# source:lemon-0.x/doc/maps.dox@1083:8043b93e5973

Last change on this file since 1083:8043b93e5973 was 1083:8043b93e5973, checked in by Alpar Juttner, 17 years ago

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1namespace lemon{
2/*!
3
4\page maps-page Maps
5
6Maps play central role in LEMON. As their name suggests, they map a
7certain range of \e keys to certain \e values. Each map has two
8<tt>typedef</tt>'s to determine the types of keys and values, like this:
9
10\code
11  typedef Edge Key;
12  typedef double Value;
13\endcode
14
16\e writable (\ref lemon::concept::WriteMap "WriteMap") or both
18There also exists a special type of
19ReadWrite map called \ref lemon::concept::ReferenceMap "reference map".
21read and write the values of a key, a reference map
22can also give you a reference to the
24where it is stored.
25
26Each graph structure in LEMON provides two standard map templates called
27\c EdgeMap and \c NodeMap. Both are reference maps and you can easily
28assign data to the nodes and to the edges of the graph. For example if you
29have a graph \c G defined as
30\code
31  ListGraph G;
32\endcode
33and you want to assign a floating point value to each edge, you can do
34it like this.
35\code
36  ListGraph::EdgeMap<double> length(G);
37\endcode
38Note that you must give the underlying graph to the constructor.
39
40The value of a readable map can be obtained by <tt>operator[]</tt>.
41\code
42  d=length[e];
43\endcode
44where \c e is an instance of \c ListGraph::Edge.
45(Or anything else
46that converts to \c ListGraph::Edge, like  \c ListGraph::EdgeIt or
47\c ListGraph::OutEdgeIt etc.)
48
49There are two ways the assign a new value to a key
50
51- In case of a <em>reference map</em> <tt>operator[]</tt>
52gives you a reference to the
53value, thus you can use this.
54\code
55  length[e]=3.5;
56\endcode
57- <em>Writable maps</em> have
58a member function \c set(Key,const Value &)
59for this purpose.
60\code
61  length.set(e,3.5);
62\endcode
63
64The first case is more comfortable and if you store complex structures in your
65map, it might be more efficient. However, there are writable but
66not reference maps, so if you want to write a generic algorithm, you should
67insist on the second way.
68
69\section how-to-write-your-own-map How to Write Your Own Maps
70
72
73Readable maps are very frequently used as the input of the
74algorithms.  For this purpose the most straightforward way is the use of the
75default maps provided by LEMON's graph structures.
76Very often however, it is more
78
79You can find some examples below. In these examples \c Graph is the
80type of the particular graph structure you use.
81
82
83This simple map assigns \f$\pi\f$ to each edge.
84
85\code
86struct MyMap
87{
88  typedef double Value;
89  typedef Graph::Edge Key;
90  double operator[](Key e) const { return M_PI;}
91};
92\endcode
93
94An alternative way to define maps is to use \c MapBase
95
96\todo For this, \c MapBase seems to be a better name then \c NullMap.
97
98\code
99struct MyMap : public MapBase<Graph::Edge,double>
100{
101  Value operator[](Key e) const { return M_PI;}
102};
103\endcode
104
105Here is a bit more complex example.
106It provides a length function obtained
107from a base length function shifted by a potential difference.
108
109\code
110class ReducedLengthMap  : public MapBase<Graph::Edge,double>
111{
112  const Graph &g;
113  const Graph::EdgeMap<double> &orig_len;
114  const Graph::NodeMap<double> &pot;
115
116public:
117  Value operator[](Key e) const {
118    return orig_len.get(e)-pot.get(G.target(e))-pot.get(G.source(e));
119  }
120
121  ReducedLengthMap(const Graph &_g,
122                   const Graph::EdgeMap &o,
123                   const Graph::NodeMap &p)
124    : G(g), orig_len(o), pot(p) {};
125};
126\endcode
127
128Then, you can call e.g. Dijkstra algoritm on this map like this:
129\code
130  ...
131  ReducedLengthMap rm(g,len,pot);
132  Dijkstra<Graph,ReducedLengthMap> dij(g,rm);
133  dij.run(s);
134  ...
135\endcode
136
137
138\subsection write-maps Writable Maps
139
140To be written...
141
142\subsection side-effect-maps Maps with Side Effect
143
144To be written...
145
146*/
147}
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