how to get rows of 2d numpy array by index array

Question

I have a 2-D array that looks like this:

my_array = np.array([[1,7]
                     [2,4]
                     [3,10] 
                     [4,3] 
                     [5,23]])

and I have an index array which looks like this:

index_array = np.array([0,2,3])

as an output I want to get a matrix containing only the rows from the index array so the shape of the output matrix should be (3,2) and it should look like this:

[[1,7]
 [3,10]
 [4,3]]

The solution shouldn't use a for loop and should work with any 2-D matrix. Thanks in advance :)

Does this answer your question? How to get the values from a NumPy array using multiple indices — Equinox
– Equinox, Commented Nov 16, 2020 at 15:16

0xDECAFC0FFEE · Accepted Answer · 2020-11-16 16:17:56Z

3

you can use numpy array slicing notation to select your rows :D

import numpy as np

my_array = np.array([[1,7],
                     [2,4],
                     [3,10], 
                     [4,3],
                     [5,23]])

print(my_array[[0,2,3]])

my_array[index_array] would work as well

edited Nov 16, 2020 at 16:17

answered Nov 16, 2020 at 15:14

0xDECAFC0FFEE

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Comments

Anton Shpigunov · Accepted Answer · 2020-11-16 15:18:22Z

0

not sure if a list comprehension qualifies as a for loop, but you can have this one-line solution:

[list(my_array[i]) for i in index_array]

that will return [[1, 7], [3, 10], [4, 3]].

answered Nov 16, 2020 at 15:18

Anton Shpigunov

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