[Numpy-discussion] another broadcast/fancy indexing question
Timothy Hochberg
tim.hochberg@ieee....
Tue Jul 10 16:02:54 CDT 2007
On 7/10/07, Mark.Miller <mpmusu@cc.usu.edu> wrote:
>
> Just ran across something that doesn't quite make sense to me at the
> moment.
>
> Here's some code:
>
> >>> numpy.__version__
> '1.0.2'
> >>>
> >>> def f1(b,c):
> b=b.astype(int)
> c=c.astype(int)
> return b,c
>
> >>> b,c = numpy.fromfunction(f1,(5,5))
> >>> a=numpy.zeros((2,12,5,5),int)
> >>> a1=a[0]
> >>> a1[:,b,c].shape
> (12, 5, 5)
> >>> a[0,:,b,c].shape
> (5, 5, 12) ###why does this not return (12,5,5)?
> >>>
>
> So in a nutshell, it's not completely clear to me why these are
> returning arrays of different shapes. Can someone shed some light?
It's because you are using arrays as indices (aka Fancy-Indexing). When you
do this everything works differently. In this case, everything is being
broadcast to the same shape. As I understand it (and I try to use only the
simplest forms of fancy indexing), what you are doing is equivalent to:
--
. __
. |-\
.
. tim.hochberg@ieee.org
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