[Numpy-discussion] Matrix dot product over an axis(for a 3d array/list of matrices)

Charles R Harris charlesr.harris@gmail....
Thu Jul 15 11:58:54 CDT 2010

On Thu, Jul 15, 2010 at 10:38 AM, Emmanuel Bengio <bengioe@gmail.com> wrote:

> Hello,
> I have a list of 4x4 transformation matrices, that I want to "dot with"
> another list of the same size (elementwise).
> Making a for loop that calculates the dot product of each is extremely
> slow,
> I thought that maybe it's due to the fact that I have thousands of matrices
> and it's a python for loop and there's a high Python overhead.
> I do something like this:
> >> for a,b in izip(Rot,Trans):
> >>     c.append(numpy.dot(a,b))
> Is there a way to do this in one instruction?
> Or is there a way to do this all using weave.inline?
Yes, there is a trick for this using a multiply with properly placed newaxis
followed by a sum. It uses more memory but for stacks of small arrays that
shouldn't matter. See the post

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