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updated to match example from the book

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[SVN r20426]
This commit is contained in:
Jeremy Siek
2003-10-20 15:45:43 +00:00
parent e7a047c5b1
commit b21c4393ae
1 changed files with 78 additions and 75 deletions
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153
doc/kevin_bacon.html
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@@ -64,12 +64,12 @@ href="../example/kevin-bacon.cpp"><TT>examples/kevin-bacon.cpp</TT></a>.
We have supplied a file <TT>kevin-bacon.dat</TT> which contains a list
of actor pairs who appeared in the same movie. Each line of the file
contains an actor's name, a movie, and another actor that appeared in
the movie. The ``|'' character is used as a separator. Here is an
the movie. The ``;'' character is used as a separator. Here is an
excerpt.
<P>
<PRE>
Patrick Stewart|Prince of Egypt, The (1998)|Steve Martin
Patrick Stewart;Prince of Egypt, The (1998);Steve Martin
</PRE>
<P>
@@ -81,7 +81,7 @@ it. We use a <TT>std::ifstream</TT> to input the file.
std::ifstream datafile("./kevin-bacon.dat");
if (!datafile) {
cerr &lt;&lt; "No ./kevin-bacon.dat file" &lt;&lt; endl;
return -1;
return EXIT_FAILURE;
}
</PRE>
@@ -97,7 +97,7 @@ actors appearing together in a movie is symmetric.
using namespace boost;
typedef adjacency_list&lt;vecS, vecS, undirectedS,
property&lt;vertex_name_t, string&gt;,
property&lt;edge_name_t, string, property&lt;edge_weight_t, int&gt; &gt;
property&lt;edge_name_t, string &gt; &gt;
&gt; Graph;
</PRE>
@@ -107,22 +107,20 @@ objects from the graph. The following code shows how this is done.
<P>
<PRE>
boost::property_map&lt;Graph, vertex_name_t&gt;::type actor_name = get(vertex_name, g);
boost::property_map&lt;Graph, edge_name_t&gt;::type connecting_movie = get(edge_name, g);
boost::property_map&lt;Graph, edge_weight_t&gt;::type weight = get(edge_weight, g);
property_map&lt;Graph, vertex_name_t&gt;::type actor_name = get(vertex_name, g);
property_map&lt;Graph, edge_name_t&gt;::type connecting_movie = get(edge_name, g);
</PRE>
<P>
Next we can start to loop through the file, parsing each line into a
list of tokens. <TT>stringtok</TT> is a C++ version of the old
<TT>strtok</TT> standard C function.
sequence of tokens using the <a href="../../tokenizer/index.htm">Boost
Tokenizer Library</a>.
<P>
<PRE>
std::string line;
while (std::getline(datafile,line)) {
std::list&lt;std::string&gt; line_toks;
boost::stringtok(line_toks, line, "|");
for (std::string line; std::getline(datafile, line);) {
char_delimiters_separator&lt;char&gt; sep(false, "", ";");
tokenizer&lt;&gt; line_toks(line, sep);
...
}
</PRE>
@@ -150,7 +148,7 @@ can be used to implement this mapping.
The first token will be an actor's name. If the actor is not already
in our actor's map we will add a vertex to the graph, set the name
property of the vertex, and record the vertex descriptor in the map.
If the actor is already in the graph, we will retreive the vertex
If the actor is already in the graph, we will retrieve the vertex
descriptor from the map's <TT>pos</TT> iterator.
<P>
@@ -159,16 +157,16 @@ descriptor from the map's <TT>pos</TT> iterator.
bool inserted;
Vertex u, v;
std::list&lt;std::string&gt;::iterator i = line_toks.begin();
tokenizer&lt;&gt;::iterator i = line_toks.begin();
std::string actors_name = *i++;
boost::tie(pos, inserted) = actors.insert(make_pair(*i, Vertex()));
tie(pos, inserted) = actors.insert(std::make_pair(actors_name, Vertex()));
if (inserted) {
u = add_vertex(g);
actor_name[u] = *i;
actor_name[u] = actors_name;
pos-&gt;second = u;
} else
u = pos-&gt;second;
++i;
</PRE>
<P>
@@ -180,7 +178,7 @@ actor into the graph.
<PRE>
std::string movie_name = *i++;
boost::tie(pos, inserted) = actors.insert(make_pair(*i, Vertex()));
tie(pos, inserted) = actors.insert(std::make_pair(*i, Vertex()));
if (inserted) {
v = add_vertex(g);
actor_name[v] = *i;
@@ -190,72 +188,81 @@ actor into the graph.
</PRE>
<P>
The final step is to add an edge connecting the two actors, and
record the name of the connecting movie. We also set the weight
of the edge to one. Since we chose <TT>setS</TT> for the <TT>EdgeList</TT>
type of the <TT>adjacency_list</TT>, any parallel edges in the input
will not be inserted into the graph.
The final step is to add an edge connecting the two actors, and record
the name of the connecting movie.
