Putting one matrix a varying number of times along the diagonal of another matrix
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I have a matrix A which is 6x6. I need to create another matrix where A is along the diagonal with the other entries being 0. I know I could use blkdiag() for this, but the issue is I need to have as many entries of A as the value of another variable that appears earlier in the code. Let's call it n. Sometimes n could be 2, so the matrix would be a 12x12 with A along the diagonal. Sometimes n could be 100 so the ouput matrix would be 600x600 since A needs to appear 100 times. How could I go about automating this process so I don't have to write blkdiag(A,A,A,A...)? Thanks!
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Accepted Answer
John D'Errico
on 23 Jun 2022
A = rand(3);
n = 50;
Acell = repmat({sparse(A)},[1,n]);
B = blkdiag(Acell{:});
spy(B)
Note that I made A sparse in there, so that the final resulting matrix will also be sparse.
4 Comments
Steven Lord
on 23 Jun 2022
M = magic(3)
N = 4;
C = repmat({M}, 1, N)
B = blkdiag(C{:}) % Equivalent to blkdiag(M, M, M, M)
Search the documentation for "comma-separated list" for an explanation of how that last line works.
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