graph_tool.search
 Search algorithms¶
This module includes several search algorithms, which are customizable to arbitrary purposes. It is mostly a wrapper around the Visitor interface of the Boost Graph Library, and the respective search functions.
Summary¶
bfs_search 
Breadthfirst traversal of a directed or undirected graph. 
bfs_iterator 
Return an iterator of the edges corresponding to a breathfirst traversal of the graph. 
dfs_search 
Depthfirst traversal of a directed or undirected graph. 
dfs_iterator 
Return an iterator of the edges corresponding to a depthfirst traversal of the graph. 
dijkstra_search 
Dijsktra traversal of a directed or undirected graph, with nonnegative weights. 
dijkstra_iterator 
Return an iterator of the edges corresponding to a Dijkstra traversal of the graph. 
astar_search 
Heuristic \(A^*\) search on a weighted, directed or undirected graph for the case where all edge weights are nonnegative. 
astar_iterator 
Return an iterator of the edges corresponding to an \(A^*\) traversal of the graph. 
bellman_ford_search 
BellmanFord traversal of a directed or undirected graph, with negative weights. 
BFSVisitor 
A visitor object that is invoked at the eventpoints inside the bfs_search() algorithm. 
DFSVisitor 
A visitor object that is invoked at the eventpoints inside the dfs_search() algorithm. 
DijkstraVisitor 
A visitor object that is invoked at the eventpoints inside the dijkstra_search() algorithm. 
BellmanFordVisitor 
A visitor object that is invoked at the eventpoints inside the bellman_ford_search() algorithm. 
AStarVisitor 
A visitor object that is invoked at the eventpoints inside the astar_search() algorithm. 
StopSearch 
If this exception is raised from inside any search visitor object, the search is aborted. 
Examples¶
In this module, most documentation examples will make use of the network
search_example.xml
, shown below.
>>> g = gt.load_graph("search_example.xml")
>>> name = g.vp["name"]
>>> weight = g.ep["weight"]
>>> pos = g.vp["pos"]
>>> gt.graph_draw(g, pos, vertex_text=name, vertex_font_size=12, vertex_shape="double_circle",
... vertex_fill_color="#729fcf", vertex_pen_width=3,
... edge_pen_width=weight, output="search_example.pdf")
<...>
Contents¶

class
graph_tool.search.
BFSVisitor
[source]¶ A visitor object that is invoked at the eventpoints inside the
bfs_search()
algorithm. By default, it performs no action, and should be used as a base class in order to be useful.
initialize_vertex
(u)[source]¶ This is invoked on every vertex of the graph before the start of the graph search.

examine_vertex
(u)[source]¶ This is invoked on a vertex as it is popped from the queue. This happens immediately before examine_edge() is invoked on each of the outedges of vertex u.

tree_edge
(e)[source]¶ This is invoked on each edge as it becomes a member of the edges that form the search tree.

non_tree_edge
(e)[source]¶ This is invoked on back or cross edges for directed graphs and cross edges for undirected graphs.

gray_target
(e)[source]¶ This is invoked on the subset of nontree edges whose target vertex is colored gray at the time of examination. The color gray indicates that the vertex is currently in the queue.


graph_tool.search.
bfs_search
(g, source, visitor=<graph_tool.search.BFSVisitor object>)[source]¶ Breadthfirst traversal of a directed or undirected graph.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
visitor :
BFSVisitor
(optional, default:BFSVisitor()
)A visitor object that is invoked at the event points inside the algorithm. This should be a subclass of
BFSVisitor
.See also
dfs_search
 Depthfirst search
dijkstra_search
 Dijkstra’s search algorithm
astar_search
 \(A^*\) heuristic search algorithm
Notes
A breadthfirst traversal visits vertices that are closer to the source before visiting vertices that are further away. In this context “distance” is defined as the number of edges in the shortest path from the source vertex.
The time complexity is \(O(V + E)\).
The pseudocode for the BFS algorithm is listed below, with the annotated event points, for which the given visitor object will be called with the appropriate method.
BFS(G, source) for each vertex u in V[G] initialize vertex u color[u] := WHITE d[u] := infinity end for color[source] := GRAY d[source] := 0 ENQUEUE(Q, source) discover vertex source while (Q != Ø) u := DEQUEUE(Q) examine vertex u for each vertex v in Adj[u] examine edge (u,v) if (color[v] = WHITE) (u,v) is a tree edge color[v] := GRAY ENQUEUE(Q, v) discover vertex v else (u,v) is a nontree edge if (color[v] = GRAY) ... (u,v) has a gray target else ... (u,v) has a black target end for color[u] := BLACK finish vertex u end while
References
[bfs] Edward Moore, “The shortest path through a maze”, International Symposium on the Theory of Switching, 1959 [bfsbgl] http://www.boost.org/doc/libs/release/libs/graph/doc/breadth_first_search.html [bfswikipedia] http://en.wikipedia.org/wiki/Breadthfirst_search Examples
We must define what should be done during the search by subclassing
BFSVisitor
, and specializing the appropriate methods. In the following we will keep track of the distance from the root, and the predecessor tree.class VisitorExample(gt.BFSVisitor): def __init__(self, name, pred, dist): self.name = name self.pred = pred self.dist = dist def discover_vertex(self, u): print(">", self.name[u], "has been discovered!") def examine_vertex(self, u): print(self.name[u], "has been examined...") def tree_edge(self, e): self.pred[e.target()] = int(e.source()) self.dist[e.target()] = self.dist[e.source()] + 1
With the above class defined, we can perform the BFS search as follows.
>>> dist = g.new_vertex_property("int") >>> pred = g.new_vertex_property("int64_t") >>> gt.bfs_search(g, g.vertex(0), VisitorExample(name, pred, dist)) > Bob has been discovered! Bob has been examined... > Eve has been discovered! > Chuck has been discovered! > Carlos has been discovered! > Isaac has been discovered! Eve has been examined... > Carol has been discovered! > Imothep has been discovered! Chuck has been examined... Carlos has been examined... > Alice has been discovered! Isaac has been examined... Carol has been examined... Imothep has been examined... Alice has been examined... > Oscar has been discovered! > Dave has been discovered! Oscar has been examined... Dave has been examined... >>> print(dist.a) [0 2 2 1 1 3 1 1 3 2] >>> print(pred.a) [0 3 6 0 0 1 0 0 1 6]

