MATLAB Mathematics - SERC - Index of

Viewing Sparse Matrices
Note that initially nnz has the same value as nzmax by default. That is, the
number of nonzero elements is equivalent to the number of storage locations
allocated for nonzeros. However, MATLAB does not dynamically release
memory if you zero out additional array elements. Changing the value of some
matrix elements to zero changes the value of nnz, but not that of nzmax.
However, you can add as many nonzero elements to the matrix as desired. You
are not constrained by the original value of nzmax.
Viewing Sparse Matrices Graphically
It is often useful to use a graphical format to view the distribution of the
nonzero elements within a sparse matrix. The MATLAB spy function produces
a template view of the sparsity structure, where each point on the graph
represents the location of a nonzero array element.
For example,
spy(west0479)
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0 100 200 300 400
nz = 1887
6-15