digits, and continuously sums them until you end up with a single digit.

total = 12348736

while len(str(total)) > 1:
    total = sum(map(int,str(total)))

print total

EDIT: Just to explain this further:

str(12348736): turns your number into a string
map(int, '12348736'): turns your string (list of characters) into a list of integers (applies int to every character)
sum([1, 2, 3, 4, 8, 7, 3, 6]): adds up all the digits in the list

edited Aug 22, 2015 at 16:02

answered Aug 22, 2015 at 14:48

veggie1

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1 Comment

Dan D. Over a year ago

Omit the list: sum(map(int, str(total))).

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