Running subtract using list in Python

Question

I have created a function that takes a list of integers and subtracts from left to right to return the final answer. I am not keen on using a count variable to skip through the first loop as it seems wasteful - Is there a better way of doing this in Python?

def subtract(numlist):
''' numlist -> int
    Takes a list of numbers and subtracts from left to right
'''
answer = numlist[0]
count = 0
for n in numlist:

    if count != 0:
        answer -= n

    count += 1
return answer

print(subtract([10,2,4,1]))

score 2 · Answer 1 · answered Jul 14 '13 at 00:34

2

Just sum everything but the first element, and then subtract:

>>> def subtract(numlist):
...    return numlist[0] - sum(numlist[1:])
...
>>> print(subtract([10,2,4,1]))
3

answered Jul 14 '13 at 00:34

jterrace

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Great simple answer! Thanks. – Phil Jul 14 '13 at 00:41
No problem, welcome to SO. Don't forget to mark an answer as accepted by clicking the green checkmark next to it. – jterrace Jul 14 '13 at 01:17

score 1 · Answer 2 · answered Jul 14 '13 at 00:32

You can use list slicing:

>>> def subtract(numlist):
...     result = numlist[0]
...     for n in numlist[1:]:
...             result -= n
...     return result
... 
>>> subtract(L)
3

We first get the first element in the list, as you've shown, but instead of iterating through the whole list with a counter, we can just slice that first element off and iterate as normal.

score 1 · Answer 3 · answered Jul 14 '13 at 00:58

This particular operation is built in to python2, and available in functools in python3, as reduce:

reduce(...)
    reduce(function, sequence[, initial]) -> value

    Apply a function of two arguments cumulatively to the items of a sequence,
    from left to right, so as to reduce the sequence to a single value.
    For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates
    ((((1+2)+3)+4)+5).  If initial is present, it is placed before the items
    of the sequence in the calculation, and serves as a default when the
    sequence is empty.

Hence:

>>> l = [10, 2, 4, 1]
>>> import operator
>>> reduce(operator.sub, l)
3

reduce was removed in python 3, looks like the OP is using python 3 too — TerryA, Jul 14 '13 at 01:00
@Haidro: It's in `functools` in python3: `from functools import reduce` — torek, Jul 14 '13 at 01:01

score 0 · Answer 4 · edited May 23 '17 at 12:29

0

Instead of using a count variable, you could try for n in numlist[1:]

Running subtract using list in Python

5 Answers5