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| Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000 |
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| <Head> |
| <Title>DFS Visitor</Title> |
| <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" |
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| <IMG SRC="../../../boost.png" |
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| <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 called 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> |
| |
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