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1<HTML>
2<!--
3  -- Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000
4  --
5  -- Distributed under the Boost Software License, Version 1.0.
6  -- (See accompanying file LICENSE_1_0.txt or copy at
7  -- http://www.boost.org/LICENSE_1_0.txt)
8  -->
9<Head>
10<Title>DFS Visitor</Title>
11<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" 
12        ALINK="#ff0000"> 
13<IMG SRC="../../../boost.png" 
14     ALT="C++ Boost" width="277" height="86"> 
15
16<BR Clear>
17
18<H1><img src="figs/python.gif" alt="(Python)"/>DFS Visitor Concept</H1>
19
20This concept defines the visitor interface for <a
21href="./depth_first_search.html"><tt>depth_first_search()</tt></a>.
22Users can define a class with the DFS Visitor interface and pass an
23object of the class to <tt>depth_first_search()</tt>, thereby
24augmenting the actions taken during the graph search.
25
26<h3>Refinement of</h3>
27
28<a href="../../utility/CopyConstructible.html">Copy Constructible</a>
29(copying a visitor should be a lightweight operation).
30
31<h3>Notation</h3>
32
33<Table>
34<TR>
35<TD><tt>V</tt></TD>
36<TD>A type that is a model of DFS Visitor.</TD>
37</TR>
38
39<TR>
40<TD><tt>vis</tt></TD>
41<TD>An object of type <tt>V</tt>.</TD>
42</TR>
43
44<TR>
45<TD><tt>G</tt></TD>
46<TD>A type that is a model of Graph.</TD>
47</TR>
48
49<TR>
50<TD><tt>g</tt></TD>
51<TD>An object of type <tt>G</tt>.</TD>
52</TR>
53
54<TR>
55<TD><tt>e</tt></TD>
56<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::edge_descriptor</tt>.</TD>
57</TR>
58
59<TR>
60<TD><tt>s,u</tt></TD>
61<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::vertex_descriptor</tt>.</TD>
62</TR>
63
64</table>
65
66<h3>Associated Types</h3>
67
68none
69<p>
70
71<h3>Valid Expressions</h3>
72
73<table border>
74<tr>
75<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
76</tr>
77
78<tr>
79<td>Initialize Vertex</td>
80<td><tt>vis.initialize_vertex(s, g)</tt></td>
81<td><tt>void</tt></td>
82<td>
83This is invoked on every vertex of the graph before the start of the
84graph search.
85</td>
86</tr>
87
88<tr>
89<td>Start Vertex</td>
90<td><tt>vis.start_vertex(s, g)</tt></td>
91<td><tt>void</tt></td>
92<td>
93This is invoked on the source vertex once before the start of the
94search.
95</td>
96</tr>
97
98<tr>
99<td>Discover Vertex</td>
100<td><tt>vis.discover_vertex(u, g)</tt></td>
101<td><tt>void</tt></td>
102<td>
103This is invoked when a vertex is encountered for the first time.
104</td>
105</tr>
106
107<tr>
108<td>Examine Edge</td>
109<td><tt>vis.examine_edge(e, g)</tt></td>
110<td><tt>void</tt></td>
111<td>
112This is invoked on every out-edge of each vertex after it is discovered.
113</td>
114</tr>
115
116
117<tr>
118<td>Tree Edge</td>
119<td><tt>vis.tree_edge(e, g)</tt></td>
120<td><tt>void</tt></td>
121<td>
122This is invoked on each edge as it becomes a member of the edges that
123form the search tree.</td>
124</tr>
125
126<tr>
127<td>Back Edge</td>
128<td><tt>vis.back_edge(e, g)</tt></td>
129<td><tt>void</tt></td>
130<td>
131This is invoked on the back edges in the graph.  For an undirected
132graph there is some ambiguity between tree edges and back edges since
133the edge <i>(u,v)</i> and <i>(v,u)</i> are the same edge, but both the
134<tt>tree_edge()</tt> and <tt>back_edge()</tt> functions will be
135invoked. One way to resolve this ambiguity is to record the tree
136edges, and then disregard the back-edges that are already marked as
137tree edges.  An easy way to record tree edges is to record
138predecessors at the <tt>tree_edge</tt> event point.
139</td>
140</tr>
141
142<tr>
143<td>Forward or Cross Edge</td>
144<td><tt>vis.forward_or_cross_edge(e, g)</tt></td>
145<td><tt>void</tt></td>
146<td>
147This is invoked on forward or cross edges in the graph. In an
148undirected graph this method is never called.
149</td>
150</tr>
151
152<tr>
153<td>Finish Vertex</td>
154<td><tt>vis.finish_vertex(u, g)</tt></td>
155<td><tt>void</tt></td>
156<td>
157This is invoked on vertex <tt>u</tt> after <tt>finish_vertex</tt> has
158been called for all the vertices in the DFS-tree rooted at vertex
159<tt>u</tt>. If vertex <tt>u</tt> is a leaf in the DFS-tree, then
160the <tt>finish_vertex</tt> function is call on <tt>u</tt> after
161all the out-edges of <tt>u</tt> have been examined.
162</td>
163</tr>
164
165</table>
166
167<h3>Models</h3>
168
169<ul>
170 <li><a href="./dfs_visitor.html"><tt>dfs_visitor</tt></a>
171</ul>
172
173<a name="python"></a>
174<h3>Python</h3>
175
176To implement a model of the <tt>DFSVisitor</tt> concept in Python,
177create a new class that derives from the <tt>DFSVisitor</tt> type of
178the graph, which will be
179named <tt><i>GraphType</i>.DFSVisitor</tt>. The events and syntax are
180the same as with visitors in C++. Here is an example for the
181Python <tt>bgl.Graph</tt> graph type:
182
183<pre>
184class count_tree_edges_dfs_visitor(bgl.Graph.DFSVisitor):
185  def __init__(self, name_map):
186    bgl.Graph.DFSVisitor.__init__(self)
187    self.name_map = name_map
188
189  def tree_edge(self, e, g):
190    (u, v) = (g.source(e), g.target(e))
191    print "Tree edge ",
192    print self.name_map[u],
193    print " -> ",
194    print self.name_map[v]
195</pre>
196
197<br>
198<HR>
199<TABLE>
200<TR valign=top>
201<TD nowrap>Copyright &copy 2000-2001</TD><TD>
202<A HREF="../../../people/jeremy_siek.htm">Jeremy Siek</A>,
203Indiana University (<A
204HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
205<A HREF="../../../people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br>
206<A HREF=http://www.osl.iu.edu/~lums>Andrew Lumsdaine</A>,
207Indiana University (<A
208HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
209</TD></TR></TABLE>
210
211</BODY>
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