graph_tool.search.
bfs_iterator
(g, source)[source]¶ Return an iterator of the edges corresponding to a breathfirst traversal of the graph.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
Returns: bfs_iterator : An iterator over the edges in breathfirst order.
See also
dfs_iterator
 Depthfirst search
dijkstra_iterator
 Dijkstra’s search algorithm
astar_iterator
 \(A^*\) heuristic search algorithm
Notes
See
bfs_search()
for an explanation of the algorithm.The time complexity is \(O(1)\) to create the generator and \(O(V + E)\) to traverse it completely.
References
[bfs] Edward Moore, “The shortest path through a maze”, International Symposium on the Theory of Switching, 1959 [bfsbgl] http://www.boost.org/doc/libs/release/libs/graph/doc/breadth_first_search.html [bfswikipedia] http://en.wikipedia.org/wiki/Breadthfirst_search Examples
>>> for e in gt.bfs_iterator(g, g.vertex(0)): ... print(name[e.source()], ">", name[e.target()]) Bob > Eve Bob > Chuck Bob > Carlos Bob > Isaac Eve > Carol Eve > Imothep Carlos > Alice Alice > Oscar Alice > Dave

class
graph_tool.search.
DFSVisitor
[source]¶ A visitor object that is invoked at the eventpoints inside the
dfs_search()
algorithm. By default, it performs no action, and should be used as a base class in order to be useful.
initialize_vertex
(u)[source]¶ This is invoked on every vertex of the graph before the start of the graph search.

tree_edge
(e)[source]¶ This is invoked on each edge as it becomes a member of the edges that form the search tree.

back_edge
(e)[source]¶ This is invoked on the back edges in the graph. For an undirected graph there is some ambiguity between tree edges and back edges since the edge (u,v) and (v,u) are the same edge, but both the
tree_edge()
andback_edge()
functions will be invoked. One way to resolve this ambiguity is to record the tree edges, and then disregard the backedges that are already marked as tree edges. An easy way to record tree edges is to record predecessors at the tree_edge event point.


graph_tool.search.
dfs_search
(g, source, visitor=<graph_tool.search.DFSVisitor object>)[source]¶ Depthfirst traversal of a directed or undirected graph.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
visitor :
DFSVisitor
(optional, default:DFSVisitor()
)A visitor object that is invoked at the event points inside the algorithm. This should be a subclass of
DFSVisitor
.See also
bfs_search
 Breadthfirst search
dijkstra_search
 Dijkstra’s search algorithm
astar_search
 \(A^*\) heuristic search algorithm
Notes
When possible, a depthfirst traversal chooses a vertex adjacent to the current vertex to visit next. If all adjacent vertices have already been discovered, or there are no adjacent vertices, then the algorithm backtracks to the last vertex that had undiscovered neighbors. Once all reachable vertices have been visited, the algorithm selects from any remaining undiscovered vertices and continues the traversal. The algorithm finishes when all vertices have been visited.
The time complexity is \(O(V + E)\).
The pseudocode for the DFS algorithm is listed below, with the annotated event points, for which the given visitor object will be called with the appropriate method.
DFS(G) for each vertex u in V color[u] := WHITE initialize vertex u end for time := 0 call DFSVISIT(G, source) start vertex s DFSVISIT(G, u) color[u] := GRAY discover vertex u for each v in Adj[u] examine edge (u,v) if (color[v] = WHITE) (u,v) is a tree edge call DFSVISIT(G, v) else if (color[v] = GRAY) (u,v) is a back edge ... else if (color[v] = BLACK) (u,v) is a cross or forward edge ... end for color[u] := BLACK finish vertex u
References
[dfsbgl] http://www.boost.org/doc/libs/release/libs/graph/doc/depth_first_search.html [dfswikipedia] http://en.wikipedia.org/wiki/Depthfirst_search Examples
We must define what should be done during the search by subclassing
DFSVisitor
, and specializing the appropriate methods. In the following we will keep track of the discover time, and the predecessor tree.class VisitorExample(gt.DFSVisitor): def __init__(self, name, pred, time): self.name = name self.pred = pred self.time = time self.last_time = 0 def discover_vertex(self, u): print(">", self.name[u], "has been discovered!") self.time[u] = self.last_time self.last_time += 1 def examine_edge(self, e): print("edge (%s, %s) has been examined..." % \ (self.name[e.source()], self.name[e.target()])) def tree_edge(self, e): self.pred[e.target()] = int(e.source())
With the above class defined, we can perform the DFS search as follows.
>>> time = g.new_vertex_property("int") >>> pred = g.new_vertex_property("int64_t") >>> gt.dfs_search(g, g.vertex(0), VisitorExample(name, pred, time)) > Bob has been discovered! edge (Bob, Eve) has been examined... > Eve has been discovered! edge (Eve, Bob) has been examined... edge (Eve, Carol) has been examined... > Carol has been discovered! edge (Carol, Eve) has been examined... edge (Carol, Imothep) has been examined... > Imothep has been discovered! edge (Imothep, Carol) has been examined... edge (Imothep, Carlos) has been examined... > Carlos has been discovered! edge (Carlos, Bob) has been examined... edge (Carlos, Alice) has been examined... > Alice has been discovered! edge (Alice, Oscar) has been examined... > Oscar has been discovered! edge (Oscar, Alice) has been examined... edge (Oscar, Dave) has been examined... > Dave has been discovered! edge (Dave, Alice) has been examined... edge (Dave, Oscar) has been examined... edge (Alice, Dave) has been examined... edge (Alice, Carlos) has been examined... edge (Carlos, Eve) has been examined... edge (Carlos, Imothep) has been examined... edge (Imothep, Chuck) has been examined... > Chuck has been discovered! edge (Chuck, Bob) has been examined... edge (Chuck, Eve) has been examined... edge (Chuck, Isaac) has been examined... > Isaac has been discovered! edge (Isaac, Bob) has been examined... edge (Isaac, Chuck) has been examined... edge (Isaac, Eve) has been examined... edge (Chuck, Imothep) has been examined... edge (Imothep, Eve) has been examined... edge (Eve, Carlos) has been examined... edge (Eve, Chuck) has been examined... edge (Eve, Isaac) has been examined... edge (Eve, Imothep) has been examined... edge (Bob, Chuck) has been examined... edge (Bob, Carlos) has been examined... edge (Bob, Isaac) has been examined... >>> print(time.a) [0 5 2 4 8 7 1 9 6 3] >>> print(pred.a) [0 3 6 9 9 8 0 4 1 2]

