NetworkX

Source code for networkx.algorithms.shortest_paths.generic

# -*- coding: utf-8 -*-
"""
Compute the shortest paths and path lengths between nodes in the graph.

These algorithms work with undirected and directed graphs.

For directed graphs the paths can be computed in the reverse
order by first flipping the edge orientation using R=G.reverse(copy=False).

"""
__author__ = """Aric Hagberg (hagberg@lanl.gov)"""
#    Copyright (C) 2004-2010 by 
#    Aric Hagberg <hagberg@lanl.gov>
#    Dan Schult <dschult@colgate.edu>
#    Pieter Swart <swart@lanl.gov>
#    All rights reserved.
#    BSD license.

__all__ = ['shortest_path',
           'shortest_path_length',
           'average_shortest_path_length',
           'has_path']

import networkx as nx

[docs]def has_path(G, source ,target): """Return true if G has a path from source to target. False otherwise. Parameters ---------- G : NetworkX graph source : node Starting node for path. target : node Ending node for path. """ try: sp = nx.shortest_path(G,source, target) except nx.NetworkXNoPath: return False return True
[docs]def shortest_path(G, source=None, target=None, weight=None): """Compute shortest paths in the graph. Parameters ---------- G : NetworkX graph source : node, optional Starting node for path. If not specified compute shortest paths for all connected node pairs. target : node, optional Ending node for path. If not specified compute shortest paths for every node reachable from the source. weight : None or string, optional (default = None) If None, every edge has weight/distance/cost 1. If a string, use this edge attribute as the edge weight. Any edge attribute not present defaults to 1. Returns ------- path: list or dictionary If the source and target are both specified return a single list of nodes in a shortest path. If only the source is specified return a dictionary keyed by targets with a list of nodes in a shortest path. If neither the source or target is specified return a dictionary of dictionaries with path[source][target]=[list of nodes in path]. Examples -------- >>> G=nx.path_graph(5) >>> print(nx.shortest_path(G,source=0,target=4)) [0, 1, 2, 3, 4] >>> p=nx.shortest_path(G,source=0) # target not specified >>> p[4] [0, 1, 2, 3, 4] >>> p=nx.shortest_path(G) # source,target not specified >>> p[0][4] [0, 1, 2, 3, 4] Notes ----- There may be more than one shortest path between a source and target. This returns only one of them. For digraphs this returns a shortest directed path. To find paths in the reverse direction use G.reverse(copy=False) first to flip the edge orientation. See Also -------- all_pairs_shortest_path() all_pairs_dijkstra_path() single_source_shortest_path() single_source_dijkstra_path() """ if source is None: if target is None: if weight is None: paths=nx.all_pairs_shortest_path(G) else: paths=nx.all_pairs_dijkstra_path(G,weight=weight) else: raise nx.NetworkXError(\ "Target given but no source specified.") else: # source specified if target is None: if weight is None: paths=nx.single_source_shortest_path(G,source) else: paths=nx.single_source_dijkstra_path(G,source,weight=weight) else: # shortest source-target path if weight is None: paths=nx.bidirectional_shortest_path(G,source,target) else: paths=nx.dijkstra_path(G,source,target,weight) return paths
[docs]def shortest_path_length(G, source=None, target=None, weight=None): """Compute shortest path lengths in the graph. This function can compute the single source shortest path lengths by specifying only the source or all pairs shortest path lengths by specifying neither the source or target. Parameters ---------- G : NetworkX graph source : node, optional Starting node for path. If not specified compute shortest path lengths for all connected node pairs. target : node, optional Ending node for path. If not specified compute shortest path lengths for every node reachable from the source. weight : None or string, optional (default = None) If None, every edge has weight/distance/cost 1. If a string, use this edge attribute as the edge weight. Any edge attribute not present defaults to 1. Returns ------- length : number, or container of numbers If the source and target are both specified return a single number for the shortest path. If only the source is specified return a dictionary keyed by targets with a the shortest path as keys. If neither the source or target is specified return a dictionary of dictionaries with length[source][target]=value. Raises ------ NetworkXNoPath If no path exists between source and target. Examples -------- >>> G=nx.path_graph(5) >>> print(nx.shortest_path_length(G,source=0,target=4)) 4 >>> p=nx.shortest_path_length(G,source=0) # target not specified >>> p[4] 4 >>> p=nx.shortest_path_length(G) # source,target not specified >>> p[0][4] 4 Notes ----- For digraphs this returns the shortest directed path. To find path lengths in the reverse direction use G.reverse(copy=False) first to flip the edge orientation. See Also -------- all_pairs_shortest_path_length() all_pairs_dijkstra_path_length() single_source_shortest_path_length() single_source_dijkstra_path_length() """ if source is None: if target is None: if weight is None: paths=nx.all_pairs_shortest_path_length(G) else: paths=nx.all_pairs_dijkstra_path_length(G, weight=weight) else: raise nx.NetworkXError(\ "Target given but no source specified.") else: # source specified if target is None: if weight is None: paths=nx.single_source_shortest_path_length(G,source) else: paths=nx.single_source_dijkstra_path_length(G,source,weight=weight) else: # shortest source-target path if weight is None: p=nx.bidirectional_shortest_path(G,source,target) paths=len(p)-1 else: paths=nx.dijkstra_path_length(G,source,target,weight) return paths
[docs]def average_shortest_path_length(G, weight=None): r"""Return the average shortest path length. The average shortest path length is .. math:: a =\sum_{s,t \in V} \frac{d(s, t)}{n(n-1)} where `V` is the set of nodes in `G`, `d(s, t)` is the shortest path from `s` to `t`, and `n` is the number of nodes in `G`. Parameters ---------- G : NetworkX graph weight : None or string, optional (default = None) If None, every edge has weight/distance/cost 1. If a string, use this edge attribute as the edge weight. Any edge attribute not present defaults to 1. Raises ------ NetworkXError: if the graph is not connected. Examples -------- >>> G=nx.path_graph(5) >>> print(nx.average_shortest_path_length(G)) 2.0 For disconnected graphs you can compute the average shortest path length for each component: >>> G=nx.Graph([(1,2),(3,4)]) >>> for g in nx.connected_component_subgraphs(G): ... print(nx.average_shortest_path_length(g)) 1.0 1.0 """ if G.is_directed(): if not nx.is_weakly_connected(G): raise nx.NetworkXError("Graph is not connected.") else: if not nx.is_connected(G): raise nx.NetworkXError("Graph is not connected.") avg=0.0 if weight is None: for node in G: path_length=nx.single_source_shortest_path_length(G, node) avg += sum(path_length.values()) else: for node in G: path_length=nx.single_source_dijkstra_path_length(G, node, weight=weight) avg += sum(path_length.values()) n=len(G) return avg/(n*(n-1))