# Changeset 58:10b6a5b7d4c0 in lemon-tutorial

Ignore:
Timestamp:
03/01/10 02:30:00 (11 years ago)
Branch:
default
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public
Message:

Improve Algorithms section (it is still under construction)

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2 edited

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• ## algorithms.dox

 r57 \todo This page is under construction. \todo The following contents are mainly ported from the LEMON 0.x tutorial, thus they have to be thoroughly revised and reworked. \warning Currently, this section may contain old or faulty contents. \todo This section should be revised and extended. In addition to the graph structures, the most important parts of LEMON are the various algorithms related to graph theory and combinatorial optimization. The library provides quite flexible and efficient implementations for well-known fundamental algorithms, such as breadth-first search (BFS), depth-first search (DFS), Dijkstra algorithm, Kruskal algorithm and methods for discovering graph properties like connectivity, bipartiteness or Euler property, as well as more complex optimization algorithms for finding maximum flows, minimum cuts, matchings, minimum cost flows and arc-disjoint paths. for well-known fundamental algorithms, such as \ref Bfs "breadth-first search (BFS)", \ref Dfs "depth-first search (DFS)", \ref Dijkstra "Dijkstra algorithm", \ref kruskal "Kruskal algorithm" and methods for discovering \ref graph_properties "graph properties" like connectivity, bipartiteness or Euler property, as well as more complex optimization algorithms for finding \ref max_flow "maximum flows", \ref min_cut "minimum cuts", \ref matching "matchings", \ref min_cost_flow_algs "minimum cost flows" etc. In this section, we present only some of the most fundamental algorithms. [SEC]sec_graph_search[SEC] Graph Search See \ref Bfs, \ref Dfs and \ref graph_properties. Both \ref lemon::Bfs "Bfs" and \ref lemon::Dfs "Dfs" are highly adaptable and efficient implementations of the well known algorithms. The algorithms are placed most cases in separated files named after the algorithm itself but lower case as all other header file names. For example the next Bfs class is in the \c lemon/bfs.h. The algorithm is implemented in the \ref lemon::Bfs "Bfs" template class - rather than as function. The class has two template parameters: \b GR and \b TR.
GR is the digraph the algorithm runs on. It has \ref lemon::ListDigraph "ListDigraph" as default type. TR is a Traits class commonly used to easy the parameterization of templates. In most cases you wont need to modify the default type \ref lemon::BfsDefaultTraits "BfsDefaultTraits". To use the class, declare it! \code Bfs  bfs(gr); \endcode Note the lack of second template argument because of the default parameter. It provides a simple but powerful interface to control the execution. \code int dist = bfs.run(s,t); \endcode It finds the shortest path from node \c s to node \c t and returns it, or zero if there is no path from \c s to \c t.
If you want the shortest path from a specified node to all other node, just write: The common graph search algorithms, namely \ref Bfs "breadth-first search (BFS)" and \ref Dfs "depth-first search (DFS)" are implemented in highly adaptable and efficient algorithm classes \ref Bfs and \ref Dfs. In LEMON, the algorithms are typically placed in separated files, which are named after the algorithm itself but with lower case like all other header files. For example, we have to include bfs.h for using \ref Bfs. \code #include \endcode Basically, all algorithms are implemented in template classes. The template parameters typically specify the used graph type (for more information, see \ref sec_graph_structures) and the required map types. For example, an instance of the \ref BFs class can be created like this. \code ListDigraph g; Bfs bfs(g); \endcode This class provides a simple but powerful interface to control the execution of the algorithm and to obtain all the results. You can execute the algorithm from a given source node by calling the \ref Bfs::run() "run()" function. \code bfs.run(s); \endcode This operation finds the shortest paths from \c s to all other nodes. If you are looking for an s-t path for a certain target node \c t, you can also call the \ref Bfs::run() "run()" function with two parameters. In this case, the BFS search will terminate once it has found the shortest path from \c s to \c t. \code bfs.run(s,t); \endcode By default, the distances and the path information are stored in internal maps, which you can access with member functions like \ref lemon::Bfs::distMap "distMap()" and \ref lemon::Bfs::predMap() "predMap()" or more directly with other member functions like \ref lemon::Bfs::dist() "dist()", \ref lemon::Bfs::path() "path()", \ref lemon::Bfs::predNode() "predNode()", \ref lemon::Bfs::predArc() "predArc()". Once the execution of the algorithm is finished, these query functions can be called. For an example, let us say we want to print the shortest path of those nodes that are at most in a certain distance \c max_dist. \code bfs.run(s); \endcode Now the distances and path information are stored in maps which you can access with member functions like \ref lemon::Bfs::distMap "distMap()" or \ref lemon::Bfs::predMap "predMap()".
1. We run the dfs algorithm to all nodes.
2. Put every node into a list when processed completely.
3. Write out the list in reverse order.
\dontinclude topological_ordering.cc First of all we will need an own \ref lemon::Dfs::ProcessedMap "ProcessedMap". The ordering will be done through it. \skip MyOrdererMap \until }; The class meets the \ref concepts::WriteMap "WriteMap" concept. In it's \c set() method the only thing we need to do is insert the key - that is the node whose processing just finished - into the beginning of the list.
Although we implemented this needed helper class ourselves it was not necessary. The \ref lemon::FrontInserterBoolMap "FrontInserterBoolMap" class does exactly what we needed. To be correct it's more general - and it's all in \c LEMON. But we wanted to show you, how easy is to add additional functionality. First we declare the needed data structures: the digraph and a map to store the nodes' label. \skip ListDigraph \until label Now we build a digraph. But keep in mind that it must be DAG because cyclic digraphs has no topological ordering. \skip belt \until trousers We label them... \skip label \until trousers Then add arcs which represent the precedences between those items. \skip trousers, belt \until ); See how easy is to access the internal information of this algorithm trough maps. We only need to set our own map as the class's \ref lemon::Dfs::ProcessedMap "ProcessedMap". \skip Dfs \until run And now comes the third part. Write out the list in reverse order. But the list was composed in reverse way (with \c push_front() instead of \c push_back() so we just iterate it. \skip std \until endl The program is to be found in the \ref demo directory: \ref topological_ordering.cc \todo Check the linking of the demo file, the code samples are missing. More algorithms are described in the \ref algorithms2 "second part". } \endcode bfs.start() is only a shortcut of the following code. \code while (!bfs.emptyQueue()) { bfs.processNextNode(); } \endcode \todo Write about function-type interfaces Since the DFS algorithm is very similar to BFS with a few tiny differences, the \ref Dfs class can be used similarly to \ref Bfs. [SEC]sec_shortest_paths[SEC] Shortest Paths See \ref Dijkstra and \ref BellmanFord. If you want to solve some transportation problems in a network then you will want to find shortest paths between nodes of a graph. This is usually solved using Dijkstra's algorithm. A utility that solves this is the LEMON Dijkstra class. The following code is a simple program using the LEMON Dijkstra class: it calculates the shortest path between node s and t in a graph g. We omit the part reading the graph g and the length map len.
If you would like to solve some transportation problems in a network, then you will most likely want to find shortest paths between nodes of a graph. This is usually solved using Dijkstra's algorithm. The following code is a simple program using the LEMON \ref Dijkstra class through the function-type interface \ref dijkstra(). It calculates the shortest path between node \c s and \c t in a digraph \c g. \code dijkstra(g, length).distMap(dist).run(s,t); \endcode In LEMON, the algorithms are implemented basically as classes, but which can still be used quite flexibly due to named parameters. The original sample code could also use the class interface as follows. The above sample code could also use the class interface as follows. \code dijkstra.distMap(dist); dijsktra.init(); dijkstra.addSource(u); dijkstra.addSource(s); dijkstra.start(); \endcode \endcode LEMON provides several other algorithms for findign shortest paths in specific or more general cases. For example, \ref BellmanFord can be used instead of \ref Dijkstra when the graph contains an arc with negative cost. You may check the \ref shortest_path module of the reference manual for more details. [SEC]sec_max_flow[SEC] Maximum Flows See \ref Preflow. \todo Write this subsection. [TRAILER]
• ## maps.dox

