<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>DFS 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)"/>DFS Visitor Concept</H1> This concept defines the visitor interface for <a href="./depth_first_search.html"><tt>depth_first_search()</tt></a>. Users can define a class with the DFS Visitor interface and pass an object of the class to <tt>depth_first_search()</tt>, thereby augmenting the actions taken during the graph 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 DFS 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</tt></TD> <TD>An object of type <tt>boost::graph_traits<G>::vertex_descriptor</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(s, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on every vertex of the graph before the start of the graph search. </td> </tr> <tr> <td>Start Vertex</td> <td><tt>vis.start_vertex(s, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on the source vertex once before the start of the search. </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>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>Tree Edge</td> <td><tt>vis.tree_edge(e, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on each edge as it becomes a member of the edges that form the search tree.</td> </tr> <tr> <td>Back Edge</td> <td><tt>vis.back_edge(e, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on the back edges in the graph. For an undirected graph there is some ambiguity between tree edges and back edges since the edge <i>(u,v)</i> and <i>(v,u)</i> are the same edge, but both the <tt>tree_edge()</tt> and <tt>back_edge()</tt> functions will be invoked. One way to resolve this ambiguity is to record the tree edges, and then disregard the back-edges that are already marked as tree edges. An easy way to record tree edges is to record predecessors at the <tt>tree_edge</tt> event point. </td> </tr> <tr> <td>Forward or Cross Edge</td> <td><tt>vis.forward_or_cross_edge(e, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on forward or cross edges in the graph. In an undirected graph this method is never called. </td> </tr> <tr> <td>Finish Vertex</td> <td><tt>vis.finish_vertex(u, g)</tt></td> <td><tt>void</tt></td> <td> This is invoked on vertex <tt>u</tt> after <tt>finish_vertex</tt> has been called for all the vertices in the DFS-tree rooted at vertex <tt>u</tt>. If vertex <tt>u</tt> is a leaf in the DFS-tree, then the <tt>finish_vertex</tt> function is call on <tt>u</tt> after all the out-edges of <tt>u</tt> have been examined. </td> </tr> </table> <h3>Models</h3> <ul> <li><a href="./dfs_visitor.html"><tt>dfs_visitor</tt></a> </ul> <a name="python"></a> <h3>Python</h3> To implement a model of the <tt>DFSVisitor</tt> concept in Python, create a new class that derives from the <tt>DFSVisitor</tt> type of the graph, which will be named <tt><i>GraphType</i>.DFSVisitor</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_dfs_visitor(bgl.Graph.DFSVisitor): def __init__(self, name_map): bgl.Graph.DFSVisitor.__init__(self) self.name_map = name_map def tree_edge(self, e, g): (u, v) = (g.source(e), g.target(e)) print "Tree 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>