[Numpy-discussion] Is this an indexing bug?
Sven Schreiber
svetosch@gmx....
Wed Jun 20 15:30:12 CDT 2007
Travis Oliphant schrieb:
> Sturla Molden wrote:
>> >>> x = numpy.arange(100).reshape((1,10,10))
>>
>> >>> x[0,:,numpy.arange(5)].shape
>> (5, 10)
>>
>> >>> x[:,:,numpy.arange(5)].shape
>> (1, 10, 5)
>>
>>
>> It looks like a bug that needs to be squashed.
>>
>
> These are both correct. See my previous posts about the rule.
>
> The first case is exactly the example we saw before: we start with a
> (1,10,10)-shaped array and replace the first and last-dimension
> (1,10)-shaped array with a (5,)-shaped array. Not having a clear place
> to put the extracted (5,)-shaped subspace, it is tacked on to the front.
>
> In the second example, the last-dimension (10,)-shaped sub-space is
> replaced with a (5,)-shaped sub-space. There is no ambiguity in this
> case and the result is a (1,10,5)-shaped array.
>
> There is no bug here. Perhaps unexpected behavior with "advanced
> indexing" combined with single-integer indexing in separated dimensions
> but no bug. The result, does follow an understandable and generalizable
> rule.
>
>
Thanks for the explanation. Maybe some of these examples could be added
to the relevant section of the book. Although I must say I'm glad that I
currently don't need to use this stuff, I'm not sure I would get it
right :-)
cheers,
sven
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