How can I create a sparse matrix containing (3,3) block matrices on the main diagonal and on diagonals below and above the main diagonal without using loops?
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The idea is to create a (3m,3m) sparse matrix with small (3,3) matrices on the main diagonal and on diagonals below and above the main diagonal. By saying this, I want that each time the main diagonal of the small (3,3) matrix is on the main diagonal or on another diagonal respectively. There is no gap between the small (3,3) matrices on the diagonals. How can I use the functions sparse, spdiags, blkdiag to create this matrix? If there are other functions guaranteeing sparsity that's fine.
2 Comments
Cedric
on 19 May 2013
Did you generate these m 3x3 sparse matrices already or do you want to avoid building them and build directly the block diagonal large sparse matrix? If you have these small matrices already defined, what have you tried so far using BLKDIAG?
Accepted Answer
Cedric
on 20 May 2013
BLKDIAG will give you a sparse matrix if one of its inputs is sparse. Try building the 3x3 matrices as sparse, or converting them (or just one) to sparse, before using BLKDIAG; it should work fine.
More Answers (1)
Iain
on 20 May 2013
You can initialise a sparse matrix as:
matrix = sparse(zeros(3*m,3*m));
If you then use it as:
matrix(1:3,1:3) = [a b c; d e f; g h i];
matrix(3+1:3,3+1:3) = [j k l; m n o; p q r];
... etc you will get what I think you're asking for.
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