# How to create a 2D diagonal matrix from a 3D matrix with a generic dimension? It could be with a loop structure.

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Mariana on 16 Nov 2017
Answered: Matt J on 16 Nov 2017
%%3D matrix k
n=3;
k=zeros(n)
for a=1:n
for b=1:n
for c=1:n
k(b,c,a)=c;
end
end
end
using blkdiag could be obtained, however for a generic number of matrices n, this doesn't work.
u=blkdiag(k(:,:,1),k(:,:,2),k(:,:,3))
Stephen23 on 16 Nov 2017
What is the expected output? Can you please provide a specific example with input and output arrays.

KL on 16 Nov 2017
Use a cell array and then something like,
c = arrayfun(@(x) k(:,:,x),1:n,'uni',0)
u = blkdiag(c{:})
u =
1 2 3 0 0 0 0 0 0
1 2 3 0 0 0 0 0 0
1 2 3 0 0 0 0 0 0
0 0 0 1 2 3 0 0 0
0 0 0 1 2 3 0 0 0
0 0 0 1 2 3 0 0 0
0 0 0 0 0 0 1 2 3
0 0 0 0 0 0 1 2 3
0 0 0 0 0 0 1 2 3

Matt J on 16 Nov 2017
kcell=num2cell(k,[1,2]);
u=blkdiag(kcell{:});

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