python cumsum with reduce function

Question

I'm trying to write a version of cumulative sum in python using the reduce function. Here is my code so far:

from functools import reduce

def my_cum_sum(arg):
    return reduce(lambda a, x: (a.append(a[-1] + x)) if len(a) > 0 else a.append(x), arg, [])

assert(my_cum_sum([1, 1, 1, 1]) == [1, 2, 3, 4]))

But the problem is that in my lambda function, python doesn't know that a (my accumulator parameter) is a list object, and that I want my reduce function to return a list. In other functional programming languages, it might ask me to specify the type of a and x. But I'm new to python, and haven't quite figured out how it handles types and stuff. What is the pythonic way of solving this problem?

Jean-François Fabre · Accepted Answer · 2017-12-18 07:59:49Z

2

append returns None, so you cannot get the list back into reduce like this. Just use the addition between the list and a list made of a single element, or just a list made of initial element if list is empty:

from functools import reduce

def my_cum_sum(arg):
    return reduce(lambda a, x: a + [a[-1]+x] if a else [x], arg, [])

print (my_cum_sum([1, 1, 1, 1]))

result:

[1, 2, 3, 4]

(note that if len(a)>0 is better written if a as well)

answered Dec 18, 2017 at 7:59

Jean-François Fabre♦

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4 Comments

skan Over a year ago

Why do you use a[-1]?

Jean-François Fabre Over a year ago

this is the current last element of a

skan Over a year ago

But isn't "a" already a single element?

Jean-François Fabre Over a year ago

no it's the parameter of the lambda passed by reduce so it's a list. Try it out

Hylke · Accepted Answer · 2024-03-20 09:11:12Z

0

Why not use Python's accumulate (although this doesn't explicitly use reduce)?

from itertools import accumulate

def my_cum_sum(arg) -> list:
  result = accumulate(arg)
  return list(result)

assert(my_cum_sum([1, 1, 1, 1]) == [1, 2, 3, 4])

answered Mar 20, 2024 at 9:11

Hylke

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Comments

no comment · Accepted Answer · 2024-03-20 16:20:18Z

0

You can make the lambda do the append and then return the list.

from functools import reduce

def my_cum_sum(arg):
    return reduce(lambda a, x: a.append(a[-1] + x if a else x) or a, arg, [])

assert(my_cum_sum([1, 1, 1, 1]) == [1, 2, 3, 4])

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answered Mar 20, 2024 at 16:20

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