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<Title>Boost Graph Library: Random Spanning Tree</Title>
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<TT>random_spanning_tree</TT>
</H1>

<P>
<PRE>
<i>// named parameter version</i>
template &lt;class Graph, class Gen, class class P, class T, class R&gt;
void random_spanning_tree(Graph&amp; G,
  Gen&amp; gen,
  const bgl_named_params&lt;P, T, R&gt;&amp; params);

<i>// non-named parameter versions</i>
template &lt;class Graph, class Gen, class PredMap, class WeightMap, class ColorMap&gt;
void random_spanning_tree(const Graph&amp; g, Gen&amp; gen, vertex_descriptor root,
  PredMap pred, WeightMap weight, ColorMap color);
</PRE>

<p>
The <tt>random_spanning_tree()</tt> function generates a random spanning tree
on a directed or undirected graph.  The algorithm used is Wilson's algorithm (<a
href="bibliography.html#wilson96generating">73</a>, based on <!-- (FIXME: add
documentation for loop_erased_random_walk()) <a
href="loop_erased_random_walk.html"> -->loop-erased random walks<!-- </a> -->.  There must
be a path from every non-root vertex of the graph to the root;
the algorithm typically enters an infinite loop when
given a graph that does not satisfy this property, but may also throw the
exception <tt>loop_erased_random_walk_stuck</tt> if the search reaches a vertex
with no outgoing edges.  Both weighted and unweighted versions of
<tt>random_spanning_tree()</tt> are
implemented.  In the unweighted version, all spanning trees are equally likely.
In the weighted version, the probability of a particular spanning tree being
selected is the product of its edge weights.
In the non-named-parameter
version of the algorithm, the unweighted version can be selected by passing an
object of type <tt>static_property_map&lt;double&gt;</tt> as the weight map.
In the named-parameter version, leaving off the <tt>weight_map</tt> parameter
has the same effect.
</p>

<H3>Where Defined</H3>

<P>
<a href="../../../boost/graph/random_spanning_tree.hpp"><TT>boost/graph/random_spanning_tree.hpp</TT></a>

<h3>Parameters</h3>

IN: <tt>const Graph&amp; g</tt>
<blockquote>
  An undirected graph. The graph type must
  be a model of <a href="./IncidenceGraph.html">Incidence Graph</a>
  and <a href="./VertexListGraph.html">Vertex List Graph</a>.<br>
</blockquote>

IN: <tt>Gen&amp; gen</tt>
<blockquote>
  A random number generator. The generator type must
  be a model of <a
  href="../../../doc/html/boost_random/reference.html#boost_random.reference.concepts.uniform_random_number_generator">Uniform
  Random Number Generator</a> or a pointer or reference to such a type.<br>
</blockquote>


<h3>Named Parameters</h3>

IN: <tt>root_vertex(vertex_descriptor root)</tt>
<blockquote>
  This parameter, whose type must be the vertex descriptor type of
  <tt>Graph</tt>, gives the root of the tree to be generated.  The default is
  <tt>*vertices(g).first</tt>.<br>
</blockquote>

UTIL: <tt>color_map(ColorMap color)</tt>
<blockquote>
  This is used by the algorithm to keep track of its progress through
  the graph. The type <tt>ColorMap</tt> must be a model of <a
  href="../../property_map/doc/ReadWritePropertyMap.html">Read/Write
  Property Map</a> and its key type must be the graph's vertex
  descriptor type and the value type of the color map must model
  <a href="./ColorValue.html">ColorValue</a>.<br>
  <b>Default:</b> a <tt>two_bit_color_map</tt> of size
  <tt>num_vertices(g)</tt> and using the <tt>i_map</tt> for the index
  map.<br>
</blockquote>

IN: <tt>vertex_index_map(VertexIndexMap i_map)</tt>
<blockquote>
  This maps each vertex to an integer in the range <tt>[0,
  num_vertices(g))</tt>. This parameter is only necessary when the
  default color property map is used. The type <tt>VertexIndexMap</tt>
  must be a model of <a
  href="../../property_map/doc/ReadablePropertyMap.html">Readable Property
  Map</a>. The value type of the map must be an integer type. The
  vertex descriptor type of the graph needs to be usable as the key
  type of the map.<br>
</blockquote>

OUT: <tt>predecessor_map(PredMap pred)</tt>
<blockquote>
  This map, on output, will contain the predecessor of each vertex in the graph
  in the spanning tree.  The value
  <tt>graph_traits&lt;Graph&gt;::null_vertex()</tt> will be used as the
  predecessor of the root of the tree.  The type <tt>PredMap</tt> must be a
  model of
  <a
  href="../../property_map/doc/ReadWritePropertyMap.html">Read/Write Property
  Map</a>. The key and value types of the map must both be the graph's vertex type.<br>
</blockquote>

IN: <tt>weight_map(WeightMap weight)</tt>
<blockquote>
  This map contains the weight of each edge in the graph.  The probability of
  any given spanning tree being produced as the result of the algorithm is
  proportional to the product of its edge weights.  If the weight map is
  omitted, a default that gives an equal weight to each edge will be used; a
  faster algorithm that relies on constant weights will also be invoked.
  The type <tt>WeightMap</tt> must be a
  model of
  <a
  href="../../property_map/doc/ReadablePropertyMap.html">Readable Property
  Map</a>. The key type of the map must be the graph's edge type, and the value
  type must be a real number type (such as <tt>double</tt>).<br>
</blockquote>

<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy; 2000-2001</TD><TD>
<A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>,
Indiana University (<A
HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
<A HREF="http://www.boost.org/people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br>
<A HREF="http://www.osl.iu.edu/~lums">Andrew Lumsdaine</A>,
Indiana University (<A
HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
</TD></TR></TABLE>

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