4 \page basic_concepts Basic concepts
6 \section basic_graph The graph classes
7 The most important classes in LEMON are the graph classes. An instance of a graph
8 class is the representation of the mathematical graph. It holds the topology and
9 every structural information of the graph. The structural manipulations are also
10 provided by the graph object. There is no universal graph class instead we have
11 different classes for different purposes. They can differ in many ways, but all
12 have to satisfy one or more \ref concept "graph concepts" which are standardized
13 interfaces to work with the rest of the library. The most basic concept is the
15 A good example is the \ref ListGraph which we already know from Hello World and
16 will be used in our examples as well.
18 One main advantage of the templates are, that you can write your own graph classes.
19 As long as they provide the interface a concept is defining all the LEMON algorithms
20 and classes will work with it properly - no representation or implementation is
24 \subsection basic_node Nodes
25 To refer to a node of a graph we need some kind of typed variable. Graph classes
26 have the Node public type for this purpose. Stacking by the last example:
27 \code lemon::ListGraph::Node \endcode
29 If the graph fits the ExtendableGraphComponent concept, then you can add new nodes
30 to the graph with the addNode() member function. It returns the newly added node
31 (as value). So if you need the new node to do something useful with, for example
32 create an edge, assign a value to it through \ref map1 maps.
33 \code lemon::ListGraph::Node new_node = graph.addNode(); \endcode
35 If the graph fits into the ErasableGraphComponent concept you can also remove nodes
36 from the graph with the erase() member function.
37 \code graph.erase( new_node ); \endcode
39 You don't have to store every node in a variable, you can access individual nodes
40 with node iterators discussed in the next section. But how do you know which
42 The graph class has the id( Node n ) member function providing an unique identifier
43 assigned to every node.
46 \subsection basic_edge Edges
47 An Edge is what you think it is. It goes from one node to another node (they can
48 be identical if the edge is a loop). If the graph class is directed, the Edge is directed too. Otherwise
49 the edge is considered undirected and called UEdge.
50 \code lemon::ListUGraph::UEdge \endcode
52 The addEdge() member function will create a new edge. It has two arguments, the
53 source node and the target node. The graph class must be extendable.
54 \code lemon::ListGraph::Edge new_edge = graph.addEdge( src_node, trg_node ); \endcode
55 You can handle edges similar as nodes. The erase() member function has an edge taking
58 You can ask for the source or target node of the edge by the corresponding member
61 graph.source( new_edge );
62 lemon::ListGraph::Node n = graph.target( new_edge ); \endcode
63 These functions are always legal even if the graph is undirected. UEdge has a
67 \section basic_iterators Iterators
68 Graphs are some kind of containers. And as you expect they have iterator types.
69 One for nodes and a couple for edges - and special classes can have additional
70 iterators too. An example:
71 \code lemon::ListGraph::NodeIt \endcode
72 This is a node iterator. Every iterator type starts with a name that refers to
73 the iterated object, and ends with 'It'.
75 LEMON style iterators differ from \c stl or \c boost iterators in a very tasty
76 way. A graph has no begin or end - or at least a generic graph class has none.
77 If by some topology you could pick a good begin node, it would be misleading and
78 incorrect. A LEMON style iterator must be initialized at construction time.
79 The constructor takes the needed parameters - by a node iterator it's the graph
80 object. And will be compared to the lemon::INVALID to check if it's still valid.
81 Every iterator can be compared to INVALID. No \c begin() or \c end() needed.<br>
82 Let's see these things working together:
84 for( ListGraph::NodeIt n(graph); n != INVALID; ++n )
85 do_useful_things_with_node(n);
87 Note that the function \c do_useful_things_with_node() expects a Node type argument
88 ad we just gave him the iterator. LEMON style iterators must provide "on demand
89 dereferencing". For example a NodeIt can be used everywhere a Node could. (In some
90 graph classes Node is the base class of NodeIt. But in other cases this is implemented
91 through typecast operator.)
93 <b>Very important!</b> The iteration has no defined order. There is absolutely no
94 warranty that the next time the iteration will give us the nodes in the same order.
95 Don't use this order to identify nodes! Use the \c id() member function of the
96 graph class described above. (There is a powerful technique using maps right in
99 The \ref EdgeIt works exactly the same - nothing more to say. But there are \ref InEdgeIt
100 and \ref OutEdgeIt by directed graphs and \ref IncEdgeIt by undirected graphs.
101 They take two arguments. The first is a graph, the second is certain node of the
102 graph. InEdgeIt iterates on the incoming edges of that node and OutEdgeIt does it
103 on the outgoing edges. The IncEdgeIt of course iterates every edge connecting to
107 for( ListGraph::NodeIt n(graph); n != INVALID; ++n ) {
109 for( ListGraph::InEdgeIt e(graph,n); e != INVALID; ++e ) ++in;
110 for( ListGraph::OutEdgeIt e(graph,n); e != INVALID; ++e ) ++out;
112 std::cout << "#" << graph.id(n) << " node has " << in << " incoming and "
113 << out << "outgoing edges." << std::endl;
118 \section basic_ListGraph ListGraph - a versatile directed graph
119 As you see ListGraph satisfies most of the basic concepts and ideal for general
120 graph representations. It has an undirected version too: ListUGraph.