3 \page quicktour Quick Tour to LEMON
5 Let us first answer the question <b>"What do I want to use LEMON for?"
7 LEMON is a C++ library, so you can use it if you want to write C++
8 programs. What kind of tasks does the library LEMON help to solve?
9 It helps to write programs that solve optimization problems that arise
10 frequently when <b>designing and testing certain networks</b>, for example
11 in telecommunication, computer networks, and other areas that I cannot
12 think of now. A very natural way of modelling these networks is by means
13 of a <b> graph</b> (we will always mean a directed graph by that and say
14 <b> undirected graph </b> otherwise).
15 So if you want to write a program that works with
16 graphs then you might find it useful to use our library LEMON. LEMON
17 defines various graph concepts depending on what you want to do with the
18 graph: a very good description can be found in the page
19 about \ref graphs "graphs".
21 You will also want to assign data to the edges or nodes of the graph, for
22 example a length or capacity function defined on the edges. You can do this in
23 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".
25 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.
26 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".
30 <ul> <li> The first thing to discuss is the way one can create data structures
31 like graphs and maps in a program using LEMON.
32 //There are more graph types
33 //implemented in LEMON and you can implement your own graph type just as well:
34 //read more about this in the already mentioned page on \ref graphs "graphs".
36 First we show how to add nodes and edges to a graph manually. We will also
37 define a map on the edges of the graph. After this we show the way one can
38 read a graph (and perhaps maps on it) from a stream (e.g. a file). Of course
39 we also have routines that write a graph (and perhaps maps) to a stream
40 (file): this will also be shown. LEMON supports the DIMACS file formats to
41 store network optimization problems, but more importantly we also have our own
42 file format that gives a more flexible way to store data related to network
45 <ol> <li>The following code shows how to build a graph from scratch
46 and iterate on its nodes and edges. This example also shows how to
47 give a map on the edges of the graph. The type Listgraph is one of
48 the LEMON graph types: the typedefs in the beginning are for
49 convenience and we will assume them later as well.
51 \include hello_lemon.cc
53 See the whole program in file \ref hello_lemon.cc in the \c demo subdir of
56 If you want to read more on the LEMON graph structures and
57 concepts, read the page about \ref graphs "graphs".
60 <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
61 short example file in this format: read the detailed description of the LEMON
62 graph file format and input-output routines here: \ref graph-io-page.
64 So here is a file describing a graph of 6 nodes (0 to 5), two nodemaps
65 (called \c coordinates_x and \c coordinates_y), several edges, an edge map
66 called \c capacity and two designated nodes (called \c source and \c target).
70 Finally let us give a simple example that reads a graph from a file and writes
71 it to the standard output.
73 \include reader_writer_demo.cc
75 See the whole program in file \ref reader_writer_demo.cc.
77 <li> The following code shows how to read a graph from a stream
78 (e.g. a file) in the DIMACS file format (find the documentation of the
79 DIMACS file formats on the web).
83 std::ifstream f("graph.dim");
87 One can also store network (graph+capacity on the edges) instances and
88 other things (minimum cost flow instances etc.) in DIMACS format and
89 use these in LEMON: to see the details read the documentation of the
90 \ref dimacs.h "Dimacs file format reader". There you will also find
91 the details about the output routines into files of the DIMACS format.
94 <li> If you want to solve some transportation problems in a network then
95 you will want to find shortest paths between nodes of a graph. This is
96 usually solved using Dijkstra's algorithm. A utility
97 that solves this is the \ref lemon::Dijkstra "LEMON Dijkstra class".
98 The following code is a simple program using the
99 \ref lemon::Dijkstra "LEMON Dijkstra class": it calculates the shortest path between node \c s and \c t in a graph \c g.
100 We omit the part reading the graph \c g and the length map \c len.
102 \dontinclude dijkstra_demo.cc
106 \skip Dijkstra algorithm
107 \until std::cout << g.id(s)
109 See the whole program in \ref dijkstra_demo.cc.
111 Some explanation: after instantiating a member of the Dijkstra class
112 we run the Dijkstra algorithm from node \c s. After this we read some
113 of the results. You can do much more with the Dijkstra class, for
114 example you can run it step by step and gain full control of the
115 execution. For a detailed description, see the documentation of the
116 \ref lemon::Dijkstra "LEMON Dijkstra class".
119 <li> If you want to design a network and want to minimize the total length
120 of wires then you might be looking for a <b>minimum spanning tree</b> in
121 an undirected graph. This can be found using the Kruskal algorithm: the
122 function \ref lemon::kruskal "LEMON Kruskal ..." does this job for you.
123 The following code fragment shows an example:
125 Ide Zsuzska fog irni!
127 <li>Many problems in network optimization can be formalized by means
128 of a linear programming problem (LP problem, for short). In our
129 library we decided not to write an LP solver, since such packages are
130 available in the commercial world just as well as in the open source
131 world, and it is also a difficult task to compete these. Instead we
132 decided to develop an interface that makes it easier to use these
133 solvers together with LEMON. The advantage of this approach is
134 twofold. Firstly our C++ interface is more comfortable than the
135 solvers' native interface. Secondly, changing the underlying solver in
136 a certain software using LEMON's LP interface needs zero effort. So,
137 for example, one may try his idea using a free solver, demonstrate its
138 usability for a customer and if it works well, but the performance
139 should be improved, then one may decide to purchase and use a better
143 interface for the commercial LP solver software \b CPLEX (developed by ILOG)
144 and for the open source solver \b GLPK (a shorthand for Gnu Linear Programming
147 We will show two examples, the first one shows how simple it is to formalize
148 and solve an LP problem in LEMON, while the second one shows how LEMON
149 facilitates solving network optimization problems using LP solvers.
152 <li>The following code shows how to solve an LP problem using the LEMON lp
153 interface. The code together with the comments is self-explanatory.
155 \dontinclude lp_demo.cc
156 \skip A default solver is taken
157 \until End of LEMON style code
159 See the whole code in \ref lp_demo.cc.
161 <li>The second example shows how easy it is to formalize a max-flow
162 problem as an LP problem using the LEMON LP interface: we are looking
163 for a real valued function defined on the edges of the digraph
164 satisfying the nonnegativity-, the capacity constraints and the
165 flow-conservation constraints and giving the largest flow value
166 between to designated nodes.
168 In the following code we suppose that we already have the graph \c g,
169 the capacity map \c cap, the source node \c s and the target node \c t
170 in the memory. We will also omit the typedefs.
172 \dontinclude lp_maxflow_demo.cc
173 \skip Define a map on the edges for the variables of the LP problem
175 \skip Solve with the underlying solver
179 The complete program can be found in file \ref lp_maxflow_demo.cc. After compiling run it in the form:
181 <tt>./lp_maxflow_demo < sample.lgf</tt>
183 where sample.lgf is a file in the lemon format containing a maxflow instance (designated "source", "target" nodes and "capacity" map on the edges).