[Numpy-discussion] broacasting question

Olivier Delalleau shish@keba...
Thu Jun 30 12:36:15 CDT 2011


2011/6/30 Thomas K Gamble <tkgamble@windstream.net>

> I'm trying to convert some IDL code to python/numpy and i'm having some
> trouble understanding the rules for boradcasting during some operations.
> example:
>
> given the following arrays:
> a = array((2048,3577), dtype=float)
> b = array((256,25088), dtype=float)
> c = array((2048,3136), dtype=float)
> d = array((2048,3136), dtype=float)
>
> do:
> a = b * c + d
>
> In IDL, the computation is done without complaint and all array sizes are
> preserved.  In ptyhon I get a value error concerning broadcasting.  I can
> force it to work by taking slices, but the resulting size would be a =
> (256x3136) rather than (2048x3577).  I admit that I don't understand IDL
> (or
> python to be honest) well enough to know how it handles this to be able to
> replicate the result properly.  Does it only operate on the smallest
> dimensions ignoring the larger indices leaving their values unchanged?  Can
> someone explain this to me?
>
> --
> Thomas K. Gamble tkgamble@windstream.net
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion@scipy.org
> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>

It should be all explained here:
http://docs.scipy.org/doc/numpy/user/basics.broadcasting.html

-=- Olivier
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