python reduce method works with length of 2 list but not with length with 3 or more

user_16322445 0
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i have a set of points like this:

points = [[1, 1], [2, 3]]

i want to add up the x-components and y-components separately so i wrote this code

sum_x = reduce(lambda a, b: a[0] + b[0] ,points)
sum_y = reduce(lambda a, b: a[1] + b[1], points)

if i have only 2 points it works but if i add any more points then i get this error:

Traceback (most recent call last):
  File "linear-regression.py", line 29, in <module>
    sum_x = reduce(lambda a, b: a[0] + b[0], points)
  File "linear-regression.py", line 29, in <lambda>
    sum_x = reduce(lambda a, b: a[0] + b[0], points)
TypeError: 'int' object is not subscriptable
python functional-programming
3 Answers

Your accumulator should be an integer, not a list. You can initialize it with zero to get it started. If you don't initialize it, the initial value will be the first value in points ([1, 1]), which is not what you want here.

from functools import reduce

points = [[1, 1], [2, 3], [2, 3]]


sum_x = reduce(lambda a, b: a + b[0], points, 0)
# 5
sum_y = reduce(lambda a, b: a + b[1], points, 0)
# 7

Alternatively, you can do this in a single step and return a list of sums:

sum_x, sum_y = reduce(lambda a, b: [a[0] + b[0], a[1] + b[1]], points)

Now passing the initial value in as an accumulator makes sense because the final value is a list.

user_38746230

Mark's answer fixes your immediate problem, but I want to point something more general out, especially since you tagged functional-programming (which, at its core, is about decomposing functionality into its irreducible parts). You're performing two operations that you try to treat as one. Your computation can be summarized as

  1. Get the first element of each vector
  2. Sum them

We can split these two steps up.

sum_x = reduce(lambda a, b: a + b, map(lambda x: x[0], points))

And reduce with addition as the operation has a name in Python: sum

sum_x = sum(map(lambda x: x[0], points))

Now our code reads as "get the first elements and sum them".

user_22886590

why not just

points = [[1, 1], [2, 3]]
sum_x, sum_y = map(sum, zip(*points))

this works with arbitrary length list points.

user_40466320
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