MATLAB Answers

indexing diagonals out of a 3d matrix

Asked by Alberto Navarro on 9 Jul 2013
Accepted Answer by Matt J

I thought this would be fairly simple. I thought the diag function should be able to do this task. I have a 14x14x1045 matrix Ht, t=1,...,1045 simbolizes the number of observations. I merely want to extract the main daigonals of Ht for all t and store them in a new 3d matrix Dt which is also 14x14x1045 which contains the diagonal elements of every Ht and zeros in all other positions. However, when using the function diag(Ht) I get the following error: Error using diag First input must be 2D.

Is there a function that works like diag but for 3d matrices? I have no experience programing any functions. Could someone please provide a simple loop to make a function that fulfills this task in case no such function exists already?

I´m sorry if this question is too trivial, but I haven´t been able to find a specific answer and I seem not to be able to program such a function with my expertise level...

Thanks a lot for your help!



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3 Answers

Matt J
Answer by Matt J
on 9 Jul 2013
Edited by Matt J
on 9 Jul 2013
 Accepted answer
D=bsxfun(@times, eye(size(H(:,:,1))), H)


Matt J
on 9 Jul 2013

You could also use ndSparse (Available Here) to improve memory efficiency,

    D=bsxfun(@times, ndSparse(speye(size(H(:,:,1)))), H);
Alberto Navarro on 9 Jul 2013

Great! Thanks a lot for the super fast and efficient answer!

Best regards!

Answer by Evan
on 9 Jul 2013
Edited by Evan
on 9 Jul 2013
A = rand(14,14,1045);
B = A(logical(repmat(eye(size(A(:,:,1))),[1 1 size(A,3)])))


Answer by Kiran Sagar
on 9 Jul 2013

I think this would work elegantly. But I don't know if this would take up alot of time/memory for your purposes.

for i=1:1045

  1 Comment

Alberto Navarro on 9 Jul 2013

Memory/time wasn´t an issue. That worked fine as well. I solved it with the first answer, but with this one, I now know how to use the for loop in the command window. Thanks for the suggestion!

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