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numpy unique without sort

Asked 2012-10-17T03:58:57.333
52

How can I use numpy unique without sorting the result but just in the order they appear in the sequence? Something like this?

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

np.unique(a) = [4,2,1,3]

rather than

np.unique(a) = [1,2,3,4]

Use naive solution should be fine to write a simple function. But as I need to do this multiple times, are there any fast and neat way to do this?

python numpy
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77

You can do this with the return_index parameter:

>>> import numpy as np
>>> a = [4,2,1,3,1,2,3,4]
>>> np.unique(a)
array([1, 2, 3, 4])
>>> indexes = np.unique(a, return_index=True)[1]
>>> [a[index] for index in sorted(indexes)]
[4, 2, 1, 3]
answered 2012-10-17T04:11:20.400

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