graph_tool.search.
dfs_iterator
(g, source)[source]¶ Return an iterator of the edges corresponding to a depthfirst traversal of the graph.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
Returns: dfs_iterator : An iterator over the edges in detpthfirst order.
See also
bfs_iterator
 Breadthfirst search
dijkstra_iterator
 Dijkstra’s search algorithm
astar_iterator
 \(A^*\) heuristic search algorithm
Notes
See
dfs_search()
for an explanation of the algorithm.The time complexity is \(O(1)\) to create the generator and \(O(V + E)\) to traverse it completely.
References
[dfsbgl] http://www.boost.org/doc/libs/release/libs/graph/doc/depth_first_search.html [dfswikipedia] http://en.wikipedia.org/wiki/Depthfirst_search Examples
>>> for e in gt.dfs_iterator(g, g.vertex(0)): ... print(name[e.source()], ">", name[e.target()]) Bob > Eve Eve > Carol Carol > Imothep Imothep > Carlos Carlos > Alice Alice > Oscar Oscar > Dave Imothep > Chuck Chuck > Isaac

class
graph_tool.search.
DijkstraVisitor
[source]¶ A visitor object that is invoked at the eventpoints inside the
dijkstra_search()
algorithm. By default, it performs no action, and should be used as a base class in order to be useful.
initialize_vertex
(u)[source]¶ This is invoked on every vertex of the graph before the start of the graph search.

examine_vertex
(u)[source]¶ This is invoked on a vertex as it is popped from the queue. This happens immediately before
examine_edge()
is invoked on each of the outedges of vertex u.

edge_relaxed
(e)[source]¶ Upon examination, if the following condition holds then the edge is relaxed (its distance is reduced), and this method is invoked.
(u, v) = tuple(e) assert(compare(combine(d[u], weight[e]), d[v]))


