Extracting elements from one matrix according to criteria from another matrix

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MiauMiau 2014-6-28

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评论： MiauMiau 2014-6-28

I have the following problem: I have say 100 3-dimensional points, and a correlation matrix A with the correlation of all the points to each other, hence A has the dimensions 100 x 100.

For all my points I have another matrix B with the coordinates of each point, such that B is a 3 x 100 matrix.

Now I want the following to be done: From my correlation matrix A, I only want to have the correlation of points extracted, where the x coordinate of the point is 0 or bigger than zero, such that I arrive at a new correlation matrix, and also for the matrix B I only want to keep the points which fullfill this criterion. The x values are stored in the first row of the matrix B for each of the 100 points.

How could I do that?

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Answer 1

the cyclist 2014-6-28

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编辑：the cyclist 2014-6-28

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If I understand correctly what you want, I think this does it.

index = B(1,:)>=0;
A = A(index,:);
A = A(:,index);
B = B(:,index);

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the cyclist 2014-6-28

Sorry, I thought your B was 100x3. I corrected my code.

MiauMiau 2014-6-28

Ok, thanks! B is correct now, but A isn't. So for A, since in B only column 1 and 4 are kept, we would need to keep only the correlation between the first point and the fourth. These are encoded in A in (1,1) (correlation of the first point with itself), (4,4) (correlation of the fourth point with itself), then (1,4) and (4,1). How would we do that?

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