MATLAB: Creating a repeating diagonal matrix

matrix

I'm trying to create a diagonal matrix with the following sequence on the main diagonal line where v is the vector v=[4,1,0,1] to get the following matrix
[4 0 0 0 0 0 0
0 1 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 1 0 0 0
0 0 0 0 4 0 …]
but I want it to repeat the same sequence over an over again for a 100 by 100 matrix how would I be able to do this?

Best Answer

You want to repeat that sequence on the main diagonal?
v = [4 1 0 1];
A = diag(repmat(v,1,25));
That is, use repmat to replicate the sequece 25 times. Then use diag to create the associated diagonal matrix.
If you wanted the matrix to be a sparse matrix, you would use spdiags to form the matrix.
v = [4;1;0;1];
A = spdiags(repmat(v,25,1),0,100,100);
In the second form, I used spdiags. We can visualize the matrix as created using spy.
spy(A)
A dot is shown for all non-zero elements. You can even visualize those zero elements on the diagonal, because no dots were shown there.
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