[Numpy-discussion] Numpy array slicing

Eirik Gjerløw eirik.gjerlow@astro.uio...
Thu Feb 9 07:28:12 CST 2012


Ok, that was an enlightening discussion, I guess I signed up for this 
list a couple of days too late!

Thanks,
Eirik

On 09. feb. 2012 12:55, Olivier Delalleau wrote:
> This was actually discussed very recently (for more details: 
> http://mail.scipy.org/pipermail/numpy-discussion/2012-February/060232.html).
>
> It's caused by mixing slicing with advanced indexing. The resulting 
> shape is the concatenation of a first part obtained by broadcasting of 
> the non-slice items (in your case, 0 and mask[0] being broadcasted to 
> shape (12285,), the number of non-zero elements in mask[0]), followed 
> by a second part obtained by the slice items (in your case extracting 
> dimensions #1 and #3 of a, i.e. shape (4, 2)).
> So the final shape is (12285, 4, 2).
>
> -=- Olivier
>
>
> Le 9 février 2012 06:32, Eirik Gjerløw <eirik.gjerlow@astro.uio.no 
> <mailto:eirik.gjerlow@astro.uio.no>> a écrit :
>
>     Hello,
>
>     (also sent to Scipy-User, sorry for duplicates).
>
>     This is (I think) a rather basic question about numpy slicing. I have
>     the following code:
>
>     In [29]: a.shape
>     Out[29]: (3, 4, 12288, 2)
>
>     In [30]: mask.shape
>     Out[30]: (3, 12288)
>
>     In [31]: mask.dtype
>     Out[31]: dtype('bool')
>
>     In [32]: sum(mask[0])
>     Out[32]: 12285
>
>     In [33]: a[[0] + [slice(None)] + [mask[0]] + [slice(None)]].shape
>     Out[33]: (12285, 4, 2)
>
>     My question is: Why is not the final shape (4, 12285, 2) instead of
>     (12285, 4, 2)?
>
>     Eirik Gjerløw
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>
>
>
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