[Numpy-discussion] Indexing bug

[Ivan Oseledets]

Message: 2
Date: Sat, 30 Mar 2013 11:13:35 -0700
From: Jaime Fern?ndez del R?o <jaime.frio at gmail.com>
Subject: Re: [Numpy-discussion] Indexing bug?
To: Discussion of Numerical Python <numpy-discussion at scipy.org>
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        <CAPOWHWk+mL6KN6F2FHTPn5HTiU0UEQPj6KdxjNK_+T1E-YRiBg at mail.gmail.com>
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On Sat, Mar 30, 2013 at 11:01 AM, Ivan Oseledets
<ivan.oseledets at gmail.com>wrote:

> I am using numpy 1.6.1,
> and encountered a wierd fancy indexing bug:
>
> import numpy as np
> c = np.random.randn(10,200,10);
>
> In [29]: print c[[0,1],:200,:2].shape
> (2, 200, 2)
>
> In [30]: print c[[0,1],:200,[0,1]].shape
> (2, 200)
>
> It means, that here fancy indexing is not working right for a 3d array.
>

On Sat, Mar 30, 2013 at 11:01 AM, Ivan Oseledets
<ivan.oseledets at gmail.com>wrote:

> I am using numpy 1.6.1,
> and encountered a wierd fancy indexing bug:
>
> import numpy as np
> c = np.random.randn(10,200,10);
>
> In [29]: print c[[0,1],:200,:2].shape
> (2, 200, 2)
>
> In [30]: print c[[0,1],:200,[0,1]].shape
> (2, 200)
>
> It means, that here fancy indexing is not working right for a 3d array.
>
-->
It is working fine, review the docs:

http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#advanced-indexing

In your return, item [0, :] is c[0, :, 0] and item[1, :]is c[1, :, 1].

If you want a return of shape (2, 200, 2) where item [i, :, j] is c[i, :,
j] you could use slicing:

 c[:2, :200, :2]

or something more elaborate like:

c[np.arange(2)[:, None, None], np.arange(200)[:, None], np.arange(2)]

Jaime
--->


Oh!  So it is not a bug, it is a feature, which is completely
incompatible with other array based languages (MATLAB and Fortran). To
me, I can not find a single explanation why it is so in numpy.
Taking submatrices from a matrix is a common operation and the syntax
above is very natural to take submatrices, not a weird diagonal stuff.
i.e.,

c = np.random.randn(100,100)
d = c[[0,3],[2,3]]

should NOT produce two numbers! (and you can not do it using slices!)

In MATLAB and Fortran
c(indi,indj)
will produce a 2 x 2 matrix.
How it can be done in numpy (and why the complications?)

So, please consider this message as a feature request.

Ivan