BellmanFord< GR, LEN, TR > Class Template Reference

---

Detailed Description

template<typename GR, typename LEN, typename TR>
class lemon::BellmanFord< GR, LEN, TR >

This class provides an efficient implementation of the Bellman-Ford algorithm. The maximum time complexity of the algorithm is O(ne).

The Bellman-Ford algorithm solves the single-source shortest path problem when the arcs can have negative lengths, but the digraph should not contain directed cycles with negative total length. If all arc costs are non-negative, consider to use the Dijkstra algorithm instead, since it is more efficient.

The arc lengths are passed to the algorithm using a ReadMap, so it is easy to change it to any kind of length. The type of the length values is determined by the Value type of the length map.

There is also a function-type interface for the Bellman-Ford algorithm, which is convenient in the simplier cases and it can be used easier.

Template Parameters:

GR	The type of the digraph the algorithm runs on. The default type is ListDigraph.
LEN	A readable arc map that specifies the lengths of the arcs. The default map type is GR::ArcMap<int>.
TR	The traits class that defines various types used by the algorithm. By default, it is BellmanFordDefaultTraits<GR, LEN>. In most cases, this parameter should not be set directly, consider to use the named template parameters instead.

#include <lemon/bellman_ford.h>

List of all members.

Classes
class	ActiveIt
	LEMON iterator for getting the active nodes. More...
struct	SetDistMap
	Named parameter for setting DistMap type. More...
struct	SetOperationTraits
	Named parameter for setting OperationTraits type. More...
struct	SetPredMap
	Named parameter for setting PredMap type. More...
Public Types
typedef TR::Digraph	Digraph
	The type of the underlying digraph.
typedef TR::LengthMap::Value	Value
	The type of the arc lengths.
typedef TR::LengthMap	LengthMap
	The type of the map that stores the arc lengths.
typedef TR::PredMap	PredMap
	The type of the map that stores the last arcs of the shortest paths.
typedef TR::DistMap	DistMap
	The type of the map that stores the distances of the nodes.
typedef PredMapPath< Digraph, PredMap >	Path
	The type of the paths.
typedef TR::OperationTraits	OperationTraits
	The operation traits class of the algorithm.
typedef TR	Traits
	The traits class of the algorithm.
Public Member Functions
	BellmanFord (const Digraph &g, const LengthMap &length)
	Constructor.
	~BellmanFord ()
	Destructor.
BellmanFord &	lengthMap (const LengthMap &map)
	Sets the length map.
BellmanFord &	predMap (PredMap &map)
	Sets the map that stores the predecessor arcs.
BellmanFord &	distMap (DistMap &map)
	Sets the map that stores the distances of the nodes.
Execution Control
The simplest way to execute the Bellman-Ford algorithm is to use one of the member functions called run(). If you need better control on the execution, you have to call init() first, then you can add several source nodes with addSource(). Finally the actual path computation can be performed with start(), checkedStart() or limitedStart().
void	init (const Value value=OperationTraits::infinity())
	Initializes the internal data structures.
void	addSource (Node source, Value dst=OperationTraits::zero())
	Adds a new source node.
bool	processNextRound ()
	Executes one round from the Bellman-Ford algorithm.
bool	processNextWeakRound ()
	Executes one weak round from the Bellman-Ford algorithm.
void	start ()
	Executes the algorithm.
bool	checkedStart ()
	Executes the algorithm and checks the negative cycles.
void	limitedStart (int num)
	Executes the algorithm with arc number limit.
void	run (Node s)
	Runs the algorithm from the given root node.
void	run (Node s, int num)
	Runs the algorithm from the given root node with arc number limit.
Query Functions
The result of the Bellman-Ford algorithm can be obtained using these functions. Either run() or init() should be called before using them.
Path	path (Node t) const
	The shortest path to the given node.
Value	dist (Node v) const
	The distance of the given node from the root(s).
Arc	predArc (Node v) const
	Returns the 'previous arc' of the shortest path tree for the given node.
Node	predNode (Node v) const
	Returns the 'previous node' of the shortest path tree for the given node.
const DistMap &	distMap () const
	Returns a const reference to the node map that stores the distances of the nodes.
const PredMap &	predMap () const
	Returns a const reference to the node map that stores the predecessor arcs.
bool	reached (Node v) const
	Checks if a node is reached from the root(s).
lemon::Path< Digraph >	negativeCycle () const
	Gives back a negative cycle.

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Constructor & Destructor Documentation

BellmanFord	(	const Digraph &	g,
		const LengthMap &	length
	)		[inline]

Constructor.

Parameters:

g	The digraph the algorithm runs on.
length	The length map used by the algorithm.

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Member Function Documentation

BellmanFord& lengthMap

(

const LengthMap &

map

)

[inline]

Sets the length map.

Returns:

(*this)

BellmanFord& predMap

(

PredMap &

map

)

[inline]

Sets the map that stores the predecessor arcs. If you don't use this function before calling run() or init(), an instance will be allocated automatically. The destructor deallocates this automatically allocated map, of course.

Returns:

(*this)

BellmanFord& distMap

(

DistMap &

map

)

[inline]

Sets the map that stores the distances of the nodes calculated by the algorithm. If you don't use this function before calling run() or init(), an instance will be allocated automatically. The destructor deallocates this automatically allocated map, of course.

