# [SciPy-User] Sparse matrix multiply

Jaakko Luttinen jaakko.luttinen@aalto...
Thu Mar 22 05:08:47 CDT 2012

```On 03/22/2012 12:07 PM, Jaakko Luttinen wrote:
> 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.

TYPOFIX: I thought that multiply computes element-wise multiplication.

-Jaakko
```