doc/quicktour.dox
author kpeter
Tue, 05 Feb 2008 11:24:32 +0000
changeset 2563 5841132a89fd
parent 2391 14a343be7a5a
permissions -rw-r--r--
Small fixes in README.
     1 /* -*- C++ -*-
     2  *
     3  * This file is a part of LEMON, a generic C++ optimization library
     4  *
     5  * Copyright (C) 2003-2008
     6  * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport
     7  * (Egervary Research Group on Combinatorial Optimization, EGRES).
     8  *
     9  * Permission to use, modify and distribute this software is granted
    10  * provided that this copyright notice appears in all copies. For
    11  * precise terms see the accompanying LICENSE file.
    12  *
    13  * This software is provided "AS IS" with no warranty of any kind,
    14  * express or implied, and with no claim as to its suitability for any
    15  * purpose.
    16  *
    17  */
    18 
    19 /**
    20 
    21 \page quicktour Quick Tour to LEMON
    22 
    23 Let us first answer the question <b>"What do I want to use LEMON for?"</b>. 
    24 LEMON is a C++ library, so you can use it if you want to write C++ 
    25 programs. What kind of tasks does the library LEMON help to solve? 
    26 It helps to write programs that solve optimization problems that arise
    27 frequently when <b>designing and testing certain networks</b>, for example
    28 in telecommunication, computer networks, and other areas that I cannot
    29 think of now. A very natural way of modelling these networks is by means
    30 of a <b> graph</b> (we will always mean a directed graph by that and say
    31 <b> undirected graph </b> otherwise). 
    32 So if you want to write a program that works with 
    33 graphs then you might find it useful to use our library LEMON. LEMON 
    34 defines various graph concepts depending on what you want to do with the 
    35 graph: a very good description can be found in the page
    36 about \ref graphs "graphs".
    37 
    38 You will also want to assign data to the edges or nodes of the graph, for
    39 example a length or capacity function defined on the edges. You can do this in
    40 LEMON using so called \b maps. You can define a map on the nodes or on the edges of the graph and the value of the map (the range of the function) can be practically almost of any type. Read more about maps \ref maps-page "here".
    41 
    42 In this quick tour we want to show you some facilities LEMON library can provide through examples (simple demo programs). The examples will only show part of the functionality, but links will always be given to reach complete details. 
    43 You will find links next to the code fragments that help to download full demo programs: save them on your computer and compile them according to the description in the page about \ref getstart "How to start using LEMON". 
    44 
    45 Have fun!
    46 
    47 <ul> <li> The first thing to discuss is the way one can create data structures
    48 like graphs and maps in a program using LEMON. 
    49 //There are more graph types
    50 //implemented in LEMON and you can implement your own graph type just as well:
    51 //read more about this in the already mentioned page on \ref graphs "graphs".
    52 
    53 First we show how to add nodes and edges to a graph manually. We will also
    54 define a map on the edges of the graph. After this we show the way one can
    55 read a graph (and perhaps maps on it) from a stream (e.g. a file). Of course
    56 we also have routines that write a graph (and perhaps maps) to a stream
    57 (file): this will also be shown. LEMON supports the DIMACS file formats to
    58 read network optimization problems, but more importantly we also have our own
    59 file format that gives a more flexible way to store data related to network
    60 optimization.
    61 
    62 <ol> <li>The following code shows how to build a graph from scratch
    63 and iterate on its nodes and edges.  This example also shows how to
    64 give a map on the edges of the graph.  The type Listgraph is one of
    65 the LEMON graph types: the typedefs in the beginning are for
    66 convenience and we will assume them later as well.
    67 
    68 \dontinclude hello_lemon.cc
    69 \skip include
    70 \until }
    71 
    72 See the whole program in file \ref hello_lemon.cc in the \c demo subdir of
    73 LEMON package.
    74 
    75     If you want to read more on the LEMON graph structures and
    76 concepts, read the page about \ref graphs "graphs".
    77 
    78 
    79 <li>LEMON has an own file format for storing graphs, maps on edges/nodes and some other things. Instead of any explanation let us give a
    80 short example file in this format: read the detailed description of the LEMON
    81 graph file format and input-output routines here: \ref graph-io-page.
    82 
    83 So here is a file describing a graph of 6 nodes (0 to 5), two nodemaps
    84 (called \c coordinates_x and \c coordinates_y), several edges, an edge map
    85 called \c capacity and two designated nodes (called \c source and \c target).
    86 
    87 \verbatim
    88 @nodeset
    89 id      coordinates_x   coordinates_y
    90 5       796.398 208.035
    91 4       573.002 63.002
    92 3       568.549 401.748
    93 2       277.889 68.476
    94 1       288.248 397.327
    95 0       102.239 257.532
    96 @edgeset
    97                 id      capacity
    98 4       5       6       8
    99 3       5       5       8
   100 2       4       4       5
   101 1       4       3       8
   102 1       3       2       5
   103 0       2       1       10
   104 0       1       0       10
   105 #This is a comment here
   106 @nodes
   107 source 0
   108 target 5
   109 @edges 
   110 @attributes 
   111 author "Attila BERNATH"
   112 @end
   113 \endverbatim
   114 
   115 Finally let us give a simple example that reads a graph from a file and writes
   116 it to the standard output.
