doc/quicktour.dox
author alpar
Thu, 28 Jul 2005 19:09:39 +0000
changeset 1605 4fa618f7aa69
parent 1580 a9e4208cf4e3
child 1640 9c7834ac5e64
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Some (awful) toolbar images added.
The inline image data (guipixbufs.h) are automatically generated, but
I don't know how to use it.
     1 /**
     2 
     3 \page quicktour Quick Tour to LEMON
     4 
     5 Let us first answer the question <b>"What do I want to use LEMON for?"</b>. 
     6 LEMON is a C++ library, so you can use it if you want to write C++ 
     7 programs. What kind of tasks does the library LEMON help to solve? 
     8 It helps to write programs that solve optimization problems that arise
     9 frequently when <b>designing and testing certain networks</b>, for example
    10 in telecommunication, computer networks, and other areas that I cannot
    11 think of now. A very natural way of modelling these networks is by means
    12 of a <b> graph</b> (we will always mean a directed graph by that and say
    13 <b> undirected graph </b> otherwise). 
    14 So if you want to write a program that works with 
    15 graphs then you might find it useful to use our library LEMON. LEMON 
    16 defines various graph concepts depending on what you want to do with the 
    17 graph: a very good description can be found in the page
    18 about \ref graphs "graphs".
    19 
    20 You will also want to assign data to the edges or nodes of the graph, for
    21 example a length or capacity function defined on the edges. You can do this in
    22 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".
    23 
    24 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. 
    25 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". 
    26 
    27 Have fun!
    28 
    29 <ul> <li> The first thing to discuss is the way one can create data structures
    30 like graphs and maps in a program using LEMON. 
    31 //There are more graph types
    32 //implemented in LEMON and you can implement your own graph type just as well:
    33 //read more about this in the already mentioned page on \ref graphs "graphs".
    34 
    35 First we show how to add nodes and edges to a graph manually. We will also
    36 define a map on the edges of the graph. After this we show the way one can
    37 read a graph (and perhaps maps on it) from a stream (e.g. a file). Of course
    38 we also have routines that write a graph (and perhaps maps) to a stream
    39 (file): this will also be shown. LEMON supports the DIMACS file formats to
    40 read network optimization problems, but more importantly we also have our own
    41 file format that gives a more flexible way to store data related to network
    42 optimization.
    43 
    44 <ol> <li>The following code shows how to build a graph from scratch
    45 and iterate on its nodes and edges.  This example also shows how to
    46 give a map on the edges of the graph.  The type Listgraph is one of
    47 the LEMON graph types: the typedefs in the beginning are for
    48 convenience and we will assume them later as well.
    49 
    50 \include hello_lemon.cc
    51 
    52 See the whole program in file \ref hello_lemon.cc in the \c demo subdir of
    53 LEMON package.
    54 
    55     If you want to read more on the LEMON graph structures and
    56 concepts, read the page about \ref graphs "graphs".
    57 
    58 
    59 <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
    60 short example file in this format: read the detailed description of the LEMON
    61 graph file format and input-output routines here: \ref graph-io-page.
    62 
    63 So here is a file describing a graph of 6 nodes (0 to 5), two nodemaps
    64 (called \c coordinates_x and \c coordinates_y), several edges, an edge map
    65 called \c capacity and two designated nodes (called \c source and \c target).
    66 
    67 \verbatim
    68 @nodeset
    69 id      coordinates_x   coordinates_y
    70 5       796.398 208.035
    71 4       573.002 63.002
    72 3       568.549 401.748
    73 2       277.889 68.476
    74 1       288.248 397.327
    75 0       102.239 257.532
    76 @edgeset
    77                 id      capacity
    78 4       5       6       8
    79 3       5       5       8
    80 2       4       4       5
    81 1       4       3       8
    82 1       3       2       5
    83 0       2       1       10
    84 0       1       0       10
    85 #This is a comment here
    86 @nodes
    87 source 0
    88 target 5
    89 @edges 
    90 @attributes 
    91 author "Attila BERNATH"
    92 @end
    93 \endverbatim
    94 
    95 Finally let us give a simple example that reads a graph from a file and writes
    96 it to the standard output.
    97 
    98 \include reader_writer_demo.cc
    99 
   100 See the whole program in file \ref reader_writer_demo.cc.
   101 
   102 <li> The following code shows how to read a graph from a stream
   103 (e.g. a file) in the DIMACS file format (find the documentation of the
   104 DIMACS file formats on the web).
   105 
   106 \code
   107 Graph g;
   108 std::ifstream f("graph.dim");
   109 readDimacs(f, g);
   110 \endcode
   111 
   112 One can also store network (graph+capacity on the edges) instances and
   113 other things (minimum cost flow instances etc.) in DIMACS format and
   114 read these in LEMON: to see the details read the documentation of the
   115 \ref dimacs.h "Dimacs file format reader". 
