CODEWITHSUNDEEP
pythonlistloops
KevinOelen
KevinOelen

Reputation: 779

Iterating list with given index list in python

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]

Upvotes: 0

Views: 44

Answers (2)

falsetru
falsetru

Reputation: 368944

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.

Upvotes: 3

Rahul K P
Rahul K P

Reputation: 16081

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]

Upvotes: 1

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