   117 
   118 \dontinclude reader_writer_demo.cc
   119 \skip include
   120 \until return
   121 \until }
   122 
   123 See the whole program in file \ref reader_writer_demo.cc.
   124 
   125 <li> The following code shows how to read a graph from a stream
   126 (e.g. a file) in the DIMACS file format (find the documentation of the
   127 DIMACS file formats on the web).
   128 
   129 \code
   130 Graph g;
   131 std::ifstream f("graph.dim");
   132 readDimacs(f, g);
   133 \endcode
   134 
   135 One can also store network (graph+capacity on the edges) instances and
   136 other things (minimum cost flow instances etc.) in DIMACS format and
   137 read these in LEMON: to see the details read the documentation of the
   138 \ref dimacs.h "Dimacs file format reader". 
   139 
   140 </ol>
   141 <li> If you want to solve some transportation problems in a network then 
   142 you will want to find shortest paths between nodes of a graph. This is 
   143 usually solved using Dijkstra's algorithm. A utility
   144 that solves this is  the \ref lemon::Dijkstra "LEMON Dijkstra class".
   145 The following code is a simple program using the 
   146 \ref lemon::Dijkstra "LEMON Dijkstra class": it calculates the shortest path between node \c s and \c t in a graph \c g.
   147 We omit the part reading the graph  \c g and the length map \c len.
   148 
   149 \dontinclude dijkstra_demo.cc
   150 \skip ListGraph
   151 \until Graph g
   152 ...
   153 \skip Dijkstra algorithm
   154 \until std::cout << g.id(s)
   155 
   156 See the whole program in \ref dijkstra_demo.cc.
   157 
   158 Some explanation: after instantiating a member of the Dijkstra class
   159 we run the Dijkstra algorithm from node \c s. After this we read some
   160 of the results.  You can do much more with the Dijkstra class, for
   161 example you can run it step by step and gain full control of the
   162 execution. For a detailed description, see the documentation of the
   163 \ref lemon::Dijkstra "LEMON Dijkstra class".
   164 
   165 
   166 <li> If you want to design a network and want to minimize the total
   167 length of wires then you might be looking for a <b>minimum spanning
   168 tree</b> in an undirected graph. This can be found using the Kruskal
   169 algorithm: the function \ref lemon::kruskal "LEMON Kruskal " does this
   170 job for you.  
   171 
   172 First make a graph \c g and a cost map \c
   173 edge_cost_map, then make a bool edgemap \c tree_map or a vector \c
   174 tree_edge_vec for the algorithm output. After calling the function it
   175 gives back the weight of the minimum spanning tree and the \c tree_map or
   176 the \c tree_edge_vec contains the edges of the tree.
   177 
   178 If you want to store the edges in a bool edgemap, then use the
   179 function as follows:
   180 
   181 \dontinclude kruskal_demo.cc
   182 \skip Kruskal with boolmap; 
   183 \until  std::endl
   184 
   185 And if you rather use a vector instead of a bool map:
   186 
   187 \skip Kruskal with vector; 
   188 \until std::endl
   189 
   190 See the whole program in \ref kruskal_demo.cc.
   191 
   192 
   193 
   194 <li>Many problems in network optimization can be formalized by means
   195 of a linear programming problem (LP problem, for short). In our
   196 library we decided not to write an LP solver, since such packages are
   197 available in the commercial world just as well as in the open source
   198 world, and it is also a difficult task to compete these. Instead we
   199 decided to develop an interface that makes it easier to use these
   200 solvers together with LEMON. The advantage of this approach is
   201 twofold. Firstly our C++ interface is more comfortable than the
   202 solvers' native interface. Secondly, changing the underlying solver in
   203 a certain software using LEMON's LP interface needs zero effort. So,
   204 for example, one may try his idea using a free solver, demonstrate its
   205 usability for a customer and if it works well, but the performance
   206 should be improved, then one may decide to purchase and use a better
   207 commercial solver.
   208 
   209 So far we have an
   210 interface for the commercial LP solver software \b CPLEX (developed by ILOG)
   211 and for the open source solver \b GLPK (a shorthand for Gnu Linear Programming
   212 Toolkit).
   213 
   214 We will show two examples, the first one shows how simple it is to formalize
   215 and solve an LP problem in LEMON, while the second one shows how LEMON
   216 facilitates solving network optimization problems using LP solvers.
   217 
   218 <ol>
   219 <li>The following code shows how to solve an LP problem using the LEMON lp
   220 interface. The code together with the comments is self-explanatory.
   221 
   222 \dontinclude lp_demo.cc
   223 \skip A default solver is taken
   224 \until End of LEMON style code
   225 
   226 See the whole code in \ref lp_demo.cc.
   227 
   228 <li>The second example shows how easy it is to formalize a max-flow
   229 problem as an LP problem using the LEMON LP interface: we are looking
   230 for a real valued function defined on the edges of the digraph
   231 satisfying the nonnegativity-, the capacity constraints and the
   232 flow-conservation constraints and giving the largest flow value
   233 between to designated nodes.
   234 
   235 In the following code we suppose that we already have the graph \c g,
   236 the capacity map \c cap, the source node \c s and the target node \c t
   237 in the memory. We will also omit the typedefs.
   238 
   239 \dontinclude lp_maxflow_demo.cc
   240 \skip Define a map on the edges for the variables of the LP problem
   241 \until lp.max();
   242 \skip Solve with the underlying solver
   243 \until lp.solve();
   244 
   245 
   246 The complete program can be found in file \ref lp_maxflow_demo.cc. After compiling run it in the form:
   247 
   248 <tt>./lp_maxflow_demo < sample.lgf</tt>
   249 
   250 where sample.lgf is a file in the lemon format containing a maxflow instance (designated "source", "target" nodes and "capacity" map on the edges).
   251 
   252 
   253 
   254 </ol>
   255 </ul>
   256 
   257 */