Using elements of numpy array as index of another numpy array

Question

I have an unknown 2D numpy array like the following:

a = np.array([
    [1, 2],
    [0, 2],
])

I want to use the elements of the array as an index to another array b to produce the following effect:

b = np.random.randn(5, 5)
c = b[[1, 2], [0, 2]]

How can I use the variable a to replace the hardcoded values in the index?

Using the following code did not work:

b = np.random.randn(5, 5)
c = [*a]

As the * expression is used in an index.

no it is a 2D array where the size is not known when writing the program — SdahlSean
– SdahlSean, Commented Sep 6, 2021 at 19:29

BatWannaBe · Accepted Answer · 2021-09-06 19:35:54Z

4

Just change the outer layer of a to a tuple to trigger advanced indexing: b[tuple(a)].

This is pretty much how the index syntax of b[[1, 2], [0, 2]] is interpreted; the elements are changed to NumPy arrays and are put into a tuple.

edited Sep 6, 2021 at 19:35

answered Sep 6, 2021 at 19:29

BatWannaBe

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Agustin Rodriguez Cantalapiedr · Accepted Answer · 2021-09-06 19:39:06Z

2

I don't know if I got it but, try using a list for the index instead of an np array:

import numpy as np

a = [
    [1, 2],
    [0, 2]
    ]

b = np.random.randn(5, 5)
c = b[a]

edited Sep 6, 2021 at 19:39

answered Sep 6, 2021 at 19:33

Agustin Rodriguez Cantalapiedr

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1 Comment

BatWannaBe Over a year ago

This will work for now, but NumPy is planning to change the default treatment of non-tuples arr[tuple(not_a_tuple)] to arr[np.array(not_a_tuple)], which does something different. This code currently prints a FutureWarning that suggests you write arr[tuple(not_a_tuple)] for future versions.

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