3 \page algorithms Algorithms
5 \section algo_bfs_dfs Bfs/Dfs
6 Both \ref lemon::Bfs "Bfs" and \ref lemon::Dfs "Dfs" are highly adaptable and efficient
7 implementations of the well known algorithms. The algorithms are placed most cases in
8 separated files named after the algorithm itself but lower case as all other header file names.
9 For example the next Bfs class is in the \c lemon/bfs.h.
12 The algorithm is implemented in the \ref lemon::Bfs "Bfs" template class - rather than as function.
13 The class has two template parameters: \b GR and \TR.<br>
14 GR is the graph the algorithm runs on. It has \ref lemon::ListGraph "ListGraph" as default type.
15 TR is a Traits class commonly used to easy the parametrization of templates. In most cases you
16 wont need to modify the default type \ref lemon::BfsDefaultTraits "BfsDefaultTraits<GR>".
18 To use the class, declare it!
20 Bfs<ListUGraph> bfs(gr);
22 Note the lack of second template argument because of the default parameter.
24 It provides a simple but powerful interface to control the execution.
26 int dist = bfs.run(s,t);
28 It finds the shortest path from node \c s to node \c t and returns it, or zero
29 if there is no path from \c s to \c t.<br>
30 If you want the shortest path from a specified node to all other node, just write:
34 Now the distances and path information are stored in maps which you can access with
35 member functions like \ref lemon::Bfs::distMap "distMap()" or \ref lemon::Bfs::predMap "predMap()".<br>
36 Or more directly whit other member functions like \c predNode(). Once the algorithm
37 is finished (or to be precise reached that node) \ref lemon::Bfs::dist "dist()" or \ref lemon::Bfs::predNode
38 "predNode()" can be called.
40 For an example let's say we want to print the shortest path of those nodes which
41 are in a certain distance.
45 for( ListUGraph::NodeIt n(gr); n != INVALID; ++n ) {
46 if( bfs.reached(n) && bfs.dist(n) <= max_dist ) {
47 std::cout << gr.id(n);
49 Node prev = bfs.prevNode(n);
50 while( prev != INVALID ) {
51 std::cout << "<-" << gr.id(prev);
52 prev = bfs.prevNode(n);
55 std::cout << std::endl;
60 \subsubsection bfs_adv_control Advanced control
61 In the previous code we only used \c run(). Now we introduce the way you can directly
62 control the execution of the algorithm.
64 First you have to initialize the variables with \ref lemon::Bfs::init "init()".
69 Then you add one or more source nodes to the queue. They will be processed, as they would
70 be reached by the algorithm before. And yes - you can add more sources during the execution.
72 bfs.addSource(node_1);
73 bfs.addSource(node_2);
77 And finally you can start the process with \ref lemon::Bfs::start "start()", or
78 you can write your own loop to process the nodes one-by-one.
80 \todo demo for bfs advanced control
83 Since Dfs is very similar to Bfs with a few tiny differences we only see a bit more complex example
84 to demonstrate Dfs's capabilities.
86 We will see a program, which solves the problem of <b>topological ordering</b>.
87 We need to know in which order we should put on our clothes. The program will do the following:
89 <li>We run the dfs algorithm to all nodes.
90 <li>Put every node into a list when processed completely.
91 <li>Write out the list in reverse order.
94 \dontinclude topological_ordering.cc
95 First of all we will need an own \ref lemon::Dfs::ProcessedMap "ProcessedMap". The ordering
96 will be done through it.
99 The class meets the \ref lemon::WriteMap "WriteMap" concept. In it's \c set() method the only thing
100 we need to do is insert the key - that is the node who's processing just finished - into the beginning
102 Although we implemented this needed helper class ourselves it was not necessary.
103 The \ref lemon::FrontInserterBoolMap "FrontInserterBoolMap" class does exactly
104 what we needed. To be correct it's more general - and it's all in \c LEMON. But
105 we wanted to show you, how easy is to add additional functionality.
107 First we declare the needed data structures: the graph and a map to store the nodes' label.
111 Now we build a graph. But keep in mind that it must be DAG because cyclic graphs has no topological
118 Then add directed edges which represent the precedences between those items.
122 See how easy is to access the internal information of this algorithm trough maps.
123 We only need to set our own map as the class's \ref lemon::Dfs::ProcessedMap "ProcessedMap".
127 And now comes the third part. Write out the list in reverse order. But the list was
128 composed in reverse way (with \c push_front() instead of \c push_back() so we just iterate it.
132 The program is to be found in the \ref demo directory: \ref topological_ordering.cc
134 More algorithms are described in the \ref algorithms2 "second part".