graph_tool.search.
dijkstra_search
(g, source, weight, visitor=<graph_tool.search.DijkstraVisitor object>, dist_map=None, pred_map=None, combine=<function <lambda>>, compare=<function <lambda>>, zero=0, infinity=inf)[source]¶ Dijsktra traversal of a directed or undirected graph, with nonnegative weights.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
weight :
PropertyMap
Edge property map with weight values.
visitor :
DijkstraVisitor
(optional, default:DijkstraVisitor()
)A visitor object that is invoked at the event points inside the algorithm. This should be a subclass of
DijkstraVisitor
.dist_map :
PropertyMap
(optional, default:None
)A vertex property map where the distances from the source will be stored.
pred_map :
PropertyMap
(optional, default:None
)A vertex property map where the predecessor map will be stored (must have value type “int64_t”).
combine : binary function (optional, default:
lambda a, b: a + b
)This function is used to combine distances to compute the distance of a path.
compare : binary function (optional, default:
lambda a, b: a < b
)This function is use to compare distances to determine which vertex is closer to the source vertex.
zero : int or float (optional, default:
0
)Value assumed to correspond to a distance of zero by the combine and compare functions.
infinity : int or float (optional, default:
numpy.inf
)Value assumed to correspond to a distance of infinity by the combine and compare functions.
Returns: dist_map :
PropertyMap
A vertex property map with the computed distances from the source.
pred_map :
PropertyMap
A vertex property map with the predecessor tree.
See also
bfs_search
 Breadthfirst search
dfs_search
 Depthfirst search
astar_search
 \(A^*\) heuristic search algorithm
Notes
Dijkstra’s algorithm finds all the shortest paths from the source vertex to every other vertex by iteratively “growing” the set of vertices S to which it knows the shortest path. At each step of the algorithm, the next vertex added to S is determined by a priority queue. The queue contains the vertices in V  S prioritized by their distance label, which is the length of the shortest path seen so far for each vertex. The vertex u at the top of the priority queue is then added to S, and each of its outedges is relaxed: if the distance to u plus the weight of the outedge (u,v) is less than the distance label for v then the estimated distance for vertex v is reduced. The algorithm then loops back, processing the next vertex at the top of the priority queue. The algorithm finishes when the priority queue is empty.
The time complexity is \(O(E + V \log V)\).
The pseudocode for Dijkstra’s algorithm is listed below, with the annotated event points, for which the given visitor object will be called with the appropriate method.
DIJKSTRA(G, source, weight) for each vertex u in V initialize vertex u d[u] := infinity p[u] := u end for d[source] := 0 INSERT(Q, source) discover vertex s while (Q != Ø) u := EXTRACTMIN(Q) examine vertex u for each vertex v in Adj[u] examine edge (u,v) if (weight[(u,v)] + d[u] < d[v]) edge (u,v) relaxed d[v] := weight[(u,v)] + d[u] p[v] := u DECREASEKEY(Q, v) else edge (u,v) not relaxed ... if (d[v] was originally infinity) INSERT(Q, v) discover vertex v end for finish vertex u end while return d
References
[dijkstra] E. Dijkstra, “A note on two problems in connexion with graphs”, Numerische Mathematik, 1:269271, 1959. [dijkstrabgl] http://www.boost.org/doc/libs/release/libs/graph/doc/dijkstra_shortest_paths_no_color_map.html [dijkstrawikipedia] http://en.wikipedia.org/wiki/Dijkstra’s_algorithm Examples
We must define what should be done during the search by subclassing
DijkstraVisitor
, and specializing the appropriate methods. In the following we will keep track of the discover time, and the predecessor tree.class VisitorExample(gt.DijkstraVisitor): def __init__(self, name, time): self.name = name self.time = time self.last_time = 0 def discover_vertex(self, u): print(">", self.name[u], "has been discovered!") self.time[u] = self.last_time self.last_time += 1 def examine_edge(self, e): print("edge (%s, %s) has been examined..." % \ (self.name[e.source()], self.name[e.target()])) def edge_relaxed(self, e): print("edge (%s, %s) has been relaxed..." % \ (self.name[e.source()], self.name[e.target()]))
With the above class defined, we can perform the Dijkstra search as follows.
>>> time = g.new_vertex_property("int") >>> dist, pred = gt.dijkstra_search(g, g.vertex(0), weight, VisitorExample(name, time)) > Bob has been discovered! edge (Bob, Eve) has been examined... edge (Bob, Eve) has been relaxed... > Eve has been discovered! edge (Bob, Chuck) has been examined... edge (Bob, Chuck) has been relaxed... > Chuck has been discovered! edge (Bob, Carlos) has been examined... edge (Bob, Carlos) has been relaxed... > Carlos has been discovered! edge (Bob, Isaac) has been examined... edge (Bob, Isaac) has been relaxed... > Isaac has been discovered! edge (Eve, Bob) has been examined... edge (Eve, Carol) has been examined... edge (Eve, Carol) has been relaxed... > Carol has been discovered! edge (Eve, Carlos) has been examined... edge (Eve, Chuck) has been examined... edge (Eve, Isaac) has been examined... edge (Eve, Imothep) has been examined... edge (Eve, Imothep) has been relaxed... > Imothep has been discovered! edge (Isaac, Bob) has been examined... edge (Isaac, Chuck) has been examined... edge (Isaac, Eve) has been examined... edge (Chuck, Bob) has been examined... edge (Chuck, Eve) has been examined... edge (Chuck, Isaac) has been examined... edge (Chuck, Imothep) has been examined... edge (Carlos, Bob) has been examined... edge (Carlos, Alice) has been examined... edge (Carlos, Alice) has been relaxed... > Alice has been discovered! edge (Carlos, Eve) has been examined... edge (Carlos, Imothep) has been examined... edge (Imothep, Carol) has been examined... edge (Imothep, Carlos) has been examined... edge (Imothep, Chuck) has been examined... edge (Imothep, Eve) has been examined... edge (Alice, Oscar) has been examined... edge (Alice, Oscar) has been relaxed... > Oscar has been discovered! edge (Alice, Dave) has been examined... edge (Alice, Dave) has been relaxed... > Dave has been discovered! edge (Alice, Carlos) has been examined... edge (Carol, Eve) has been examined... edge (Carol, Imothep) has been examined... edge (Oscar, Alice) has been examined... edge (Oscar, Dave) has been examined... edge (Dave, Alice) has been examined... edge (Dave, Oscar) has been examined... >>> print(time.a) [0 7 5 3 2 9 1 4 8 6] >>> print(pred.a) [0 3 6 0 0 1 0 0 1 6] >>> print(dist.a) [ 0. 8.91915887 9.27141329 4.29277116 4.02118246 12.23513866 3.23790211 3.45487436 11.04391549 7.74858396]

