Mapping locations of similar rows in two matrices
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Patrick Mboma
le 1 Nov 2014
Commenté : Patrick Mboma
le 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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Guillaume
le 1 Nov 2014
[~, out] = ismember(A, B, 'rows');
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Guillaume
le 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
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