[Numpy-discussion] Right way to do fancy indexing from argsort() result?

[Robert Kern]

On Mon, Mar 26, 2018 at 11:24 AM, Benjamin Root <ben.v.root at gmail.com>
wrote:
>
> I seem to be losing my mind... I can't seem to get this to work right.
>
> I have a (N, k) array `distances` (along with a bunch of other arrays of
the same shape). I need to resort the rows, so I do:
>
> indexs = np.argsort(distances, axis=1)
>
> How do I use this index array correctly to get back distances sorted
along rows? Note, telling me to use `np.sort()` isn't going to work because
I need to apply the same indexing to a couple of other arrays.
>
> new_dists = distances[indexs]
>
> gives me a (N, k, k) array, while
>
> new_dists = np.take(indexs, axis=1)
>
> gives me a (N, N, k) array.
>
> What am I missing?

Broadcasting!

  new_dists = distances[np.arange(N)[:, np.newaxis], indexs]

--
Robert Kern
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