graph_tool.search.
dijkstra_iterator
(g, source, weight, dist_map=None, combine=None, compare=None, zero=0, infinity=inf)[source]¶ Return an iterator of the edges corresponding to a Dijkstra traversal of the graph.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
weight :
PropertyMap
Edge property map with weight values.
dist_map :
PropertyMap
(optional, default:None
)A vertex property map where the distances from the source will be stored.
combine : binary function (optional, default:
lambda a, b: a + b
)This function is used to combine distances to compute the distance of a path.
compare : binary function (optional, default:
lambda a, b: a < b
)This function is use to compare distances to determine which vertex is closer to the source vertex.
zero : int or float (optional, default:
0
)Value assumed to correspond to a distance of zero by the combine and compare functions.
infinity : int or float (optional, default:
numpy.inf
)Value assumed to correspond to a distance of infinity by the combine and compare functions.
Returns: djk_iterator : An iterator over the edges in Dijkstra order.
See also
bfs_iterator
 Breadthfirst search
dfs_iterator
 Depthfirst search
astar_iterator
 \(A^*\) heuristic search algorithm
Notes
See
dijkstra_search()
for an explanation of the algorithm.The time complexity is \(O(1)\) to create the generator and \(O(E + V\log V)\) to traverse it completely.
References
[dijkstra] E. Dijkstra, “A note on two problems in connexion with graphs”, Numerische Mathematik, 1:269271, 1959. [dijkstrabgl] http://www.boost.org/doc/libs/release/libs/graph/doc/dijkstra_shortest_paths_no_color_map.html [dijkstrawikipedia] http://en.wikipedia.org/wiki/Dijkstra’s_algorithm Examples
>>> for e in gt.dijkstra_iterator(g, g.vertex(0), weight): ... print(name[e.source()], ">", name[e.target()]) Bob > Eve Bob > Chuck Bob > Carlos Bob > Isaac Eve > Carol Eve > Imothep Carlos > Alice Alice > Oscar Alice > Dave

class
graph_tool.search.
BellmanFordVisitor
[source]¶ A visitor object that is invoked at the eventpoints inside the
bellman_ford_search()
algorithm. By default, it performs no action, and should be used as a base class in order to be useful.
edge_relaxed
(e)[source]¶ This is invoked when the distance label for the target vertex is decreased. The edge (u,v) that participated in the last relaxation for vertex v is an edge in the shortest paths tree.

edge_not_relaxed
(e)[source]¶ This is invoked if the distance label for the target vertex is not decreased.


graph_tool.search.
bellman_ford_search
(g, source, weight, visitor=<graph_tool.search.BellmanFordVisitor object>, dist_map=None, pred_map=None, combine=<function <lambda>>, compare=<function <lambda>>, zero=0, infinity=inf)[source]¶ BellmanFord traversal of a directed or undirected graph, with negative weights.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
weight :
PropertyMap
Edge property map with weight values.
visitor :
DijkstraVisitor
(optional, default:DijkstraVisitor()
)A visitor object that is invoked at the event points inside the algorithm. This should be a subclass of
DijkstraVisitor
.dist_map :
PropertyMap
(optional, default:None
)A vertex property map where the distances from the source will be stored.
pred_map :
PropertyMap
(optional, default:None
)A vertex property map where the predecessor map will be stored (must have value type “int64_t”).
combine : binary function (optional, default:
lambda a, b: a + b
)This function is used to combine distances to compute the distance of a path.
compare : binary function (optional, default:
lambda a, b: a < b
)This function is use to compare distances to determine which vertex is closer to the source vertex.
zero : int or float (optional, default:
0
)Value assumed to correspond to a distance of zero by the combine and compare functions.
infinity : int or float (optional, default:
float('inf')
)Value assumed to correspond to a distance of infinity by the combine and compare functions.
Returns: minimized : bool
True if all edges were successfully minimized, or False if there is a negative loop in the graph.
dist_map :
PropertyMap
A vertex property map with the computed distances from the source.
pred_map :
PropertyMap
A vertex property map with the predecessor tree.
See also
bfs_search
 Breadthfirst search
dfs_search
 Depthfirst search
dijsktra_search
 Dijkstra search
astar_search
 \(A^*\) heuristic search
Notes
The BellmanFord algorithm [bellmanford] solves the singlesource shortest paths problem for a graph with both positive and negative edge weights. If you only need to solve the shortest paths problem for positive edge weights,
dijkstra_search()
provides a more efficient alternative. If all the edge weights are all equal, thenbfs_search()
provides an even more efficient alternative.The BellmanFord algorithm proceeds by looping through all of the edges in the graph, applying the relaxation operation to each edge. In the following pseudocode,
v
is a vertex adjacent tou
,w
maps edges to their weight, andd
is a distance map that records the length of the shortest path to each vertex seen so far.p
is a predecessor map which records the parent of each vertex, which will ultimately be the parent in the shortest paths treeRELAX(u, v, w, d, p) if (w(u,v) + d[u] < d[v]) d[v] := w(u,v) + d[u] relax edge (u,v) p[v] := u else ... edge (u,v) is not relaxed
The algorithm repeats this loop
V
times after which it is guaranteed that the distances to each vertex have been reduced to the minimum possible unless there is a negative cycle in the graph. If there is a negative cycle, then there will be edges in the graph that were not properly minimized. That is, there will be edges(u,v)
such thatw(u,v) + d[u] < d[v]
. The algorithm loops over the edges in the graph one final time to check if all the edges were minimized, returning true if they were and returning false otherwise.BELLMANFORD(G) for each vertex u in V d[u] := infinity p[u] := u end for for i := 1 to V1 for each edge (u,v) in E examine edge (u,v) RELAX(u, v, w, d, p) end for end for for each edge (u,v) in E if (w(u,v) + d[u] < d[v]) return (false, , ) edge (u,v) was not minimized else ... edge (u,v) was minimized end for return (true, p, d)
The time complexity is \(O(V E)\).
References
[bellmanford] (1, 2) R. Bellman, “On a routing problem”, Quarterly of Applied Mathematics, 16(1):8790, 1958. [bellmanfordbgl] http://www.boost.org/doc/libs/release/libs/graph/doc/bellman_ford_shortest.html [bellmanfordwikipedia] http://en.wikipedia.org/wiki/BellmanFord_algorithm Examples
We must define what should be done during the search by subclassing
BellmanFordVisitor
, and specializing the appropriate methods. In the following we will keep track of the edge minimizations.class VisitorExample(gt.BellmanFordVisitor): def __init__(self, name): self.name = name def edge_minimized(self, e): print("edge (%s, %s) has been minimized..." % \ (self.name[e.source()], self.name[e.target()])) def edge_not_minimized(self, e): print("edge (%s, %s) has not been minimized..." % \ (self.name[e.source()], self.name[e.target()]))
With the above class defined, we can perform the BellmanFord search as follows.
>>> nweight = g.copy_property(weight) >>> nweight.a[6] *= 1 # include negative weight in edge (Carlos, Alice) >>> minimized, dist, pred = gt.bellman_ford_search(g, g.vertex(0), nweight, VisitorExample(name)) edge (Bob, Eve) has been minimized... edge (Bob, Chuck) has been minimized... edge (Bob, Carlos) has been minimized... edge (Bob, Isaac) has been minimized... edge (Alice, Oscar) has been minimized... edge (Alice, Dave) has been minimized... edge (Alice, Carlos) has been minimized... edge (Carol, Eve) has been minimized... edge (Carol, Imothep) has been minimized... edge (Carlos, Eve) has been minimized... edge (Carlos, Imothep) has been minimized... edge (Chuck, Eve) has been minimized... edge (Chuck, Isaac) has been minimized... edge (Chuck, Imothep) has been minimized... edge (Dave, Oscar) has been minimized... edge (Eve, Isaac) has been minimized... edge (Eve, Imothep) has been minimized... >>> print(minimized) True >>> print(pred.a) [3 3 9 1 6 1 3 6 1 3] >>> print(dist.a) [28.42555934 37.34471821 25.20438243 41.97110592 35.20316571 34.02873843 36.58860946 33.55645565 35.2199616 36.0029274 ]

