doc/quicktour.dox
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     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 \dontinclude hello_lemon.cc
    51 \skip include
    52 \until }
    53 
    54 See the whole program in file \ref hello_lemon.cc in the \c demo subdir of
    55 LEMON package.
    56 
    57     If you want to read more on the LEMON graph structures and
    58 concepts, read the page about \ref graphs "graphs".
    59 
    60 
    61 <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
    62 short example file in this format: read the detailed description of the LEMON
    63 graph file format and input-output routines here: \ref graph-io-page.
    64 
    65 So here is a file describing a graph of 6 nodes (0 to 5), two nodemaps
    66 (called \c coordinates_x and \c coordinates_y), several edges, an edge map
    67 called \c capacity and two designated nodes (called \c source and \c target).
    68 
    69 \verbatim
    70 @nodeset
    71 id      coordinates_x   coordinates_y
    72 5       796.398 208.035
    73 4       573.002 63.002
    74 3       568.549 401.748
    75 2       277.889 68.476
    76 1       288.248 397.327
    77 0       102.239 257.532
    78 @edgeset
    79                 id      capacity
    80 4       5       6       8
    81 3       5       5       8
    82 2       4       4       5
    83 1       4       3       8
    84 1       3       2       5
    85 0       2       1       10
    86 0       1       0       10
    87 #This is a comment here
    88 @nodes
    89 source 0
    90 target 5
    91 @edges 
    92 @attributes 
    93 author "Attila BERNATH"
    94 @end
    95 \endverbatim
    96 
    97 Finally let us give a simple example that reads a graph from a file and writes
    98 it to the standard output.
    99 
   100 \dontinclude reader_writer_demo.cc
   101 \skip include
   102 \until return
   103 \until }
   104 
   105 See the whole program in file \ref reader_writer_demo.cc.
   106 
   107 <li> The following code shows how to read a graph from a stream
   108 (e.g. a file) in the DIMACS file format (find the documentation of the
   109 DIMACS file formats on the web).
   110 
   111 \code
   112 Graph g;
   113 std::ifstream f("graph.dim");
   114 readDimacs(f, g);
   115 \endcode
   116 
   117 One can also store network (graph+capacity on the edges) instances and
   118 other things (minimum cost flow instances etc.) in DIMACS format and
   119 read these in LEMON: to see the details read the documentation of the
   120 \ref dimacs.h "Dimacs file format reader". 
   121 
   122 </ol>
   123 <li> If you want to solve some transportation problems in a network then 
   124 you will want to find shortest paths between nodes of a graph. This is 
   125 usually solved using Dijkstra's algorithm. A utility
   126 that solves this is  the \ref lemon::Dijkstra "LEMON Dijkstra class".
   127 The following code is a simple program using the 
   128 \ref lemon::Dijkstra "LEMON Dijkstra class": it calculates the shortest path between node \c s and \c t in a graph \c g.
   129 We omit the part reading the graph  \c g and the length map \c len.
   130 
   131 \dontinclude dijkstra_demo.cc
   132 \skip ListGraph
   133 \until Graph g
   134 ...
   135 \skip Dijkstra algorithm
   136 \until std::cout << g.id(s)
   137 
   138 See the whole program in \ref dijkstra_demo.cc.
   139 
   140 Some explanation: after instantiating a member of the Dijkstra class
   141 we run the Dijkstra algorithm from node \c s. After this we read some
   142 of the results.  You can do much more with the Dijkstra class, for
   143 example you can run it step by step and gain full control of the
   144 execution. For a detailed description, see the documentation of the
   145 \ref lemon::Dijkstra "LEMON Dijkstra class".
   146 
   147 
   148 <li> If you want to design a network and want to minimize the total
   149 length of wires then you might be looking for a <b>minimum spanning
   150 tree</b> in an undirected graph. This can be found using the Kruskal
   151 algorithm: the function \ref lemon::kruskal "LEMON Kruskal " does this
   152 job for you.  
   153 
   154 First make a graph \c g and a cost map \c
   155 edge_cost_map, then make a bool edgemap \c tree_map or a vector \c
   156 tree_edge_vec for the algorithm output. After calling the function it
   157 gives back the weight of the minimum spanning tree and the \c tree_map or
   158 the \c tree_edge_vec contains the edges of the tree.
   159 
   160 If you want to store the edges in a bool edgemap, then use the
   161 function as follows:
   162 
   163 \dontinclude kruskal_demo.cc
   164 \skip Kruskal with boolmap; 
   165 \until  std::endl
   166 
   167 And if you rather use a vector instead of a bool map:
   168 
   169 \skip Kruskal with vector; 
   170 \until std::endl
   171 
   172 See the whole program in \ref kruskal_demo.cc.
   173 
   174 
   175 
   176 <li>Many problems in network optimization can be formalized by means
   177 of a linear programming problem (LP problem, for short). In our
   178 library we decided not to write an LP solver, since such packages are
   179 available in the commercial world just as well as in the open source
   180 world, and it is also a difficult task to compete these. Instead we
   181 decided to develop an interface that makes it easier to use these
   182 solvers together with LEMON. The advantage of this approach is
   183 twofold. Firstly our C++ interface is more comfortable than the
   184 solvers' native interface. Secondly, changing the underlying solver in
   185 a certain software using LEMON's LP interface needs zero effort. So,
   186 for example, one may try his idea using a free solver, demonstrate its
   187 usability for a customer and if it works well, but the performance
   188 should be improved, then one may decide to purchase and use a better
   189 commercial solver.
   190 
   191 So far we have an
   192 interface for the commercial LP solver software \b CPLEX (developed by ILOG)
   193 and for the open source solver \b GLPK (a shorthand for Gnu Linear Programming
   194 Toolkit).
   195 
   196 We will show two examples, the first one shows how simple it is to formalize
   197 and solve an LP problem in LEMON, while the second one shows how LEMON
   198 facilitates solving network optimization problems using LP solvers.
   199 
   200 <ol>
   201 <li>The following code shows how to solve an LP problem using the LEMON lp
   202 interface. The code together with the comments is self-explanatory.
   203 
   204 \dontinclude lp_demo.cc
   205 \skip A default solver is taken
   206 \until End of LEMON style code
   207 
   208 See the whole code in \ref lp_demo.cc.
   209 
   210 <li>The second example shows how easy it is to formalize a max-flow
   211 problem as an LP problem using the LEMON LP interface: we are looking
   212 for a real valued function defined on the edges of the digraph
   213 satisfying the nonnegativity-, the capacity constraints and the
   214 flow-conservation constraints and giving the largest flow value
   215 between to designated nodes.
   216 
   217 In the following code we suppose that we already have the graph \c g,
   218 the capacity map \c cap, the source node \c s and the target node \c t
   219 in the memory. We will also omit the typedefs.
   220 
   221 \dontinclude lp_maxflow_demo.cc
   222 \skip Define a map on the edges for the variables of the LP problem
   223 \until lp.max();
   224 \skip Solve with the underlying solver
   225 \until lp.solve();
   226 
   227 
   228 The complete program can be found in file \ref lp_maxflow_demo.cc. After compiling run it in the form:
   229 
   230 <tt>./lp_maxflow_demo < sample.lgf</tt>
   231 
   232 where sample.lgf is a file in the lemon format containing a maxflow instance (designated "source", "target" nodes and "capacity" map on the edges).
   233 
   234 
   235 
   236 </ol>
   237 </ul>
   238 
   239 */