## SPY Visualize Sparsity Pattern of a Sparse Matrix

Section: Sparse Matrix Support

### Usage

Plots the sparsity pattern of a sparse matrix. The syntax for its use is

       spy(x)

which uses a default color and symbol. Alternately, you can use

       spy(x,colspec)

where `colspec` is any valid color and symbol spec accepted by `plot`.

### Example

First, an example of a random sparse matrix.

    --> y = sprand(1000,1000,.001);
    --> spy(y,'ro')

which is shown here

Here is a sparse matrix with a little more structure. First we build a sparse matrix with block diagonal structure, and then use `spy` to visualize the structure.

    --> A = sparse(1000,1000);
    --> for i=1:25; A((1:40) + 40*(i-1),(1:40) + 40*(i-1)) = 1; end;
    --> spy(A,'gx')

with the result shown here