How to create a matrix containing the diagonal matrix of another one?

Hi,

I have a diagonal block matrix and I would like to create another matrix that contains only the blocks of the diagonal (eliminating blocks out of the diagonal block). For example

A=[1 2 0 0; 3 4 0 0; 0 0 5 6; 0 0 7 8];

And I want

B=[1 2; 3 4; 5 6; 7 8];

How can I do this? Thanks,

Answer 1

[Azzi Abdelmalek]

EDIT

A=[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]
[n,m]=size(A);
%----------pn and pm are sub-block dimensions---------------
pn=2;
pm=2;
%-----------------------------------------------------------
ii=repmat(1:n,pm,1)
jj=reshape(1:m,pm,[])'
jj=repmat(jj,1,pn)'
idx=sub2ind(size(A),ii(:),jj(:))
A(idx)
B=reshape(A(idx),pm,[])'

[Fernando]

Thanks Azzi. However, can you explain me how this works more in general. I modified the A matrix to be

A=[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]

so the block dimension parameter is the same as before, but now the output is not what I expected, and I'm not sure why.

[Azzi Abdelmalek]

Look at edit

[Fernando]

Great, thanks!