Find whether a numpy array is a subset of a larger array in Python

Question

I have 2 arrays, for the sake of simplicity let's say the original one is a random set of numbers:

import numpy as np
a=np.random.rand(N)

Then I sample and shuffle a subset from this array:

b=np.array()  <------size<N

The shuffling I do do not store the index values, so b is an unordered subset of a

Is there an easy way to get the original indexes of b, so they are in the same order as a, say, if element 2 of b has the index 4 in a, create an array of its assignation.

I could use a for cycle checking element by element, but perhaps there is a more pythonic way

Thanks

unutbu · Accepted Answer · 2012-10-24 23:37:41Z

2

I think the most computationally efficient thing to do is to keep track of the indices that associate b with a as b is created.

For example, instead of sampling a, sample the indices of a:

indices = random.sample(range(len(a)), k)   # k < N
b = a[indices]

edited Oct 24, 2012 at 23:37

answered Oct 24, 2012 at 22:42

unutbu

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1 Comment

Fred Foo Over a year ago

Or, with Numpy (which the OP is already using): indices = np.arange(len(a)); np.random.shuffle(indices); indices = indices[:k]

Bi Rico · Accepted Answer · 2012-10-25 05:47:01Z

0

On the off chance a happens to be sorted you could do:

>>> from numpy import array
>>> a = array([1, 3, 4, 10, 11])
>>> b = array([11, 1, 4])
>>> a.searchsorted(b)
array([4, 0, 2])

If a is not sorted you're probably best off going with something like @unutbu's answer.

answered Oct 25, 2012 at 5:47

Bi Rico

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