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| <Head> |
| <Title>Boost Graph Library: Bellman Ford Visitor</Title> |
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| <BR Clear> |
| |
| <H1><img src="figs/python.gif" alt="(Python)"/>Bellman Ford Visitor Concept</H1> |
| |
| This concept defines the visitor interface for <a |
| href="./bellman_ford_shortest.html"><tt>bellman_ford_shortest_paths()</tt></a>. |
| Users can define a class with the Bellman Ford Visitor interface and |
| pass and object of the class to <tt>bellman_ford_shortest_paths()</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 Bellman Ford 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>Examine Edge</td> |
| <td><tt>vis.examine_edge(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on every edge in the graph <tt>num_vertices(g)</tt> times. |
| </td> |
| </tr> |
| |
| |
| <tr> |
| <td>Edge Relaxed</td> |
| <td><tt>vis.edge_relaxed(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| Upon examination, if the following condition holds then the edge |
| is relaxed (its distance is reduced), and this method is invoked.<br> |
| <tt> |
| tie(u,v) = incident(e, g);<br> |
| D d_u = get(d, u), d_v = get(d, v);<br> |
| W w_e = get(w, e);<br> |
| assert(compare(combine(d_u, w_e), d_v));<br> |
| </tt> |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Edge Not Relaxed</td> |
| <td><tt>edge_not_relaxed(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| Upon examination, if the edge is not relaxed (see above) then |
| this method is invoked. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Edge Minimized</td> |
| <td><tt>vis.edge_minimized(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| After <tt>num_vertices(g)</tt> iterations through the edge set |
| of the graph are completed, one last iteration is made to test whether |
| each edge was minimized. If the edge is minimized then this function |
| is invoked. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Edge Not Minimized</td> |
| <td><tt>edge_not_minimized(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| If the edge is not minimized, this function is invoked. This happens |
| when there is a negative cycle in the graph. |
| </td> |
| </tr> |
| |
| </table> |
| |
| |
| <h3>Models</h3> |
| |
| <ul> |
| <li><a href="./bellman_visitor.html"><tt>bellman_visitor</tt></a> |
| </ul> |
| |
| <a name="python"></a> |
| <h3>Python</h3> |
| |
| To implement a model of the <tt>BellmanFordVisitor</tt> concept in Python, |
| create a new class that derives from the <tt>BellmanFordVisitor</tt> type of |
| the graph, which will be |
| named <tt><i>GraphType</i>.BellmanFordVisitor</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_bellman_ford_visitor(bgl.Graph.BellmanFordVisitor): |
| def __init__(self, name_map): |
| bgl.Graph.BellmanFordVisitor.__init__(self) |
| self.name_map = name_map |
| |
| def edge_relaxed(self, e, g): |
| (u, v) = (g.source(e), g.target(e)) |
| print "Relaxed 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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