Thread Subject: Prod of non zero element in a sparse matrix

Hi,
I have a sparse matrix A=[1 0 0 ; -1 0 1;1 -1 0] and I want to get the product of the non zeros element i.e. [-1 -1 1].
I have try to replace all zeros by 1 and then get the product:
A(A==0)=1
B=prod(A)

It does the job but for a 1000X2000 sparse matrix replacing all zeros by ones takes too much time. Any suggestions?

Thanks

"Nihal" wrote in message <j1rm5n$lrj$1@newscl01ah.mathworks.com>...
>
> It does the job but for a 1000X2000 sparse matrix replacing all zeros by ones takes too much time. Any suggestions?
==================


Something like this maybe:

real(exp(sum(spfun(@log,A))))

"Nihal " <nihalsameh@yahoo.com> wrote in message
news:j1rm5n$lrj$1@newscl01ah.mathworks.com...
> Hi,
> I have a sparse matrix A=[1 0 0 ; -1 0 1;1 -1 0] and I want to get the
> product of the non zeros element i.e. [-1 -1 1].
> I have try to replace all zeros by 1 and then get the product:
> A(A==0)=1
> B=prod(A)
>
> It does the job but for a 1000X2000 sparse matrix replacing all zeros by
> ones takes too much time. Any suggestions?

A = sparse([1 2 3 3 2], [1 1 1 2 3], [1, -1, 1, -1, 1])
prod(nonzeros(A))

--
Steve Lord
slord@mathworks.com
To contact Technical Support use the Contact Us link on
http://www.mathworks.com

"Steven_Lord" <slord@mathworks.com> wrote in message <j1ro2e$rod$1@newscl01ah.mathworks.com>...
>
> > It does the job but for a 1000X2000 sparse matrix replacing all zeros by
> > ones takes too much time. Any suggestions?
>
> A = sparse([1 2 3 3 2], [1 1 1 2 3], [1, -1, 1, -1, 1])
> prod(nonzeros(A))
================

Don't think so. The OP asked for the _columnwise_ product of non-zero matrix values.

"Matt J " <mattjacREMOVE@THISieee.spam> wrote in message
news:j1rs10$ca6$1@newscl01ah.mathworks.com...
> "Steven_Lord" <slord@mathworks.com> wrote in message
> <j1ro2e$rod$1@newscl01ah.mathworks.com>...
>>
>> > It does the job but for a 1000X2000 sparse matrix replacing all zeros
>> > by ones takes too much time. Any suggestions?
>>
>> A = sparse([1 2 3 3 2], [1 1 1 2 3], [1, -1, 1, -1, 1])
>> prod(nonzeros(A))
> ================
>
> Don't think so. The OP asked for the _columnwise_ product of non-zero
> matrix values.

You're right; I skimmed the OP's prose and based my response off that. My
mistake.

--
Steve Lord
slord@mathworks.com
To contact Technical Support use the Contact Us link on
http://www.mathworks.com

"Nihal" wrote in message <j1rm5n$lrj$1@newscl01ah.mathworks.com>...
> Hi,
> I have a sparse matrix A=[1 0 0 ; -1 0 1;1 -1 0] and I want to get the product of the non zeros element i.e. [-1 -1 1].
> I have try to replace all zeros by 1 and then get the product:
> A(A==0)=1
> B=prod(A)
>
> It does the job but for a 1000X2000 sparse matrix replacing all zeros by ones takes too much time. Any suggestions?
>
> Thanks

 [i j s] = find(A);
B = accumarray(j,s,[size(A,1) 1], @prod)

% Bruno