Avoid multiplying with 1 in element wise multiplication for large matrices

Hello, I have really large matrices (100000x100000) I want to multiply them element wise. One of them has %90 ones and I would like to avoid multiplying with those. Is there a quick way of doing that? That will save a lot of cpu time.

Answer 1

[Walter Roberson]

Constructing the logical mask and extracting the subset and storing it back, would probably take more time than just having it proceed. However, if the positions of the non-1's is not changing, perhaps it would be worth constructing a sparse logical vector.

Answer 2

[Guillaume]

I'm really not convinced you'll have any gain in speed. Identifying the ones and filtering both input matrix is probably going to take just as long as the multiplication.

%A: a matrix
%B: another matrix the same size as A, with lots of 1.
%C = A .* B
C = A; %default resut is multiplication by 1
isnotone = B ~= 1; %identify the 1s in B
C(isnotone) = A(isnotone) .* B(isnotone); %multiply only those elements for which B is not one.