How to extract elements in an array with regard to an index?

Question

I have a row A = [0 1 2 3 4] and an index I = [0 0 1 0 1]. I would like to extract the elements in A indexed by I, i.e. [2, 4].

My attempt:

import numpy as np
A = np.array([0, 1, 2, 3, 4])
index = np.array([0, 0, 1, 0, 1])
print(A[index])

The result is not as I expected:

[0 0 1 0 1]

Could you please elaborate on how to achieve my goal?

Does this answer your question? Iterate over numpy with index (numpy equivalent of python enumerate) — DaVinci
– DaVinci, Commented Jun 2, 2020 at 15:29
Actually, @Quang Hoang's answer solves my problem efficiently ^^. I work with very large matrix, so I think the solution in the linked answer is not as efficient. — Akira
– Akira, Commented Jun 2, 2020 at 15:31

Quang Hoang · Accepted Answer · 2020-06-02 15:25:15Z

2

I think you want boolean indexing:

A[index.astype(bool)]
# array([2, 4])

answered Jun 2, 2020 at 15:25

Quang Hoang

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Comments

match · Accepted Answer · 2020-06-02 15:31:10Z

0

A non-numpy way to achieve this, in case its useful - it uses zip to combine each pair of elements, and returns the first if the second is true:

[x[0] for x in zip(a, i) if x[1]]

answered Jun 2, 2020 at 15:31

match

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