/* -*- C++ -*-

 *

 * This file is a part of LEMON, a generic C++ optimization library

 *

 * Copyright (C) 2003-2008

 * Egervary Jeno Kombinatorikus Optimalizalasi Kutatocsoport

 * (Egervary Research Group on Combinatorial Optimization, EGRES).

 *

 * Permission to use, modify and distribute this software is granted

 * provided that this copyright notice appears in all copies. For

 * precise terms see the accompanying LICENSE file.

 *

 * This software is provided "AS IS" with no warranty of any kind,

 * express or implied, and with no claim as to its suitability for any

 * purpose.

 *

 */

/**

\page quicktour Quick Tour to LEMON

Let us first answer the question <b>"What do I want to use LEMON for?"</b>. 

LEMON is a C++ library, so you can use it if you want to write C++ 

programs. What kind of tasks does the library LEMON help to solve? 

It helps to write programs that solve optimization problems that arise

frequently when <b>designing and testing certain networks</b>, for example

in telecommunication, computer networks, and other areas that I cannot

think of now. A very natural way of modelling these networks is by means

of a <b> graph</b> (we will always mean a directed graph by that and say

<b> undirected graph </b> otherwise). 

So if you want to write a program that works with 

graphs then you might find it useful to use our library LEMON. LEMON 

defines various graph concepts depending on what you want to do with the 

graph: a very good description can be found in the page

about \ref graphs "graphs".

You will also want to assign data to the edges or nodes of the graph, for

example a length or capacity function defined on the edges. You can do this in

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".

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. 

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". 

Have fun!

<ul> <li> The first thing to discuss is the way one can create data structures

like graphs and maps in a program using LEMON. 

//There are more graph types

//implemented in LEMON and you can implement your own graph type just as well:

//read more about this in the already mentioned page on \ref graphs "graphs".

First we show how to add nodes and edges to a graph manually. We will also

define a map on the edges of the graph. After this we show the way one can

read a graph (and perhaps maps on it) from a stream (e.g. a file). Of course

we also have routines that write a graph (and perhaps maps) to a stream

(file): this will also be shown. LEMON supports the DIMACS file formats to

read network optimization problems, but more importantly we also have our own

file format that gives a more flexible way to store data related to network

optimization.

<ol> <li>The following code shows how to build a graph from scratch

and iterate on its nodes and edges.  This example also shows how to

give a map on the edges of the graph.  The type Listgraph is one of

the LEMON graph types: the typedefs in the beginning are for

convenience and we will assume them later as well.

\dontinclude hello_lemon.cc

\skip include

\until }

See the whole program in file \ref hello_lemon.cc in the \c demo subdir of

LEMON package.

    If you want to read more on the LEMON graph structures and

concepts, read the page about \ref graphs "graphs".

<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

short example file in this format: read the detailed description of the LEMON

graph file format and input-output routines here: \ref graph-io-page.

So here is a file describing a graph of 6 nodes (0 to 5), two nodemaps

(called \c coordinates_x and \c coordinates_y), several edges, an edge map

called \c capacity and two designated nodes (called \c source and \c target).

\verbatim

@nodeset

id      coordinates_x   coordinates_y

5       796.398 208.035

4       573.002 63.002

3       568.549 401.748

2       277.889 68.476

1       288.248 397.327

0       102.239 257.532

@edgeset

                id      capacity

4       5       6       8

3       5       5       8

2       4       4       5

1       4       3       8

1       3       2       5

0       2       1       10

0       1       0       10

#This is a comment here

@nodes

source 0

target 5

@edges 

@attributes 

author "Attila BERNATH"

@end

\endverbatim

Finally let us give a simple example that reads a graph from a file and writes

it to the standard output.

\dontinclude reader_writer_demo.cc

\skip include

\until return

\until }

See the whole program in file \ref reader_writer_demo.cc.

<li> The following code shows how to read a graph from a stream

(e.g. a file) in the DIMACS file format (find the documentation of the

DIMACS file formats on the web).

\code

Graph g;

std::ifstream f("graph.dim");

readDimacs(f, g);

\endcode

One can also store network (graph+capacity on the edges) instances and

other things (minimum cost flow instances etc.) in DIMACS format and

read these in LEMON: to see the details read the documentation of the

\ref dimacs.h "Dimacs file format reader". 