<P>
<PRE>
Edge e;
boost::tie(e, inserted) = add_edge(u, v, g);
if (inserted) {
graph_traits&lt;Graph&gt;::edge_descriptor e;
tie(e, inserted) = add_edge(u, v, g);
if (inserted)
connecting_movie[e] = movie_name;
weight[e] = 1;
</PRE>
<P>
<H2>A Custom Visitor Class</H2>
<P>
We create a custom visitor class to record the Kevin Bacon
numbers. The Kevin Bacon number will be the shortest distances from
each actor to Kevin Bacon. This distance has also been referred to as
the ``Bacon Number'' in memory of the ``Erdos Number'' used to rank
mathematicians according to how many publications separate them from
Paul Erdos. The shortest distance to from Kevin Bacon to each actor
will follow the breadth-first tree. The BFS algorithm identifies edges
that belong to the breadth-first tree and calls our visitor's
<tt>tree_edge</tt> function. We record the distance to the target
vertex as one plus the distance to the source vertex.
<PRE>
template &lt;typename DistanceMap&gt;
class bacon_number_recorder : public default_bfs_visitor
{
public:
bacon_number_recorder(DistanceMap dist) : d(dist) { }
template &lt;typename Edge, typename Graph&gt;
void tree_edge(Edge e, const Graph& g) const
{
typename graph_traits&lt;Graph&gt;::vertex_descriptor
u = source(e, g), v = target(e, g);
d[v] = d[u] + 1;
}
private:
DistanceMap d;
};
// Convenience function
template &lt;typename DistanceMap&gt;
bacon_number_recorder&lt;DistanceMap&gt;
record_bacon_number(DistanceMap d)
{
return bacon_number_recorder&lt;DistanceMap&gt;(d);
}
</PRE>
<P>
<H2>Applying Breadth-First Search</H2>
<P>
The documentation for <A
HREF="./breadth_first_search.html"><tt>breadth_first_search()</tt></a>
algorithm shows that we will need to provide property maps for color
and vertex ID. In addition, we will be using visitors to record
predecessors and distances so we will need property maps for these as
well. The distance will be the shortest distances from each actor to
Kevin Bacon calculated by the algorithm. This distance has also been
referred to as the ``Bacon Number'' in memory of the ``Erdos Number''
used to rank mathematicians according to how many publications
separate them from Paul Erdos. We will use a vector to store the bacon
numbers, and another vector to store the color property needed by
the BFS algorithm. The <TT>predecessor</TT> vector will be used to
record the shortest path from each actor to Kevin. For each vertex,
the <TT>predecessor</TT> vector will contain a pointer to the next
vertex on the shortest path towards Kevin Bacon.
We will use a vector to store the bacon numbers.
<P>
<PRE>
std::vector&lt;int&gt; bacon_number(num_vertices(g));
std::vector&lt;int&gt; color(num_vertices(g));
std::vector&lt;Vertex&gt; predecessor(num_vertices(g));
</PRE>
<H2>Apply the Breadth-First Search</H2>
<P>
The BFS algorithm also requires a source vertex as input; of course
this will be Kevin Bacon. We now call the BGL BFS algorithm, passing
in the graph, source vertex, the property maps, and a visitor composed
of a predecessor and distance recorder. We use the <a
href="./predecessor_recorder.html"><TT>predecessor_recorder</TT></a>
to record the predecessors and the <a
href="./distance_recorder.html"><tt>distance_recorder</tt></a> to
record distances.
The BFS algorithm requires a source vertex as input; of course this
will be Kevin Bacon. We now call the BGL BFS algorithm, passing in the
graph, source vertex, and the visitor.
<P>
<PRE>
Vertex src = actors["Kevin Bacon"];
breadth_first_search
(g, src,
visitor(make_bfs_visitor
(make_list(record_predecessors(&amp;predecessor[0],
on_examine_edge()),
record_distances(&amp;bacon_number[0],
on_examine_edge()))))
);
Vertex src = actors["Kevin Bacon"];
bacon_number[src] = 0;
breadth_first_search(g, src, visitor(record_bacon_number(&amp;bacon_number[0])));
</PRE>
<P>
@@ -265,10 +272,10 @@ through all the vertices in the graph and use them to index into the
<P>
<PRE>
graph_traits&lt;Graph&gt;::vertex_iterator i, end;
for (boost::tie(i, end) = vertices(g); i != end; ++i)
std::cout &lt;&lt; actor_name[*i] &lt;&lt; "'s bacon number is "
&lt;&lt; bacon_number[*i] &lt;&lt; std::endl;
graph_traits&lt;Graph&gt;::vertex_iterator i, end;
for (boost::tie(i, end) = vertices(g); i != end; ++i)
std::cout &lt;&lt; actor_name[*i] &lt;&lt; "'s bacon number is "
&lt;&lt; bacon_number[*i] &lt;&lt; std::endl;
</PRE>
<P>
@@ -283,10 +290,6 @@ HREF="../../property_map/iterator_property_map.html"><TT>iterator_property_map</
<P>
Here are some excepts from the output of the program.
<PRE>
William Shatner was in Loaded Weapon 1 (1993) with Denise Richards
Denise Richards was in Wild Things (1998) with Kevin Bacon
Patrick Stewart was in Prince of Egypt, The (1998) with Steve Martin
...
William Shatner's bacon number is 2
Denise Richards's bacon number is 1
Kevin Bacon's bacon number is 0