[Numpy-discussion] is it a bug?

Robert Kern robert.kern@gmail....
Thu Mar 12 20:07:04 CDT 2009


On Thu, Mar 12, 2009 at 01:34, Stéfan van der Walt <stefan@sun.ac.za> wrote:
> 2009/3/12 Robert Kern <robert.kern@gmail.com>:
>>> idx = np.array([0,1])
>>> e = x[0,:,idx]
>>> print e.shape
>>>
>>> #-----> return (2,3). I think the right answer should be (3,2). Is
>>> #       it a bug here? my numpy version is 1.2.1.
>>
>> It's certainly weird, but it's working as designed. Fancy indexing via
>> arrays is a separate subsystem from indexing via slices. Basically,
>> fancy indexing decides the outermost shape of the result (e.g. the
>> leftmost items in the shape tuple). If there are any sliced axes, they
>> are *appended* to the end of that shape tuple.
>
> This was my understanding, but now I see:
>
> In [31]: x = np.random.random([4,5,6,7])
>
> In [32]: idx = np.array([1,2])
>
> In [33]: x[:, idx, idx, :].shape
> Out[33]: (4, 2, 7)

Hmm. Well, your guess is as good as mine at this point.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco


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