</ol>

<li> 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 \ref lemon::Dijkstra "LEMON Dijkstra class".

The following code is a simple program using the 

\ref lemon::Dijkstra "LEMON Dijkstra class": it calculates the shortest path between node \c s and \c t in a graph \c g.

We omit the part reading the graph  \c g and the length map \c len.

\dontinclude dijkstra_demo.cc

\skip ListGraph

\until Graph g

...

\skip Dijkstra algorithm

\until std::cout << g.id(s)

See the whole program in \ref dijkstra_demo.cc.

Some explanation: after instantiating a member of the Dijkstra class

we run the Dijkstra algorithm from node \c s. After this we read some

of the results.  You can do much more with the Dijkstra class, for

example you can run it step by step and gain full control of the

execution. For a detailed description, see the documentation of the

\ref lemon::Dijkstra "LEMON Dijkstra class".

<li> If you want to design a network and want to minimize the total

length of wires then you might be looking for a <b>minimum spanning

tree</b> in an undirected graph. This can be found using the Kruskal

algorithm: the function \ref lemon::kruskal "LEMON Kruskal " does this

job for you.  

First make a graph \c g and a cost map \c

edge_cost_map, then make a bool edgemap \c tree_map or a vector \c

tree_edge_vec for the algorithm output. After calling the function it

gives back the weight of the minimum spanning tree and the \c tree_map or

the \c tree_edge_vec contains the edges of the tree.

If you want to store the edges in a bool edgemap, then use the

function as follows:

\dontinclude kruskal_demo.cc

\skip Kruskal with boolmap; 

\until  std::endl

And if you rather use a vector instead of a bool map:

\skip Kruskal with vector; 

\until std::endl

See the whole program in \ref kruskal_demo.cc.

<li>Many problems in network optimization can be formalized by means

of a linear programming problem (LP problem, for short). In our

library we decided not to write an LP solver, since such packages are

available in the commercial world just as well as in the open source

world, and it is also a difficult task to compete these. Instead we

decided to develop an interface that makes it easier to use these

solvers together with LEMON. The advantage of this approach is

twofold. Firstly our C++ interface is more comfortable than the

solvers' native interface. Secondly, changing the underlying solver in

a certain software using LEMON's LP interface needs zero effort. So,

for example, one may try his idea using a free solver, demonstrate its

usability for a customer and if it works well, but the performance

should be improved, then one may decide to purchase and use a better

commercial solver.

So far we have an

interface for the commercial LP solver software \b CPLEX (developed by ILOG)

and for the open source solver \b GLPK (a shorthand for Gnu Linear Programming

Toolkit).

We will show two examples, the first one shows how simple it is to formalize

and solve an LP problem in LEMON, while the second one shows how LEMON

facilitates solving network optimization problems using LP solvers.

<ol>

<li>The following code shows how to solve an LP problem using the LEMON lp

interface. The code together with the comments is self-explanatory.

\dontinclude lp_demo.cc

\skip A default solver is taken

\until End of LEMON style code

See the whole code in \ref lp_demo.cc.

<li>The second example shows how easy it is to formalize a max-flow

problem as an LP problem using the LEMON LP interface: we are looking

for a real valued function defined on the edges of the digraph

satisfying the nonnegativity-, the capacity constraints and the

flow-conservation constraints and giving the largest flow value

between to designated nodes.

In the following code we suppose that we already have the graph \c g,

the capacity map \c cap, the source node \c s and the target node \c t

in the memory. We will also omit the typedefs.

\dontinclude lp_maxflow_demo.cc

\skip Define a map on the edges for the variables of the LP problem

\until lp.max();

\skip Solve with the underlying solver

\until lp.solve();

The complete program can be found in file \ref lp_maxflow_demo.cc. After compiling run it in the form:

<tt>./lp_maxflow_demo < sample.lgf</tt>

where sample.lgf is a file in the lemon format containing a maxflow instance (designated "source", "target" nodes and "capacity" map on the edges).

</ol>

</ul>

*/