Sum the digits of a number - python


Question

If I want to find the sum of the digits of a number, i.e. :

  • Input: 932
  • Output: 14, which is (9 + 3 + 2)

What is the fastest way of doing this?

I instinctively did:

sum(int(digit) for digit in str(number))

and I found this online:

sum(map(int, str(number)))

Which is best to use for speed, and are there any other methods which are even faster?

1
61
1/6/2017 5:44:48 PM

Accepted Answer

You can do it purely in integers, and it will be the most efficient:

def sum_digits(n):
    s = 0
    while n:
        s += n % 10
        n //= 10
    return s

or with divmod:

def sum_digits2(n):
    s = 0
    while n:
        n, remainder = divmod(n, 10)
        s += remainder
    return s

But both lines you posted are fine.

 

Even faster is the version without augmented assignments:

def sum_digits3(n):
   r = 0
   while n:
       r, n = r + n % 10, n // 10
   return r

 

> %timeit sum_digits(n)
1000000 loops, best of 3: 574 ns per loop

> %timeit sum_digits2(n)
1000000 loops, best of 3: 716 ns per loop

> %timeit sum_digits3(n)
1000000 loops, best of 3: 479 ns per loop

> %timeit sum(map(int, str(n)))
1000000 loops, best of 3: 1.42 us per loop

> %timeit sum([int(digit) for digit in str(n)])
100000 loops, best of 3: 1.52 us per loop

> %timeit sum(int(digit) for digit in str(n))
100000 loops, best of 3: 2.04 us per loop
95
1/7/2016 5:32:31 PM

If you want to keep summing the digits until you get a single-digit number (one of my favorite characteristics of numbers divisible by 9) you can do:

def digital_root(n):
    x = sum(int(digit) for digit in str(n))
    if x < 10:
        return x
    else:
        return digital_root(x)

Which actually turns out to be pretty fast itself...

%timeit digital_root(12312658419614961365)

10000 loops, best of 3: 22.6 ┬Ás per loop

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