Python SUMPRODUCT of elements in nested list

user_7824826 0
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I have two nested lists:

a = [[1,2,3],[2,4,2]]
b = [[5,5,5],[1,1,1]]

I want to multiply and SUMPRODUCT each group of elements to get

c = [[30],[8]]

Which result from = [[1*2+2*5+3*5],[2*1,4*1,2*1]]

I´ve tried doing:

c = sum(x * y for x, y in zip(a, b))

But I get "can't multiply sequence by non-int of type 'list'"

Is there a simple list comprehension way to do this avoiding for loops?

python
4 Answers

You can implement the dotproduct itertools recipes:

import operator


def dotproduct(vec1, vec2):
    return sum(map(operator.mul, vec1, vec2))

Code

a = [[1,2,3], [2,4,2]] 
b = [[5,5,5], [1,1,1]]

[[dotproduct(x, y)] for x, y in zip(a, b)]
# [[30], [8]]
user_45312700

Numpy solution

import numpy as np

c = map(lambda x: sum(np.prod((np.array(x)), axis=0)), zip(a,b))

[30, 8]

If you need to produce [[30], [8]] then

c = map(lambda x: [sum(np.prod((np.array(x)), axis=0))], zip(a,b))
user_68178350

Without numpy

A solution with list comprehension, maybe will this:

a = [[1,2,3],[2,4,2]]
b = [[5,5,5],[1,1,1]]

c = [[sum(map(lambda m: reduce(lambda h,i: h * i, m), n))] for n in [zip(x, y) for x, y in zip(a, b)]]

And show me:

[[30], [8]]

Another more clean solution is a simple for loop

a = [[1,2,3],[2,4,2]]
b = [[5,5,5],[1,1,1]]

c = []

for x, y in zip(a, b):
    temp = []
    for m, n in zip(x,y):
        temp.append(m * n)
    c.append([sum(temp)])

Result:

[[30], [8]]

Sorry for not readable variable names, I recommend the simple for in this cases

user_65919290

You could do it with a nested list comprehension but it would be complicated. Here it is one step at a time.

a = [[1,2,3],[2,4,2]]
b = [[5,5,5],[1,1,1]]
result1 = [[x*y for x, y in zip(r1, r2)] for r1, r2 in zip(a, b)]
c = [[sum(r)] for r in result1]

c
[[30], [8]]
user_16095140
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