[Numpy-discussion] how to use argsort result?

Stefan van der Walt stefan at sun.ac.za
Tue Jul 11 05:20:03 CDT 2006

On Tue, Jul 11, 2006 at 11:32:48AM +0200, Emanuele Olivetti wrote:
> Hi,
> I don't understand how to use argsort results. I have a 2D matrix and
> I want to sort values in each row and obtain the index array of that
> sorting. Argsort(1) is what I need, but the problem is how to use its
> result in order to obtain a sorted matrix. Here is the simple example:
> A = array([[2,3,1],[5,4,6]])
> indexes = a.argsort(1)
> now indexes is:
> array([[2, 0, 1],
>         [1, 0, 2]])
> I'd like to apply indexes to A and obtain:
> array([[1, 2, 3],
>         [4, 5, 6]])
> or better, I'm interested both in a subset of indexes, i.e. indexes[:,1:], and
> the related values of A matrix.
> How can I do this? If I simpy say: A[indexes] I get an IndexError.

Something's not quite right here.  The argsort docstring states that:

    argsort(a,axis=-1) return the indices into a of the sorted array
    along the given axis, so that take(a,result,axis) is the sorted array.


N.take(A,A.argsort()) breaks.  Either this is a bug, or the docstring
needs to be updated.


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