[Numpy-discussion] broacasting question

[Charles R Harris]

On Fri, Jul 1, 2011 at 6:38 PM, Thomas K Gamble <tkgamble at windstream.net>wrote:

> > 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.
>
>
Ah, what that does in make (2,) arrays with the given elements and '*' and
'+' are element-wise multiplication and addition. To get arrays with the
dimensions you need something like

In [19]: a = numpy.zeros((2048,3577), dtype=float)

In [20]: b = numpy.zeros((256,25088), dtype=float)

In [21]: c = numpy.zeros((2048,3136), dtype=float)

In [22]: d = numpy.zeros((2048,3136), dtype=float)

However, broadcasting b and c won't work, it isn't enough that 256 divides
2048, 256 must actually equal 1, which you can omit since it is a leading
index. Same with 3136 and 25088 except the one won't automagically be added.

So you can do things like

 In [24]: numpy.zeros((5,7))*numpy.zeros((7,))
Out[24]:
array([[ 0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.]])

second array will be broadcast over the first.

Chuck
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/numpy-discussion/attachments/20110701/1335bb45/attachment.html>