class
graph_tool.search.
AStarVisitor
[source]¶ A visitor object that is invoked at the eventpoints inside the
astar_search()
algorithm. By default, it performs no action, and should be used as a base class in order to be useful.
initialize_vertex
(u)[source]¶ This is invoked on every vertex of the graph before the start of the graph search.

examine_vertex
(u)[source]¶ This is invoked on a vertex as it is popped from the queue (i.e. it has the lowest cost on the
OPEN
list). This happens immediately before examine_edge() is invoked on each of the outedges of vertex u.

discover_vertex
(u)[source]¶ This is invoked when a vertex is first discovered and is added to the
OPEN
list.

edge_relaxed
(e)[source]¶ Upon examination, if the following condition holds then the edge is relaxed (its distance is reduced), and this method is invoked.
(u, v) = tuple(e) assert(compare(combine(d[u], weight[e]), d[v]))

edge_not_relaxed
(e)[source]¶ Upon examination, if the edge is not relaxed (see
edge_relaxed()
) then this method is invoked.


graph_tool.search.
astar_search
(g, source, weight, visitor=<graph_tool.search.AStarVisitor object>, heuristic=<function <lambda>>, dist_map=None, pred_map=None, cost_map=None, combine=<function <lambda>>, compare=<function <lambda>>, zero=0, infinity=inf, implicit=False)[source]¶ Heuristic \(A^*\) search on a weighted, directed or undirected graph for the case where all edge weights are nonnegative.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
weight :
PropertyMap
Edge property map with weight values.
visitor :
AStarVisitor
(optional, default:AStarVisitor()
)A visitor object that is invoked at the event points inside the algorithm. This should be a subclass of
AStarVisitor
.heuristic : unary function (optional, default:
lambda v: 1
)The heuristic function that guides the search. It should take a single argument which is a
Vertex
, and output an estimated distance from the supplied vertex to the target vertex.dist_map :
PropertyMap
(optional, default:None
)A vertex property map where the distances from the source will be stored.
pred_map :
PropertyMap
(optional, default:None
)A vertex property map where the predecessor map will be stored (must have value type “int64_t”).
cost_map :
PropertyMap
(optional, default:None
)A vertex property map where the vertex costs will be stored. It must have the same value type as
dist_map
. This parameter is only used ifimplicit
is True.combine : binary function (optional, default:
lambda a, b: a + b
)This function is used to combine distances to compute the distance of a path.
compare : binary function (optional, default:
lambda a, b: a < b
)This function is use to compare distances to determine which vertex is closer to the source vertex.
implicit : bool (optional, default:
False
)If true, the underlying graph will be assumed to be implicit (i.e. constructed during the search).
zero : int or float (optional, default:
0
)Value assumed to correspond to a distance of zero by the combine and compare functions.
infinity : int or float (optional, default:
float('inf')
)Value assumed to correspond to a distance of infinity by the combine and compare functions.
Returns: dist_map :
PropertyMap
A vertex property map with the computed distances from the source.
pred_map :
PropertyMap
A vertex property map with the predecessor tree.
See also
bfs_search
 Breadthfirst search
dfs_search
 Depthfirst search
dijkstra_search
 Dijkstra’s search algorithm
Notes
The \(A^*\) algorithm is a heuristic graph search algorithm: an \(A^*\) search is “guided” by a heuristic function. A heuristic function \(h(v)\) is one which estimates the cost from a nongoal state (v) in the graph to some goal state, t. Intuitively, \(A^*\) follows paths (through the graph) to the goal that are estimated by the heuristic function to be the best paths. Unlike bestfirst search, \(A^*\) takes into account the known cost from the start of the search to v; the paths \(A^*\) takes are guided by a function \(f(v) = g(v) + h(v)\), where \(h(v)\) is the heuristic function, and \(g(v)\) (sometimes denoted \(c(s, v)\)) is the known cost from the start to v. Clearly, the efficiency of \(A^*\) is highly dependent on the heuristic function with which it is used.
The time complexity is \(O((E + V)\log V)\).
The pseudocode for the \(A^*\) algorithm is listed below, with the annotated event points, for which the given visitor object will be called with the appropriate method.
A*(G, source, h) for each vertex u in V initialize vertex u d[u] := f[u] := infinity color[u] := WHITE end for color[s] := GRAY d[s] := 0 f[s] := h(source) INSERT(Q, source) discover vertex source while (Q != Ø) u := EXTRACTMIN(Q) examine vertex u for each vertex v in Adj[u] examine edge (u,v) if (w(u,v) + d[u] < d[v]) d[v] := w(u,v) + d[u] edge (u,v) relaxed f[v] := d[v] + h(v) if (color[v] = WHITE) color[v] := GRAY INSERT(Q, v) discover vertex v else if (color[v] = BLACK) color[v] := GRAY INSERT(Q, v) reopen vertex v end if else ... edge (u,v) not relaxed end for color[u] := BLACK finish vertex u end while
References
[astar] Hart, P. E.; Nilsson, N. J.; Raphael, B. “A Formal Basis for the Heuristic Determination of Minimum Cost Paths”. IEEE Transactions on Systems Science and Cybernetics SSC4 4 (2): 100107, 1968. DOI: 10.1109/TSSC.1968.300136 [astarbgl] http://www.boost.org/doc/libs/release/libs/graph/doc/astar_search.html [astarwikipedia] http://en.wikipedia.org/wiki/A*_search_algorithm Examples
