[Numpy-discussion] Inverting argsort(a, axis=0) to obtain column-wise ranks

[John Schulman]

Argsort twice and you get the rank.

a1.argsort(axis=0).argsort(axis=0)

That's because argsort is it's own inverse when applied to the ranks.

On Tue, Sep 7, 2010 at 1:01 PM, Alexander Michael <lxander.m at gmail.com> wrote:
> Calculating ranks by inverting the results of an argsort is
> straightforward and fast for 1D arrays:
>
> indices = argsort(a1)
> ranks = zeros_like(indices)
> ranks[indices] = arange(len(indices))
>
> I was wondering if there was an equally pithy way to do this for
> multiple data samples stored column-wise in a 2D array. That is, is
> there a trick to invert the results of argsort(a2, axis=0) without
> iterating (in python) over the columns?
>
> Thanks,
> Alex
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