This doesn't exactly answer your question but still may be useful. This example:
shows how to create sparse finite difference matrices for the 1D, 2D, and 3D Poisson equation. For the 3D case, the 7-point stencil is used. For the 1D case, the use of spdiags is straightforward:
K1D = spdiags(ones(n,1)*[-1 2 -1],-1:1,n,n); % 1d Poisson matrix
Possibly you've already seen this simple example. But you'll notice that the kron function is used to create the 2D and 3D matrices. Use of spciags to create these is not nearly as straightforward as in the 1D case.
You might also find this blog useful:
As it explains, the key to efficiently creating sparse matrices in MATLAB is to pre-compute an array of row, column, value triplets and then use a single call to the sparse function to convert these to a sparse matrix.
Bill
