1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
|
<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>
|
