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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<TITLE>SPY Visualize Sparsity Pattern of a Sparse Matrix
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<H2>SPY Visualize Sparsity Pattern of a Sparse Matrix
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Section: <A HREF=sec_sparse.html> Sparse Matrix Support </A>
<H3>Usage</H3>
Plots the sparsity pattern of a sparse matrix. The syntax for its use is
<PRE>
spy(x)
</PRE>
<P>
which uses a default color and symbol. Alternately, you can use
<PRE>
spy(x,colspec)
</PRE>
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where <code>colspec</code> is any valid color and symbol spec accepted by <code>plot</code>.
<H3>Example</H3>
First, an example of a random sparse matrix.
<PRE>
--> y = sprand(1000,1000,.001);
--> spy(y,'ro')
</PRE>
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which is shown here
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<DIV ALIGN="CENTER">
<IMG SRC="spy1.png">
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<P>
Here is a sparse matrix with a little more structure. First we build a
sparse matrix with block diagonal structure, and then use <code>spy</code> to
visualize the structure.
<PRE>
--> A = sparse(1000,1000);
--> for i=1:25; A((1:40) + 40*(i-1),(1:40) + 40*(i-1)) = 1; end;
--> spy(A,'gx')
</PRE>
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with the result shown here
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<DIV ALIGN="CENTER">
<IMG SRC="spy2.png">
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