How to create a block diagonal matrix from the slices of a 3D matrix
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I have a large 3D matrix (K) composed of 200 slices , and I want to create a block diagonal matrix from the slices of it without looping. For example
K(:,:,1) =[1 2;3 4]
K(:,:,2) =[5 6;7 8]
K(:,:,3) =[9 10;11 12]
.
.
.
K(:,:,200) =[1 10;13 15]
if we assume the number of slice is 3, the output should be
1 2 0 0 0 0
3 4 0 0 0 0
0 0 5 6 0 0
0 0 7 8 0 0
0 0 0 0 9 10
0 0 0 0 11 12
But how the block diagonal matrix created from all the 200 slices without looping?
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Accepted Answer
Azzi Abdelmalek
on 2 Oct 2014
k(:,:,1) =[1 2;3 4]
k(:,:,2) =[5 6;7 8]
k(:,:,3) =[9 10;11 12]
q=num2cell(k,[1,2])
blkdiag(q{:})
4 Comments
Matt J
on 2 Oct 2014
Note that this still uses a for-loop inside num2cell.m, though how much that hurts you, I don't know.
Note also that my 2nd answer here was the same as this, except that it returns the result in sparse form, which is normally more desirable for block diagonal matrices with small blocks.
More Answers (3)
Azzi Abdelmalek
on 2 Oct 2014
k(:,:,1) =[1 2;3 4]
k(:,:,2) =[5 6;7 8]
k(:,:,3) =[9 10;11 12]
out=[];
for ii=1:size(k,3)
out=blkdiag(out,k(:,:,ii))
end
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