[SciPy-User] Sparse matrix multiply

[Christopher Mutel]

On Thu, Mar 22, 2012 at 10:34 AM, Jaakko Luttinen
<jaakko.luttinen at aalto.fi> wrote:
> Hi!
>
> Why do I get two different results for the code below?
>
> import numpy as np
> import scipy.sparse as sp
> A = sp.rand(20,20,density=0.1)
> B = sp.rand(20,20,density=0.1)
> np.multiply(A,B).sum()
> # out: 21.058793740984925
> A.multiply(B).sum()
> # out: 0.76482546226069481
>
> Am I missing something?
> I think numpy.multiply should either return the correct answer or an
> error that it can't compute the correct answer.

np.multiply performs element-wise multiplication, while A.multiply is
matrix multiplication. They are both "correct", but answer different
questions.

See:
http://en.wikipedia.org/wiki/Matrix_multiplication
http://docs.scipy.org/doc/numpy/reference/generated/numpy.multiply.html

> Regards,
> Jaakko
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Chris Mutel
Ökologisches Systemdesign - Ecological Systems Design
Institut f.Umweltingenieurwissenschaften - Institute for Environmental
Engineering
ETH Zürich - HIF C 44 - Schafmattstr. 6
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