[Numpy-discussion] dot documentation

T J tjhnson@gmail....
Fri Aug 7 16:24:24 CDT 2009


Oh.  b.shape = (2,).  So I suppose the second to last dimension is, in
fact, the last dimension...and 2 == 2.

nvm

On Fri, Aug 7, 2009 at 2:19 PM, T J<tjhnson@gmail.com> wrote:
> Hi,  the documentation for dot says that a value error is raised if:
>
>    If the last dimension of a is not the same size as the
> second-to-last dimension of b.
>
> (http://docs.scipy.org/doc/numpy/reference/generated/numpy.dot.htm)
>
> This doesn't appear to be the case:
>
>>>> a = array([[1,2],[3,4]])
>>>> b = array([1,2])
>>>> dot(a,b)
> array([5,11])
>
> I can see *how* 5,11 is obtained, but it seems this should have raised
> a ValueError since the 2 != 1.  So the actual code must do something
> more involved.  When I think about broadcasting, it seems that maybe b
> should have been broadcasted to:
>
> -->  array([[1,2],[1,2]])
>
> and then the multiplication done as normal (but this would give a 2x2 result).
>
> Can someone explain this to me?
>


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