[SciPy-User] Sparse matrix multiply

Jaakko Luttinen jaakko.luttinen@aalto...
Thu Mar 22 05:07:23 CDT 2012


On 03/22/2012 12:03 PM, Christopher Mutel wrote:
> On Thu, Mar 22, 2012 at 10:34 AM, Jaakko Luttinen
> <jaakko.luttinen@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.

Is it so..?

http://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.csc_matrix.multiply.html

I don't know what "point-wise multiplication" means..

Anyway, I thought that dot computes matrix multiplication and multiply
computes matrix multiplication.

-Jaakko


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