[Numpy-discussion] Strange behavior for argsort() and take()

[Charles R Harris]

On Wed, Jun 18, 2008 at 11:48 AM, Anne Archibald <peridot.faceted at gmail.com>
wrote:

> 2008/6/18 Stéfan van der Walt <stefan at sun.ac.za>:
> > 2008/6/18 Anne Archibald <peridot.faceted at gmail.com>:
> >> In [7]: x.take(x.argsort())
> >> Out[7]: array([ 0. ,  0.1,  0.2,  0.3])
> >>
> >> If you would like to think of it more mathematically, when you feed
> >> np.argsort() an array that represents a permutation of the numbers
> >> 0,1,...,n-1, you get back the inverse permutation. When you pass a
> >> permutation as the argument to x.take(), you apply the permutation to
> >> x. (You can also invert a permutation by feeding it as indices to
> >> arange(n)).
> >>
> >> I have been tempted to write some support functions for manipulating
> >> permutations, but I'm not sure how generally useful they would be.
> >
> > Do we have an easy way of grabbing the elements out of an array that
> > correspond to
> >
> > argmax(x, axis) ?
> >
> > `take` won't do the job; there `axis` has a different meaning.
>
> Well, here's a quick hack:
>
> def take_axis(X, ix, axis):
>    XX = np.rollaxis(X,axis)
>    s = XX.shape
>    return XX[(ix,)+tuple(np.indices(s[1:]))]
>
> And a version that works for argsort()ing:
>
> def take_axis_argsort(X, ix, axis):
>    XX = np.rollaxis(X,axis)
>    ixix = np.rollaxis(ix,axis)
>    s = XX.shape
>    return np.rollaxis(XX[(ixix,)+tuple(np.indices(s)[1:])],0,axis+1)
>
> I'm always a little bit leery of using indices(), since it can produce
> very large arrays:
>

And fancy indexing too, as it tends to be slow. The axis=None case also
needs to be handled. I think this is best done in C where the code stacks
for argsort and arg{max,min} are good starting points. The possibility of
different integer types for the index array adds a bit of complication.

Chuck
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