   116 
   117 </ol>
   118 <li> If you want to solve some transportation problems in a network then 
   119 you will want to find shortest paths between nodes of a graph. This is 
   120 usually solved using Dijkstra's algorithm. A utility
   121 that solves this is  the \ref lemon::Dijkstra "LEMON Dijkstra class".
   122 The following code is a simple program using the 
   123 \ref lemon::Dijkstra "LEMON Dijkstra class": it calculates the shortest path between node \c s and \c t in a graph \c g.
   124 We omit the part reading the graph  \c g and the length map \c len.
   125 
   126 \dontinclude dijkstra_demo.cc
   127 \skip ListGraph
   128 \until Graph g
   129 ...
   130 \skip Dijkstra algorithm
   131 \until std::cout << g.id(s)
   132 
   133 See the whole program in \ref dijkstra_demo.cc.
   134 
   135 Some explanation: after instantiating a member of the Dijkstra class
   136 we run the Dijkstra algorithm from node \c s. After this we read some
   137 of the results.  You can do much more with the Dijkstra class, for
   138 example you can run it step by step and gain full control of the
   139 execution. For a detailed description, see the documentation of the
   140 \ref lemon::Dijkstra "LEMON Dijkstra class".
   141 
   142 
   143 <li> If you want to design a network and want to minimize the total
   144 length of wires then you might be looking for a <b>minimum spanning
   145 tree</b> in an undirected graph. This can be found using the Kruskal
   146 algorithm: the function \ref lemon::kruskal "LEMON Kruskal " does this
   147 job for you.  
   148 
   149 First make a graph \c g and a cost map \c
   150 edge_cost_map, then make a bool edgemap \c tree_map or a vector \c
   151 tree_edge_vec for the algorithm output. After calling the function it
   152 gives back the weight of the minimum spanning tree and the \c tree_map or
   153 the \c tree_edge_vec contains the edges of the tree.
   154 
   155 If you want to store the edges in a bool edgemap, then use the
   156 function as follows:
   157 
   158 \dontinclude kruskal_demo.cc
   159 \skip Kruskal with boolmap; 
   160 \until  std::endl
   161 
   162 And if you rather use a vector instead of a bool map:
   163 
   164 \skip Kruskal with vector; 
   165 \until std::endl
   166 
   167 See the whole program in \ref kruskal_demo.cc.
   168 
   169 
   170 
   171 <li>Many problems in network optimization can be formalized by means
   172 of a linear programming problem (LP problem, for short). In our
   173 library we decided not to write an LP solver, since such packages are
   174 available in the commercial world just as well as in the open source
   175 world, and it is also a difficult task to compete these. Instead we
   176 decided to develop an interface that makes it easier to use these
   177 solvers together with LEMON. The advantage of this approach is
   178 twofold. Firstly our C++ interface is more comfortable than the
   179 solvers' native interface. Secondly, changing the underlying solver in
   180 a certain software using LEMON's LP interface needs zero effort. So,
   181 for example, one may try his idea using a free solver, demonstrate its
   182 usability for a customer and if it works well, but the performance
   183 should be improved, then one may decide to purchase and use a better
   184 commercial solver.
   185 
   186 So far we have an
   187 interface for the commercial LP solver software \b CPLEX (developed by ILOG)
   188 and for the open source solver \b GLPK (a shorthand for Gnu Linear Programming
   189 Toolkit).
   190 
   191 We will show two examples, the first one shows how simple it is to formalize
   192 and solve an LP problem in LEMON, while the second one shows how LEMON
   193 facilitates solving network optimization problems using LP solvers.
   194 
   195 <ol>
   196 <li>The following code shows how to solve an LP problem using the LEMON lp
   197 interface. The code together with the comments is self-explanatory.
   198 
   199 \dontinclude lp_demo.cc
   200 \skip A default solver is taken
   201 \until End of LEMON style code
   202 
   203 See the whole code in \ref lp_demo.cc.
   204 
   205 <li>The second example shows how easy it is to formalize a max-flow
   206 problem as an LP problem using the LEMON LP interface: we are looking
   207 for a real valued function defined on the edges of the digraph
   208 satisfying the nonnegativity-, the capacity constraints and the
   209 flow-conservation constraints and giving the largest flow value
   210 between to designated nodes.
   211 
   212 In the following code we suppose that we already have the graph \c g,
   213 the capacity map \c cap, the source node \c s and the target node \c t
   214 in the memory. We will also omit the typedefs.
   215 
   216 \dontinclude lp_maxflow_demo.cc
   217 \skip Define a map on the edges for the variables of the LP problem
   218 \until lp.max();
   219 \skip Solve with the underlying solver
   220 \until lp.solve();
   221 
   222 
   223 The complete program can be found in file \ref lp_maxflow_demo.cc. After compiling run it in the form:
   224 
   225 <tt>./lp_maxflow_demo < sample.lgf</tt>
   226 
   227 where sample.lgf is a file in the lemon format containing a maxflow instance (designated "source", "target" nodes and "capacity" map on the edges).
   228 
   229 
   230 
   231 </ol>
   232 </ul>
   233 
   234 */