Specify Order numpy array

Question

I'm trying to put an array in the order of another array. For example, if I have:

arr1 = np.array(['a', 'b', 'c'])
index = np.array([2, 1, 0])

My desired outcome, arr2, is ['c', 'b', 'a'], such that:

arr2[index[i]] == arr1[i]

jterrace · Accepted Answer · 2012-03-14 18:40:09Z

6

You can simply pass the selector array as index to the character array:

>>> import numpy as np
>>> arr1 = np.array(['a', 'b', 'c'])
>>> index = np.array([2, 1, 0])
>>> arr1[index]
array(['c', 'b', 'a'], 
      dtype='|S1')

answered Mar 14, 2012 at 18:40

jterrace

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2 Comments

mike Over a year ago

Perhaps there's an error in my code, but when I run this I get the following relationship, not the desired one: arr2[i] == arr1[index[i]]

mike Over a year ago

I think the issue might be a few lines before in the code -- thanks!

Daniel Fath · Accepted Answer · 2012-03-15 16:24:08Z

1

Try this:

[arr1[i] for i in index]

edited Mar 15, 2012 at 16:24

Daniel Fath

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answered Mar 14, 2012 at 18:41

gcbirzan

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1 Comment

JoshAdel Over a year ago

Doesn't return an array and is inefficient for numpy arrays. General rule: Don't treat numpy arrays as python lists. There is almost always a better way to do it with numpy builtins.

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