Sparse for element by element operation
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If we want to perform the product of two sparse matrices or sparse and a full matrix , sparse function is very useful as it avoids multiplications with zero. However when we do element by element multiplication of two sparse matrices or a sparse matrix and a full matrix it is observed that it takes a considerable amount of time as compared to the multiplication of full matrices. Here is a concrete example
A = sprand(1000,1000,0.005);
B = rand(1000,1000);
Af = full(A);
timeit(@() Af.*B)
ans =
5.1227e-04
timeit(@() A.*B)
ans =
0.0011
How do I tackle this issue when I am looking for a considerable lower time than the multiplication of two full matrices?
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Answers (2)
Bruno Luong
on 15 Sep 2018
Edited: Bruno Luong
on 15 Sep 2018
A = sprand(1000,1000,0.005);
B = rand(1000,1000);
tic
Af = full(A);
AB=Af.*B;
toc % Elapsed time is 0.011628 seconds.
tic
AB=A.*B;
toc % Elapsed time is 0.004143 seconds.
Recommended solution
tic
[i,j,a]=find(A);
b=B(sub2ind(size(B),i,j));
AB=sparse(i,j,a.*b);
toc % Elapsed time is 0.001570 seconds.
3 Comments
Bruno Luong
on 16 Sep 2018
Edited: Bruno Luong
on 16 Sep 2018
Why you think my code assumes some loop or not ? It just carries out the element-wise product A.*B like other methods.
Bruno Luong
on 15 Sep 2018
Edited: Bruno Luong
on 15 Sep 2018
[i,j]=find(A);
b=B(sub2ind(size(B),i,j));
AB=setsparse(A,i,j,b,@times);
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