user3641829 user3641829 - 2 months ago 11
Python Question

PYTHON: Summing up elements from one list based on indices in another list

So here is what I am trying to achieve in Python:


  • I have a list "A" with unsorted and repeated indices.

  • I have a list "B" with some float values

  • Length A = Length B

  • I want list "C" with summed values of B based on the repeated indices in A in a sorted ascending manner.



Example:

A=[0 , 1 , 0 , 3 , 2 , 1 , 2] (indicates unsorted and repeated indices)


B=[25 , 10 , 15 , 10 , 5 , 30 , 50] (values to be summed)


C=[25+15 , 10+30 , 5+50 , 15] (summed values in a sorted manner)


So far I know how to do the sorting bit with:

C= zip(*sorted(zip(A, B)))


Getting the result:

[(0, 0, 1, 1, 2, 2, 3), (15, 25, 10, 30, 5, 50, 10)]


But I do not know how to do the sum.

What would be a good way to create list C?

Jan Jan
Answer

Use zip() in combination with a dict:

A = [0 , 1 , 0 , 3 , 2 , 1 , 2] 
B = [25 , 10 , 15 , 10 , 5 , 30 , 50]

sums = {}
for key, value in zip(A,B):
    try:
        sums[key] += value
    except KeyError:
        sums[key] = value
print(sums)
# {0: 40, 1: 40, 2: 55, 3: 10}

And see a demo on ideone.com.