Returns:

(*this)

void init

(

const Value

value = OperationTraits::infinity()

)

[inline]

Initializes the internal data structures. The optional parameter is the initial distance of each node.

void addSource	(	Node	source,
		Value	dst = OperationTraits::zero()
	)		[inline]

This function adds a new source node. The optional second parameter is the initial distance of the node.

bool processNextRound

(

)

[inline]

If the algoritm calculated the distances in the previous round exactly for the paths of at most k arcs, then this function will calculate the distances exactly for the paths of at most k+1 arcs. Performing k iterations using this function calculates the shortest path distances exactly for the paths consisting of at most k arcs.

Warning:

The paths with limited arc number cannot be retrieved easily with path() or predArc() functions. If you also need the shortest paths and not only the distances, you should store the predecessor map after each iteration and build the path manually.

Returns:

true when the algorithm have not found more shorter paths.

See also:

ActiveIt

bool processNextWeakRound

(

)

[inline]

If the algorithm calculated the distances in the previous round at least for the paths of at most k arcs, then this function will calculate the distances at least for the paths of at most k+1 arcs. This function does not make it possible to calculate the shortest path distances exactly for paths consisting of at most k arcs, this is why it is called weak round.

Returns:

true when the algorithm have not found more shorter paths.

See also:

ActiveIt

void start

(

)

[inline]

Executes the algorithm.

This method runs the Bellman-Ford algorithm from the root node(s) in order to compute the shortest path to each node.

The algorithm computes

• the shortest path tree (forest),
• the distance of each node from the root(s).

Precondition:

init() must be called and at least one root node should be added with addSource() before using this function.

bool checkedStart

(

)

[inline]

Executes the algorithm and checks the negative cycles.

This method runs the Bellman-Ford algorithm from the root node(s) in order to compute the shortest path to each node and also checks if the digraph contains cycles with negative total length.

The algorithm computes

• the shortest path tree (forest),
• the distance of each node from the root(s).

Returns:

false if there is a negative cycle in the digraph.

Precondition:

init() must be called and at least one root node should be added with addSource() before using this function.

void limitedStart

(

int

num

)

[inline]

Executes the algorithm with arc number limit.

This method runs the Bellman-Ford algorithm from the root node(s) in order to compute the shortest path distance for each node using only the paths consisting of at most num arcs.

The algorithm computes

• the limited distance of each node from the root(s),
• the predecessor arc for each node.

Warning:

The paths with limited arc number cannot be retrieved easily with path() or predArc() functions. If you also need the shortest paths and not only the distances, you should store the predecessor map after each iteration and build the path manually.

Precondition:

init() must be called and at least one root node should be added with addSource() before using this function.

void run

(

Node

s

)

[inline]

This method runs the Bellman-Ford algorithm from the given root node s in order to compute the shortest path to each node.

The algorithm computes

• the shortest path tree (forest),
• the distance of each node from the root(s).

Note:

bf.run(s) is just a shortcut of the following code.

   bf.init();
   bf.addSource(s);
   bf.start();

void run	(	Node	s,
		int	num
	)		[inline]

This method runs the Bellman-Ford algorithm from the given root node s in order to compute the shortest path distance for each node using only the paths consisting of at most num arcs.

The algorithm computes

• the limited distance of each node from the root(s),
• the predecessor arc for each node.

Warning:

The paths with limited arc number cannot be retrieved easily with path() or predArc() functions. If you also need the shortest paths and not only the distances, you should store the predecessor map after each iteration and build the path manually.

Note:

bf.run(s, num) is just a shortcut of the following code.

   bf.init();
   bf.addSource(s);
   bf.limitedStart(num);

Path path

(

Node

t

)

const [inline]

Gives back the shortest path to the given node from the root(s).

Warning:

t should be reached from the root(s).

Precondition:

Either run() or init() must be called before using this function.

Value dist

(

Node

v

)

const [inline]

Returns the distance of the given node from the root(s).

Warning:

If node v is not reached from the root(s), then the return value of this function is undefined.

Precondition:

Either run() or init() must be called before using this function.

Arc predArc

(

Node

v

)

const [inline]

This function returns the 'previous arc' of the shortest path tree for node v, i.e. it returns the last arc of a shortest path from a root to v. It is INVALID if v is not reached from the root(s) or if v is a root.

The shortest path tree used here is equal to the shortest path tree used in predNode() and predMap().

Precondition:

Either run() or init() must be called before using this function.

Node predNode

(

Node

v

)

const [inline]

This function returns the 'previous node' of the shortest path tree for node v, i.e. it returns the last but one node of a shortest path from a root to v. It is INVALID if v is not reached from the root(s) or if v is a root.

The shortest path tree used here is equal to the shortest path tree used in predArc() and predMap().

Precondition:

Either run() or init() must be called before using this function.

const DistMap& distMap

(

)

const [inline]

Returns a const reference to the node map that stores the distances of the nodes calculated by the algorithm.

Precondition:

Either run() or init() must be called before using this function.

const PredMap& predMap

(

)

const [inline]

Returns a const reference to the node map that stores the predecessor arcs, which form the shortest path tree (forest).

Precondition:

Either run() or init() must be called before using this function.

bool reached

(

Node

v

)

const [inline]

Returns true if v is reached from the root(s).

Precondition:

Either run() or init() must be called before using this function.

lemon::Path<Digraph> negativeCycle

(

)

const [inline]

This function gives back a directed cycle with negative total length if the algorithm has already found one. Otherwise it gives back an empty path.