# abacus.py

# This solution by Leslie Klein as an interview practice problem on 20160421.

"""Model addition on an abacus.

Input: two lists of strings;

Output: one list of strings.

>>> abacus(['3'], ['5'])

['8']

All strings are one-character-length integers.

"""

def abacus(first=['3'], second=['5']):

"""Return a list containing the digit-by-digit sum of the input lists."""

# TODO: rewrite the rest of this function

# Get the arrays to be the same size.

nf = len(first)

ns = len(second)

diff = nf - ns

if diff < 0:

first = [0] * -diff + first

else:

second = [0] * diff + second

# Reverse both lists for ease of addition.

first = first[::-1]

second = second[::-1]

# Decide whether we need to carry a 1 or not, for each digit.

c = 0

sums = []

for a, b in zip(first, second):

# Convert string-elements to ints.

a, b = int(a), int(b)

s = a + b + c

c = 1 if s > 9 else 0

sums.append(s%10)

sums = sums[::-1]

sums = ([1] + sums) if c else sums

return [str(i) for i in sums]

if __name__ == "__main__":

import doctest

doctest.testmod()