We will use an irregular twodimensional lattice as an example, where the heuristic function will be based on the euclidean distance to the target.
The heuristic function will be defined as:
def h(v, target, pos): return sqrt(sum((pos[v].a  pos[target].a) ** 2))
where
pos
is the vertex position in the plane.We must define what should be done during the search by subclassing
AStarVisitor
, and specializing the appropriate methods. In the following we will keep track of the discovered vertices, and which edges were examined, as well as the predecessor tree. We will also abort the search when a given target vertex is found, by raising theStopSearch
exception.class VisitorExample(gt.AStarVisitor): def __init__(self, touched_v, touched_e, target): self.touched_v = touched_v self.touched_e = touched_e self.target = target def discover_vertex(self, u): self.touched_v[u] = True def examine_edge(self, e): self.touched_e[e] = True def edge_relaxed(self, e): if e.target() == self.target: raise gt.StopSearch()
With the above class defined, we can perform the \(A^*\) search as follows.
>>> points = random((500, 2)) * 4 >>> points[0] = [0.01, 0.01] >>> points[1] = [4.01, 4.01] >>> g, pos = gt.triangulation(points, type="delaunay") >>> weight = g.new_edge_property("double") # Edge weights corresponding to ... # Euclidean distances >>> for e in g.edges(): ... weight[e] = sqrt(sum((pos[e.source()].a  ... pos[e.target()].a) ** 2)) >>> touch_v = g.new_vertex_property("bool") >>> touch_e = g.new_edge_property("bool") >>> target = g.vertex(1) >>> dist, pred = gt.astar_search(g, g.vertex(0), weight, ... VisitorExample(touch_v, touch_e, target), ... heuristic=lambda v: h(v, target, pos))
We can now observe the best path found, and how many vertices and edges were visited in the process.
>>> ecolor = g.new_edge_property("string") >>> ewidth = g.new_edge_property("double") >>> ewidth.a = 1 >>> for e in g.edges(): ... ecolor[e] = "#3465a4" if touch_e[e] else "#d3d7cf" >>> v = target >>> while v != g.vertex(0): ... p = g.vertex(pred[v]) ... for e in v.out_edges(): ... if e.target() == p: ... ecolor[e] = "#a40000" ... ewidth[e] = 3 ... v = p >>> gt.graph_draw(g, pos=pos, output_size=(300, 300), vertex_fill_color=touch_v, ... vcmap=matplotlib.cm.binary, edge_color=ecolor, ... edge_pen_width=ewidth, output="astardelaunay.pdf") <...>
The \(A^*\) algorithm is very useful for searching implicit graphs, i.e. graphs which are not entirely stored in memory and are generated “onthefly” during the search. In the following example we will carry out a search in a hamming hypercube of 10 bits witch has random weights on its edges in the range \([0,1]\). The vertices of the hypercube will be created during the search.
The heuristic function will use the Hamming distance between vertices:
def h(v, target, state): return sum(abs(state[v].a  target)) / 2
In the following visitor we will keep growing the graph onthefly, and abort the search when a given target vertex is found, by raising the
StopSearch
exception.from numpy.random import random class HammingVisitor(gt.AStarVisitor): def __init__(self, g, target, state, weight, dist, cost): self.g = g self.state = state self.target = target self.weight = weight self.dist = dist self.cost = cost self.visited = {} def examine_vertex(self, u): for i in range(len(self.state[u])): nstate = list(self.state[u]) nstate[i] ^= 1 if tuple(nstate) in self.visited: v = self.visited[tuple(nstate)] else: v = self.g.add_vertex() self.visited[tuple(nstate)] = v self.state[v] = nstate self.dist[v] = self.cost[v] = float('inf') for e in u.out_edges(): if e.target() == v: break else: e = self.g.add_edge(u, v) self.weight[e] = random() self.visited[tuple(self.state[u])] = u def edge_relaxed(self, e): if self.state[e.target()] == self.target: self.visited[tuple(self.target)] = e.target() raise gt.StopSearch()
With the above class defined, we can perform the \(A^*\) search as follows.
>>> g = gt.Graph(directed=False) >>> state = g.new_vertex_property("vector<bool>") >>> v = g.add_vertex() >>> state[v] = [0] * 10 >>> target = [1] * 10 >>> weight = g.new_edge_property("double") >>> dist = g.new_vertex_property("double") >>> cost = g.new_vertex_property("double") >>> visitor = HammingVisitor(g, target, state, weight, dist, cost) >>> dist, pred = gt.astar_search(g, g.vertex(0), weight, visitor, dist_map=dist, ... cost_map=cost, heuristic=lambda v: h(v, array(target), state), ... implicit=True)
We can now observe the best path found, and how many vertices and edges were visited in the process.
>>> ecolor = g.new_edge_property("string") >>> vcolor = g.new_vertex_property("string") >>> ewidth = g.new_edge_property("double") >>> ewidth.a = 1 >>> for e in g.edges(): ... ecolor[e] = "black" >>> for v in g.vertices(): ... vcolor[v] = "white" >>> v = visitor.visited[tuple(target)] >>> while v != g.vertex(0): ... vcolor[v] = "black" ... p = g.vertex(pred[v]) ... for e in v.out_edges(): ... if e.target() == p: ... ecolor[e] = "#a40000" ... ewidth[e] = 3 ... v = p >>> vcolor[v] = "black" >>> pos = gt.graph_draw(g, output_size=(300, 300), vertex_fill_color=vcolor, edge_color=ecolor, ... edge_pen_width=ewidth, output="astarimplicit.pdf")

