[Numpy-discussion] broacasting question

[Charles R Harris]

On Thu, Jun 30, 2011 at 11:32 AM, Thomas K Gamble
<tkgamble at windstream.net>wrote:

> 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?
>
>
I don't see a problem

In [1]: datetime64('now')
Out[1]: numpy.datetime64('2011-07-01T07:18:35-0600')

In [2]: a = array((2048, 3577), float)

In [3]: b = array((256, 25088), float)

In [4]: c = array((2048, 3136), float)

In [5]: d = array((2048, 3136), float)

In [6]: a = b*c + d

In [7]: a
Out[7]: array([   526336.,  78679104.])

What is the '*' in your expression supposed to mean?

Chuck
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