[Numpy-discussion] Using array mask swaps array axes

[Chad Kidder]

Thanks, that works.  It will be nice when the original way works also.
On Oct 16, 2013 10:28 AM, "Sebastian Berg" <sebastian at sipsolutions.net>
wrote:

> On Wed, 2013-10-16 at 11:50 -0400, Benjamin Root wrote:
> >
> >
> >
> > On Wed, Oct 16, 2013 at 11:39 AM, Chad Kidder <cckidder at gmail.com>
> > wrote:
> >         Just found what should be a bug in 1.7.1.  I'm running
> >         python(x,y) on windows here:
> >
> >         >>> dataMatrix[ii,:,mask].shape
> >         (201, 23)
> >         >>> dataMatrix[ii,:,:].shape
> >         (23, 201)
> >         >>> dataMatrix.shape
> >         (24, 23, 201)
> >         >>> mask
> >         array([ True,  True,  True,  True,  True,  True,  True,  True,
> >         True,
> >                 ...
> >                 True,  True,  True], dtype=bool)
> >
> >
> >         using a mask should not change the order of the dimensions.
> >         Is there a reason for this behavior, and if so, how do I avoid
> >         it in the future?  Thanks
> >
> >
> >              --Chad Kidder
> >
> >
> >
> >
> > Chad,
> >
> > The issue here is one where there is the mixing of fancy indexing (I
> > presume that is what "ii" is), slicing and boolean indexing. If I
> > remember correctly, the changing of the dimension orders was an
> > inadvertent byproduct of handing all this array accessing methods in
> > one shot. I think this was addressed in 1.8. Sorry for being very
> > brief and vague, hopefully someone else who understands what the
> > resolution was can fill in.
> >
> Yes, in fact `ii` can just be a normal integer, since an integer *is*
> considered an advanced/fancy index (in the sense that it forces
> transposing, not in the sense that it forces a copy by itself, so
> integers are *both* advanced and view based indices!).
>
> This is how advanced/fancy indexing works, there is `np.ix_` which helps
> in some cases, but not exactly for your problem. For a more detailed
> description of fancy indexing check:
>
> http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#advanced-indexing
>
> You have a slice between the mask and the integer `arr[1, :, mask]`,
> which means the `mask` result dimension is transposed to the front. It
> would not be if it was `arr[:, 1, mask]` (since numpy can actually guess
> where it came from in that case).
>
> Since you seem to always have exactly one advanced (mask) index, in your
> example, the simplest solution is probably:
> `dataMatrix[ii,...][:,mask]`
> (first view based slicing, then the advanced boolean index. Since the
> first part (if `ii` is an integer) will not copy the data, this will
> also work for assignments).
>
> - Sebastian
>
>
> > Cheers!
> > Ben Root
> >
> >
> >
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>
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