graph_tool.search.
astar_iterator
(g, source, weight, heuristic=<function <lambda>>, dist_map=None, combine=None, compare=None, zero=0, infinity=inf)[source]¶ Return an iterator of the edges corresponding to an \(A^*\) traversal of the graph.
Parameters: g :
Graph
Graph to be used.
source :
Vertex
Source vertex.
weight :
PropertyMap
Edge property map with weight values.
heuristic : unary function (optional, default:
lambda v: 1
)The heuristic function that guides the search. It should take a single argument which is a
Vertex
, and output an estimated distance from the supplied vertex to the target vertex.dist_map :
PropertyMap
(optional, default:None
)A vertex property map where the distances from the source will be stored.
combine : binary function (optional, default:
lambda a, b: a + b
)This function is used to combine distances to compute the distance of a path.
compare : binary function (optional, default:
lambda a, b: a < b
)This function is use to compare distances to determine which vertex is closer to the source vertex.
zero : int or float (optional, default:
0
)Value assumed to correspond to a distance of zero by the combine and compare functions.
infinity : int or float (optional, default:
numpy.inf
)Value assumed to correspond to a distance of infinity by the combine and compare functions.
Returns: astar_iterator : An iterator over the edges in \(A^*\) order.
See also
bfs_iterator
 Breadthfirst search
dfs_iterator
 Depthfirst search
dijkstra_iterator
 Dijkstra search algorithm
Notes
See
astar_search()
for an explanation of the algorithm.The time complexity is \(O(1)\) to create the generator and \(O((E + V)\log V)\) to traverse it completely.
References
[astar] Hart, P. E.; Nilsson, N. J.; Raphael, B. “A Formal Basis for the Heuristic Determination of Minimum Cost Paths”. IEEE Transactions on Systems Science and Cybernetics SSC4 4 (2): 100107, 1968. DOI: 10.1109/TSSC.1968.300136 [astarbgl] http://www.boost.org/doc/libs/release/libs/graph/doc/astar_search.html [astarwikipedia] http://en.wikipedia.org/wiki/A*_search_algorithm Examples
>>> g = gt.load_graph("search_example.xml") >>> name = g.vp["name"] >>> weight = g.ep["weight"] >>> for e in gt.astar_iterator(g, g.vertex(0), weight): ... print(name[e.source()], ">", name[e.target()]) Bob > Eve Bob > Chuck Bob > Carlos Bob > Isaac Eve > Carol Eve > Imothep Carlos > Alice Alice > Oscar Alice > Dave