 r57 These classes also conform to the map %concepts, thus they can be used like standard LEMON maps. Let us suppose that we have a traffic network stored in a LEMON digraph structure with two arc maps \c length and \c speed, which denote the physical length of each arc and the maximum (or average) speed that can be achieved on the corresponding road-section, respectively. If we are interested in the best traveling times, the following code can be used. \code dijkstra(g, divMap(length, speed)).distMap(dist).run(s); \endcode Maps play a central role in LEMON. As their name suggests, they map a [SEC]sec_algs_with_maps[SEC] Using Algorithms with Special Maps The basic functionality of the algorithms can be highly extended using special purpose map types for their internal data structures. they allow specifying any subset of the parameters and in arbitrary order. Let us suppose that we have a traffic network stored in a LEMON digraph structure with two arc maps \c length and \c speed, which denote the physical length of each arc and the maximum (or average) speed that can be achieved on the corresponding road-section, respectively. If we are interested in the best traveling times, the following code can be used. \code dijkstra(g, divMap(length, speed)).distMap(dist).run(s); \endcode \code typedef vector Container; .run(s); \endcode Let us see a bit more complex example to demonstrate Dfs's capabilities. We will see a program, which solves the problem of topological ordering. We need to know in which order we should put on our clothes. The program will do the following:
1. We run the dfs algorithm to all nodes.
2. Put every node into a list when processed completely.
3. Write out the list in reverse order.
\dontinclude topological_ordering.cc First of all we will need an own \ref lemon::Dfs::ProcessedMap "ProcessedMap". The ordering will be done through it. \skip MyOrdererMap \until }; The class meets the \ref concepts::WriteMap "WriteMap" concept. In it's \c set() method the only thing we need to do is insert the key - that is the node whose processing just finished - into the beginning of the list.
Although we implemented this needed helper class ourselves it was not necessary. The \ref lemon::FrontInserterBoolMap "FrontInserterBoolMap" class does exactly what we needed. To be correct it's more general - and it's all in \c LEMON. But we wanted to show you, how easy is to add additional functionality. First we declare the needed data structures: the digraph and a map to store the nodes' label. \skip ListDigraph \until label Now we build a digraph. But keep in mind that it must be DAG because cyclic digraphs has no topological ordering. \skip belt \until trousers We label them... \skip label \until trousers Then add arcs which represent the precedences between those items. \skip trousers, belt \until ); See how easy is to access the internal information of this algorithm trough maps. We only need to set our own map as the class's \ref lemon::Dfs::ProcessedMap "ProcessedMap". \skip Dfs \until run And now comes the third part. Write out the list in reverse order. But the list was composed in reverse way (with \c push_front() instead of \c push_back() so we just iterate it. \skip std \until endl The program is to be found in the \ref demo directory: \ref topological_ordering.cc LEMON also contains visitor based algorithm classes for
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