slicing python array elements with a vector similar to matlab/R

Question

I'm new to python and wanted to do something I normally do in matlab/R all the time, but couldn't figure it out from the docs.

I'd like to slice an array not as 0:3 which includes elements 0,1,2 but as an explicit vector of indices such as 0,3 For example, say I had this data structure

a = [1, 2, 3, 4, 5]

I'd like the second and third element so I thought something like this would work

a[list(1,3)]

but that gives me this error

TypeError: list indices must be integers

This happens for most other data types as well such as numpy arrays

In matlab, you could even say a[list(2,1)] which would return this second and then the first element.

There is an alternative implementation I am considering, but I think it would be slow for large arrays. At least it would be damn slow in matlab. I'm primarily using numpy arrays.

[ a[i] for i in [1,3] ]

What's the python way oh wise ones? Thanks!!

Philipp · Accepted Answer · 2010-07-14 17:07:05Z

1

NumPy allows you to use lists as indices:

import numpy
a = numpy.array([1, 2, 3, 4, 5])
a[[1, 3]]

Note that this makes a copy instead of a view.

answered Jul 14, 2010 at 17:07

Philipp

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sunetos · Accepted Answer · 2010-07-14 17:18:22Z

1

I believe you want numpy.take:

newA = numpy.take(a, [1,3])

edited Jul 14, 2010 at 17:18

answered Jul 14, 2010 at 17:02

sunetos

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