I have a list A = [-1, 2, 1, 2, 0, 2, 1, -3, 4, 3, 0, -1] and B = [0, 7, 11]. List B shows the index of negative integer number index.
How can I return the sum of each slice of the list: For example sum of A[0+1:7] and A[7+1:11]
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2 Answers
Using zip, you can convert [0, 7, 11] to the desired slice index pairs (1:7 / 8:11):
>>> zip(B, B[1:]) # In Python 3.x, this will return an iterator.
[(0, 7), (7, 11)]
>>> [(i+1, j) for i, j in zip(B, B[1:])]
[(1, 7), (8, 11)]
>>> [A[i+1:j] for i, j in zip(B, B[1:])]
[[2, 1, 2, 0, 2, 1], [4, 3, 0]]
>>> [sum(A[i+1:j]) for i, j in zip(B, B[1:])]
[8, 7]
UPDATE
Another way to accomplish what you want without defining B using itertools.groupby:
>>> A = [-8, 3, 0, 5, -3, 12]
>>> import itertools
>>> [sum(grp) for positive, grp in itertools.groupby(A, lambda x: x >= 0) if positive]
[8, 12]
key function was used to split 0 and positive numbers and negative numbers.
5 Comments
KevinOelen
Thanks for the answer. It works but in case A = [-8, 3, 0, 5, -3, 12] and B = [0,4] it doesn't return 12?
falsetru
@KevinOelen,
A[0+1:4] == [3, 0, 5].falsetru
@KevinOelen, If you want to get
[12], you need to specify B = [4, 6] or B = [0, 4, 6].falsetru
@KevinOelen, Run this before running the code in the answer to handle such case:
if B[-1] < len(A): B.append(len(A))falsetru
@KevinOelen, I updated the answer to include other version that does not require you to define
B (indices of negative numbers) yourself.You can implement like using one line list comprehension.
[sum(A[i+1:B[B.index(i)+1]]) if i != B[-1] else sum(A[i+1:-1]) for i in B]
Result
[8, 7, 0]
If you don't want last slice sum use this.
[sum(A[i+1:B[B.index(i)+1]]) for i in B if i != B[-1]]
Result
[8, 7]
3 Comments
KevinOelen
Thanks for the answer. Can you see above comment and answer for that?
KevinOelen
It should return 12 since there is a positive integer after -3
Rahul K P
A[0+1:4] = [3, 0, 5] ? Right ?