# [SciPy-User] Sparse matrix multiply

Christopher Mutel cmutel@gmail....
Thu Mar 22 05:24:04 CDT 2012

```On Thu, Mar 22, 2012 at 11:18 AM, Jaakko Luttinen
<jaakko.luttinen@aalto.fi> 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.
>
> Actually it seems that np.multiply computes matrix multiplication in
> this case! Below, only A.multiply(B) computes element-wise multiplication.
>
> 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: 25.240683127057885
> A.multiply(B).sum()
> # out: 2.6382118196920503
> A.dot(B).sum()
> # out: 25.240683127057885
> np.dot(A,B).sum()
> # out: 25.240683127057885

Indeed. Sorry for the confusion.
```