Creating a super diagonal matrix.

Please what's a neat way of creating a matrix that looks like this?

[1 2 3 4 5 0 0 0 0 0; 0 1 2 3 4 5 0 0 0 0; 0 0 1 2 3 4 5 0 0 0; 0 0 0 1 2 3 4 5 0 0; 0 0 0 0 1 2 3 4 5 0]

Thank you.

[Cedric]

Is it just a small example or is it the real matrix that you need to build? If it is a small example, what will be the size of the real one, and what is the size of the non-zero band?

Answer 1

[John D'Errico]

A gazillion ways. A few suggestions...

1. Simple is to create this as a circulant matrix. I've got a tool for that on the file exchange as I recall.

2. Or, use spdiags. This has the virtue of making it a sparse matrix.

3. Or create it as the sum of multiple diagonal matrices, each built using diag.

4. A matrix with the pattern you show can be built with a call to sparse, where you need to create a list of non-zero elements, and the row and column indices for each. This can be easier than it sounds. Again, you get a sparse result.

5. The simple pattern shown can easily enough be built using meshgrid and a test, possibly with a call to tril or triu afterwards.

Why not play with some or more of the ideas I posed above? This is the way to learn, by playing around with a goal in mind.

[Mike]

It's a small example, the matrix I'm trying to build is a 501 x 501 matrix with the non-zeros being 1:1:100. I'll go through your suggestions to see which I can use. Thanks John!