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indexing diagonals out of a 4d matrix

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Neekar Mohammed
Neekar Mohammed 2019-6-23
评论： Matt J 2019-6-25
I would like to extract diag elemnts of a 4d matrix and put them in 2d matrix. Any help would be much appreciated.
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John D'Errico
John D'Errico 2019-6-24
Give an example, as the diagonal of a 4-d matrix has no definition. If anything, as Bjorn points out, the result would be ONE dimensional, a vector. So wanting it to become a 2-d matrix makes little sense. That means you need to be clear and specific as to your intent.
Neekar Mohammed
Neekar Mohammed 2019-6-24
Thank you very much for your comments. I have a correlation data which is a 4D matrix, let's say g(x1,y1,x2,y2) I want to calculate the intensity which is a 2D matrix f(x,y)=g where x1=x2 and y1=y2. So yes I want it to become a 2D matrix. Many thanks. Neekar

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Matt J
Matt J 2019-6-25
编辑：Matt J 2019-6-25
[M1,N1,M2,N2]=size(g);
M=min(M1,M2); N=min(N1,N2);
f=diag(reshape(g(1:M,1:N,1:M,1:N),M*N,[]));
f=reshape(f,M,N);
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Matt J
Matt J 2019-6-25
Nekar's comment moved here:
Thank you very much for fast respond. If is like g(x1,x2,y1,y2) (I changed the order) then f(x,y)=g for x1=x2 and y1=y2, Would be the same answer you suggested?
Matt J
Matt J 2019-6-25
No, you would have to pre-permute g into (x1,y1,x2,y2) ordering for it to work
g=permute(g,[1,3,2,4]);
For this reason and others, I recommend that you do not use (x1,x2,y1,y2) ordering. It will necessitate a lot of extra manipulation.

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