[Numpy-discussion] broacasting question

[Thomas K Gamble]

> 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?

My apologies for the errors in my example.  It should have been:

a = numpy.ndarray((2048,3577), dtype=float)
b = numpy.ndarray((256,25088), dtype=float)
c = numpy.ndarray((2048,3136), dtype=float)
d = numpy.ndarray((2048,3136), dtype=float)

The numbers are the array dimensions.  Data values are not provided in the 
example.

e =  b * c + d

f = a / b

Both of these expressions result in value errors in python but IDL handles 
them without complaint.

The * is a multiplication operator.  IDL also stores its data in 
Fortran/column-major order, which causes some other issues.
 
> 
> Chuck
--
Thomas K. Gamble tkgamble at windstream.net