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13<Head>
14<Title>DFS Visitor</Title>
15<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" 
16        ALINK="#ff0000"> 
17<IMG SRC="../../../boost.png" 
18     ALT="C++ Boost" width="277" height="86"> 
19
20<BR Clear>
21
22<H1><img src="figs/python.gif" alt="(Python)"/>DFS Visitor Concept</H1>
23
24This concept defines the visitor interface for <a
25href="./depth_first_search.html"><tt>depth_first_search()</tt></a>.
26Users can define a class with the DFS Visitor interface and pass an
27object of the class to <tt>depth_first_search()</tt>, thereby
28augmenting the actions taken during the graph search.
29
30<h3>Refinement of</h3>
31
32<a href="../../utility/CopyConstructible.html">Copy Constructible</a>
33(copying a visitor should be a lightweight operation).
34
35<h3>Notation</h3>
36
37<Table>
38<TR>
39<TD><tt>V</tt></TD>
40<TD>A type that is a model of DFS Visitor.</TD>
41</TR>
42
43<TR>
44<TD><tt>vis</tt></TD>
45<TD>An object of type <tt>V</tt>.</TD>
46</TR>
47
48<TR>
49<TD><tt>G</tt></TD>
50<TD>A type that is a model of Graph.</TD>
51</TR>
52
53<TR>
54<TD><tt>g</tt></TD>
55<TD>An object of type <tt>G</tt>.</TD>
56</TR>
57
58<TR>
59<TD><tt>e</tt></TD>
60<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::edge_descriptor</tt>.</TD>
61</TR>
62
63<TR>
64<TD><tt>s,u</tt></TD>
65<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::vertex_descriptor</tt>.</TD>
66</TR>
67
68</table>
69
70<h3>Associated Types</h3>
71
72none
73<p>
74
75<h3>Valid Expressions</h3>
76
77<table border>
78<tr>
79<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
80</tr>
81
82<tr>
83<td>Initialize Vertex</td>
84<td><tt>vis.initialize_vertex(s, g)</tt></td>
85<td><tt>void</tt></td>
86<td>
87This is invoked on every vertex of the graph before the start of the
88graph search.
89</td>
90</tr>
91
92<tr>
93<td>Start Vertex</td>
94<td><tt>vis.start_vertex(s, g)</tt></td>
95<td><tt>void</tt></td>
96<td>
97This is invoked on the source vertex once before the start of the
98search.
99</td>
100</tr>
101
102<tr>
103<td>Discover Vertex</td>
104<td><tt>vis.discover_vertex(u, g)</tt></td>
105<td><tt>void</tt></td>
106<td>
107This is invoked when a vertex is encountered for the first time.
108</td>
109</tr>
110
111<tr>
112<td>Examine Edge</td>
113<td><tt>vis.examine_edge(e, g)</tt></td>
114<td><tt>void</tt></td>
115<td>
116This is invoked on every out-edge of each vertex after it is discovered.
117</td>
118</tr>
119
120
121<tr>
122<td>Tree Edge</td>
123<td><tt>vis.tree_edge(e, g)</tt></td>
124<td><tt>void</tt></td>
125<td>
126This is invoked on each edge as it becomes a member of the edges that
127form the search tree.</td>
128</tr>
129
130<tr>
131<td>Back Edge</td>
132<td><tt>vis.back_edge(e, g)</tt></td>
133<td><tt>void</tt></td>
134<td>
135This is invoked on the back edges in the graph.  For an undirected
136graph there is some ambiguity between tree edges and back edges since
137the edge <i>(u,v)</i> and <i>(v,u)</i> are the same edge, but both the
138<tt>tree_edge()</tt> and <tt>back_edge()</tt> functions will be
139invoked. One way to resolve this ambiguity is to record the tree
140edges, and then disregard the back-edges that are already marked as
141tree edges.  An easy way to record tree edges is to record
142predecessors at the <tt>tree_edge</tt> event point.
143</td>
144</tr>
145
146<tr>
147<td>Forward or Cross Edge</td>
148<td><tt>vis.forward_or_cross_edge(e, g)</tt></td>
149<td><tt>void</tt></td>
150<td>
151This is invoked on forward or cross edges in the graph. In an
152undirected graph this method is never called.
153</td>
154</tr>
155
156<tr>
157<td>Finish Vertex</td>
158<td><tt>vis.finish_vertex(u, g)</tt></td>
159<td><tt>void</tt></td>
160<td>
161This is invoked on vertex <tt>u</tt> after <tt>finish_vertex</tt> has
162been called for all the vertices in the DFS-tree rooted at vertex
163<tt>u</tt>. If vertex <tt>u</tt> is a leaf in the DFS-tree, then
164the <tt>finish_vertex</tt> function is call on <tt>u</tt> after
165all the out-edges of <tt>u</tt> have been examined.
166</td>
167</tr>
168
169</table>
170
171<h3>Models</h3>
172
173<ul>
174 <li><a href="./dfs_visitor.html"><tt>dfs_visitor</tt></a>
175</ul>
176
177<a name="python"></a>
178<h3>Python</h3>
179
180To implement a model of the <tt>DFSVisitor</tt> concept in Python,
181create a new class that derives from the <tt>DFSVisitor</tt> type of
182the graph, which will be
183named <tt><i>GraphType</i>.DFSVisitor</tt>. The events and syntax are
184the same as with visitors in C++. Here is an example for the
185Python <tt>bgl.Graph</tt> graph type:
186
187<pre>
188class count_tree_edges_dfs_visitor(bgl.Graph.DFSVisitor):
189  def __init__(self, name_map):
190    bgl.Graph.DFSVisitor.__init__(self)
191    self.name_map = name_map
192
193  def tree_edge(self, e, g):
194    (u, v) = (g.source(e), g.target(e))
195    print "Tree edge ",
196    print self.name_map[u],
197    print " -> ",
198    print self.name_map[v]
199</pre>
200
201<br>
202<HR>
203<TABLE>
204<TR valign=top>
205<TD nowrap>Copyright &copy 2000-2001</TD><TD>
206<A HREF="../../../people/jeremy_siek.htm">Jeremy Siek</A>,
207Indiana University (<A
208HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
209<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>
210<A HREF=http://www.osl.iu.edu/~lums>Andrew Lumsdaine</A>,
211Indiana University (<A
212HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
213</TD></TR></TABLE>
214
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