CODEWITHSUNDEEP
pythonarraysnumpyindexing
MoTSCHIGGE
MoTSCHIGGE

Reputation: 989

Python: How to get values of an array at certain index positions?

I have a numpy array like this:

a = [0,88,26,3,48,85,65,16,97,83,91]

How can I get the values at certain index positions in ONE step? For example:

ind_pos = [1,5,7]

The result should be:

[88,85,16]

Upvotes: 31

Views: 263845

Answers (5)

user3906621
user3906621

Reputation: 107

your code would be

a = [0,88,26,3,48,85,65,16,97,83,91]

ind_pos = [a[1],a[5],a[7]]

print(ind_pos)

you get [88, 85, 16]

Upvotes: -2

Ohad Cohen
Ohad Cohen

Reputation: 6152

The one liner "no imports" version

a = [0,88,26,3,48,85,65,16,97,83,91]
ind_pos = [1,5,7]
[ a[i] for i in ind_pos ]

Upvotes: 24

chepner
chepner

Reputation: 532208

Although you ask about numpy arrays, you can get the same behavior for regular Python lists by using operator.itemgetter.

>>> from operator import itemgetter
>>> a = [0,88,26,3,48,85,65,16,97,83,91]
>>> ind_pos = [1, 5, 7]
>>> print itemgetter(*ind_pos)(a)
(88, 85, 16)

Upvotes: 8

Padraic Cunningham
Padraic Cunningham

Reputation: 180512

Just index using you ind_pos 

ind_pos = [1,5,7]
print (a[ind_pos]) 
[88 85 16]


In [55]: a = [0,88,26,3,48,85,65,16,97,83,91]

In [56]: import numpy as np

In [57]: arr = np.array(a)

In [58]: ind_pos = [1,5,7]

In [59]: arr[ind_pos]
Out[59]: array([88, 85, 16])

Upvotes: 36

Ffisegydd
Ffisegydd

Reputation: 53718

You can use index arrays, simply pass your ind_pos as an index argument as below:

a = np.array([0,88,26,3,48,85,65,16,97,83,91])
ind_pos = np.array([1,5,7])

print(a[ind_pos])
# [88,85,16]

Index arrays do not necessarily have to be numpy arrays, they can be also be lists or any sequence-like object (though not tuples).

Upvotes: 5

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