[Numpy-discussion] lost with slicing

josef.pktd@gmai... josef.pktd@gmai...
Mon Mar 30 19:21:22 CDT 2009


On Mon, Mar 30, 2009 at 7:54 PM, Partridge, Matthew BGI SYD
<Matthew.Partridge@barclaysglobal.com> wrote:
>
> I apologise if I'm asking an obvious question or one that has already
> been addressed.
>
> I've tried to understand the documentation in the numpy manual on
> slicing, but I'm a bit lost.  I'm trying to do indexing using both
> slices and index lists.  I have a problem when I do something like:
>
> x[0, :, [0,1,2]]
>
> Here are a couple of examples:
>
>>>> a = numpy.arange(6).reshape(2,3)
>>>> print a
> [[0 1 2]
>  [3 4 5]]
>>>> print a[:, [0,1,2]]   # example 1 - this works as I expected
> [[0 1 2]
>  [3 4 5]]
>>>> b = numpy.arange(6).reshape(1,2,3)
>>>> print b
> [[[0 1 2]
>  [3 4 5]]]
>>>> print b[0, :, [0,1,2]]  # example 2 - this seems to be the transpose
> of what I was expecting
> [[0 3]
>  [1 4]
>  [2 5]]
>>>> print b[0, [[0],[1]], [[0,1,2]]] # example 3 - this is what I
> expected
> [[0 1 2]
>  [3 4 5]]
>
> Am I doing something wrong?  Why do we get different behaviour in
> example 2 compared with example 1 or example 3?
>
> (I'm using numpy 1.0.3.1 on python 2.4.1 for windows, but I've tried
> some more recent versions of numpy as well.)
>
> mattp
>

that's how it works, whether we like it or not.

 see thread with title "is it a bug?" starting  march 11

Josef


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