Mapping locations of similar rows in two matrices

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Patrick Mboma on 1 Nov 2014

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Commented: Patrick Mboma on 1 Nov 2014

Dear all,

I have two matrices A and B of respective sizes [na,m] and [nb,m], with na>=nb. Both matrices have the same number of columns. All the rows in B are unique while the rows in A can be duplicated, but are all members of B. I would like to create a mapping between A and B and one very inefficient way to do that is as follows:

out=nan(na,1);
for ii=1:na
    % for each row of A, I search its location in B
    out(ii)=find(all(bsxfun(@minus,A(ii,:),B),2)==0);
end

This works well but becomes prohibitively expensive for large na and nb. And so my question is whether there is a way of getting rid of the for loop and/or creating this mapping differently.

Thanks,

P.

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Answer 1

Guillaume on 1 Nov 2014

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This is exactly what ismember is for. The 2nd output argument will give you the locations:

[~, out] = ismember(A, B, 'rows');

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Guillaume on 1 Nov 2014

For that specialised function to be any faster than ismember you would have to write it as a mex file.

And if ismember(..., 'rows') is implemented efficiently, it's possible the only saving you could get would be to skip the parsing of the 'rows' argument

Patrick Mboma on 1 Nov 2014