<HTML> <!-- Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000 Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> <Head> <Title>Boost Graph Library: Dijkstra Visitor</Title> <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" ALINK="#ff0000"> <IMG SRC="../../../boost.png" ALT="C++ Boost" width="277" height="86"> <BR Clear> <H1><img src="figs/python.gif" alt="(Python)"/>Dijkstra Visitor Concept</H1> This concept defines the visitor interface for <a href="./dijkstra_shortest_paths.html"><tt>dijkstra_shortest_paths()</tt></a> and related algorithms. The user can create a class that matches this interface, and then pass objects of the class into <tt>dijkstra_shortest_paths()</tt> to augment the actions taken during the search. <h3>Refinement of</h3> <a href="../../utility/CopyConstructible.html">Copy Constructible</a> (copying a visitor should be a lightweight operation). <h3>Notation</h3> <Table> <TR> <TD><tt>V</tt></TD> <TD>A type that is a model of Dijkstra Visitor.</TD> </TR> <TR> <TD><tt>vis</tt></TD> <TD>An object of type <tt>V</tt>.</TD> </TR> <TR> <TD><tt>G</tt></TD> <TD>A type that is a model of Graph.</TD> </TR> <TR> <TD><tt>g</tt></TD> <TD>An object of type <tt>G</tt>.</TD> </TR> <TR> <TD><tt>e</tt></TD> <TD>An object of type <tt>boost::graph_traits<G>::edge_descriptor</tt>.</TD> </TR> <TR> <TD><tt>s,u,v</tt></TD> <TD>An object of type <tt>boost::graph_traits<G>::vertex_descriptor</tt>.</TD> </TR> <TR> <TD><tt>DistanceMap</tt></TD> <TD>A type that is a model of <a href="../../property_map/doc/ReadWritePropertyMap.html">Read/Write Property Map</a>.</TD> </TR> <TR> <TD><tt>d</tt></TD> <TD>An object of type <tt>DistanceMap</tt>.</TD> </TR> <TR> <TD><tt>WeightMap</tt></TD> <TD>A type that is a model of <a href="../../property_map/doc/ReadWritePropertyMap.html">Readable Property Map</a>.</TD> </TR> <TR> <TD><tt>w</tt></TD> <TD>An object of type <tt>DistanceMap</tt>.</TD> </TR> </table> <h3>Associated Types</h3> none <p> <h3>Valid Expressions</h3> <table border> <tr> <th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th> </tr> <tr> <td>Initialize Vertex</td> <td><tt>vis.initialize_vertex(u, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked one each vertex of the graph when it is initialized. </td> </tr> <tr> <td>Examine Vertex</td> <td><tt>vis.examine_vertex(u, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on a vertex as it is popped from the queue. This happens immediately before <tt>examine_edge()</tt> is invoked on each of the out-edges of vertex <tt>u</tt>. </td> </tr> <tr> <td>Examine Edge</td> <td><tt>vis.examine_edge(e, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on every out-edge of each vertex after it is discovered. </td> </tr> <tr> <td>Discover Vertex</td> <td><tt>vis.discover_vertex(u, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked when a vertex is encountered for the first time. </td> </tr> <tr> <td>Edge Relaxed</td> <td><tt>vis.edge_relaxed(e, g)</tt></td> <td><tt>void</tt></td> <td> Upon examination, if the following condition holds then the edge is relaxed (its distance is reduced), and this method is invoked.<br> <tt> tie(u,v) = incident(e, g);<br> D d_u = get(d, u), d_v = get(d, v);<br> W w_e = get(w, e);<br> assert(compare(combine(d_u, w_e), d_v));<br> </tt> </td> </tr> <tr> <td>Edge Not Relaxed</td> <td><tt>vis.edge_not_relaxed(e, g)</tt></td> <td><tt>void</tt></td> <td> Upon examination, if the edge is not relaxed (see above) then this method is invoked. </td> </tr> <tr> <td>Finish Vertex</td> <td><tt>vis.finish_vertex(u, g)</tt></td> <td><tt>void</tt></td> <td> This invoked on a vertex after all of its out edges have been added to the search tree and all of the adjacent vertices have been discovered (but before their out-edges have been examined). </td> </tr> </table> <h3>Models</h3> <ul> <li><a href="./dijkstra_visitor.html"><tt>dijkstra_visitor</tt></a> </ul> <a name="python"></a> <h3>Python</h3> To implement a model of the <tt>DijkstraVisitor</tt> concept in Python, create a new class that derives from the <tt>DijkstraVisitor</tt> type of the graph, which will be named <tt><i>GraphType</i>.DijkstraVisitor</tt>. The events and syntax are the same as with visitors in C++. Here is an example for the Python <tt>bgl.Graph</tt> graph type: <pre> class count_tree_edges_dijkstra_visitor(bgl.Graph.DijkstraVisitor): def __init__(self, name_map): bgl.Graph.DijkstraVisitor.__init__(self) self.name_map = name_map def edge_relaxed(self, e, g): (u, v) = (g.source(e), g.target(e)) print "Relaxed edge ", print self.name_map[u], print " -> ", print self.name_map[v] </pre> <br> <HR> <TABLE> <TR valign=top> <TD nowrap>Copyright © 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> </BODY> </HTML>