using find(ismember) instead of a loop?
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I have matrix B which is has around 1 million rows and Matrix A which has 800,000 rows. I want to match the observations of A and B and then assign to A the observations of B that matches. I tried a loop which taking forever in terms of time. I am wondering if I can do it in one shot using find(ismember). below are my codes:
for i=1:size(B,1)
x0=find(A(:,2)==B(i,2) & A(:,1)==B(i,1));
B(i,8)=A(x0,9);
end
2 commentaires
Matt J
le 4 Oct 2016
Modifié(e) : Matt J
le 4 Oct 2016
I suspect you haven't shown your actual code, making it hard to interpret what you are trying to do. In the posted code, you are comparing columns of A with rows of B which should be producing errors like the following, unless you are using R2016b
>> [1 2 3] == [1;2;3]
Error using ==
Matrix dimensions must agree.
Also, the code you've shown uses only the 1st, 2nd, and 9th column of A. So, why is it significant that "A has 800000 rows"?
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Guillaume
le 4 Oct 2016
Modifié(e) : Guillaume
le 4 Oct 2016
Note that the find was completely unnecessary in your original example, you could just have used the logical array returned by the comparison:
x0=A(:,2)==B(i,2) & A(:,1)==B(i,1);
B(i,8)=A(x0,9); %would have worked exactly the same with marginal speed gain.
To answer your question:
[found, Arow] = ismember(B(:, 1:2), A(:, 1:2), 'rows');
B(found, 8) = A(Arow(found), 9);
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