How to compare some specific entries of a matrix, if I have their indices stored in 2 files?

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Aditya Wahi
Aditya Wahi 2017-7-7
编辑： José-Luis 2017-7-7
I have 2 files, that have numbers that represent rows and columns respectively, out of which I want to pick a pair at a time, and compare that corresponding element from a given matrix. For example, if I have matrices A and B, such that:-
A= [1,5,9] and B= [2,6,11]
Then I want to compare the (1,2), (5,6) and (9,11) elements of a matrix C, and return the indices of the largest value.
Originally, my matrix A and B are 19007x1 in size. And my matrix C is 5601x5601 in size.

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采纳的回答

José-Luis
José-Luis 2017-7-7
idx = sub2ind(size(C), A, B)
result = max(C(idx));
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Guillaume
Guillaume 2017-7-7
编辑：Guillaume 2017-7-7
The problem is with the find(C == max(C(idx))), you can't use find on the full C matrix, you have to do it on the subset indexed by idx, so:
find(C(idx) == max(C(idx)))
edit, after José-Luis edit while I was writing my comment: while the new code may provide correct results most of the time, this is still wrong. As it searches the whole C matrix, not just the subset provided by A and B. Try with example:
A = 1;
B = 1;
C = [0 0; 0 0];
José-Luis
José-Luis 2017-7-7
编辑：José-Luis 2017-7-7
True, didn't think of that. Yet another edit.
A = [ 1 , 2 , 7 ];
B = [ 5 , 4 , 8];
C = rand(10);
idx = sub2ind(size(C),A,B);
[idx_x, idx_y] = find(C(idx) == max(C(idx)));
x = A(idx_x);
y = B(idx_y);

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更多回答（1 个）

Guillaume
Guillaume 2017-7-7
As per José-Luis' answer, you have to use sub2ind to convert your 2D indexing from A and B into linear indices. This would work:
index = sub2ind(size(C), A, B);
[~, row] = max(C(index));
row is the row in A and B where the maximum is found. If there are several locations where it is found, then you only get the first one. If you want all of them:
Csearch = C(sub2ind(size(C), A, B));
rows = find(Csearch == max(Csearch));

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