185 lines
4.5 KiB
HTML
185 lines
4.5 KiB
HTML
<HTML>
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<!--
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Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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-->
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<Head>
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<Title>Boost Graph Library: Bellman Ford Visitor</Title>
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<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
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ALINK="#ff0000">
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<IMG SRC="../../../boost.png"
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ALT="C++ Boost" width="277" height="86">
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<BR Clear>
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<H1><img src="figs/python.gif" alt="(Python)"/>Bellman Ford Visitor Concept</H1>
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This concept defines the visitor interface for <a
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href="./bellman_ford_shortest.html"><tt>bellman_ford_shortest_paths()</tt></a>.
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Users can define a class with the Bellman Ford Visitor interface and
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pass and object of the class to <tt>bellman_ford_shortest_paths()</tt>,
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thereby augmenting the actions taken during the graph search.
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<h3>Refinement of</h3>
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<a href="../../utility/CopyConstructible.html">Copy Constructible</a>
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(copying a visitor should be a lightweight operation).
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<h3>Notation</h3>
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<Table>
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<TR>
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<TD><tt>V</tt></TD>
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<TD>A type that is a model of Bellman Ford Visitor.</TD>
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</TR>
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<TR>
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<TD><tt>vis</tt></TD>
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<TD>An object of type <tt>V</tt>.</TD>
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</TR>
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<TR>
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<TD><tt>G</tt></TD>
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<TD>A type that is a model of Graph.</TD>
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</TR>
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<TR>
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<TD><tt>g</tt></TD>
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<TD>An object of type <tt>G</tt>.</TD>
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</TR>
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<TR>
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<TD><tt>e</tt></TD>
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<TD>An object of type <tt>boost::graph_traits<G>::edge_descriptor</tt>.</TD>
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</TR>
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<TR>
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<TD><tt>s,u</tt></TD>
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<TD>An object of type <tt>boost::graph_traits<G>::vertex_descriptor</tt>.</TD>
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</TR>
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</table>
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<h3>Associated Types</h3>
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none
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<p>
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<h3>Valid Expressions</h3>
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<table border>
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<tr>
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<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
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</tr>
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<tr>
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<td>Examine Edge</td>
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<td><tt>vis.examine_edge(e, g)</tt></td>
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<td><tt>void</tt></td>
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<td>
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This is invoked on every edge in the graph <tt>num_vertices(g)</tt> times.
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</td>
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</tr>
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<tr>
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<td>Edge Relaxed</td>
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<td><tt>vis.edge_relaxed(e, g)</tt></td>
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<td><tt>void</tt></td>
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<td>
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Upon examination, if the following condition holds then the edge
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is relaxed (its distance is reduced), and this method is invoked.<br>
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<tt>
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tie(u,v) = incident(e, g);<br>
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D d_u = get(d, u), d_v = get(d, v);<br>
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W w_e = get(w, e);<br>
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assert(compare(combine(d_u, w_e), d_v));<br>
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</tt>
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</td>
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</tr>
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<tr>
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<td>Edge Not Relaxed</td>
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<td><tt>edge_not_relaxed(e, g)</tt></td>
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<td><tt>void</tt></td>
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<td>
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Upon examination, if the edge is not relaxed (see above) then
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this method is invoked.
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</td>
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</tr>
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<tr>
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<td>Edge Minimized</td>
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<td><tt>vis.edge_minimized(e, g)</tt></td>
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<td><tt>void</tt></td>
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<td>
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After <tt>num_vertices(g)</tt> iterations through the edge set
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of the graph are completed, one last iteration is made to test whether
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each edge was minimized. If the edge is minimized then this function
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is invoked.
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</td>
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</tr>
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<tr>
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<td>Edge Not Minimized</td>
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<td><tt>edge_not_minimized(e, g)</tt></td>
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<td><tt>void</tt></td>
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<td>
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If the edge is not minimized, this function is invoked. This happens
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when there is a negative cycle in the graph.
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</td>
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</tr>
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</table>
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<h3>Models</h3>
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<ul>
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<li><a href="./bellman_visitor.html"><tt>bellman_visitor</tt></a>
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</ul>
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<a name="python"></a>
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<h3>Python</h3>
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To implement a model of the <tt>BellmanFordVisitor</tt> concept in Python,
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create a new class that derives from the <tt>BellmanFordVisitor</tt> type of
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the graph, which will be
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named <tt><i>GraphType</i>.BellmanFordVisitor</tt>. The events and syntax are
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the same as with visitors in C++. Here is an example for the
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Python <tt>bgl.Graph</tt> graph type:
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<pre>
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class count_tree_edges_bellman_ford_visitor(bgl.Graph.BellmanFordVisitor):
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def __init__(self, name_map):
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bgl.Graph.BellmanFordVisitor.__init__(self)
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self.name_map = name_map
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def edge_relaxed(self, e, g):
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(u, v) = (g.source(e), g.target(e))
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print "Relaxed edge ",
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print self.name_map[u],
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print " -> ",
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print self.name_map[v]
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</pre>
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<br>
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<HR>
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<TABLE>
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<TR valign=top>
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<TD nowrap>Copyright © 2000-2001</TD><TD>
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<A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>,
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Indiana University (<A
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HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
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<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>
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<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>,
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Indiana University (<A
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HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
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</TD></TR></TABLE>
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</BODY>
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</HTML>
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