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Source code for networkx.algorithms.traversal.depth_first_search

"""
==================
Depth-first search
==================

Basic algorithms for depth-first searching.

Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
"""
import networkx as nx
from collections import defaultdict
__author__ = """\n""".join(['Aric Hagberg <aric.hagberg@gmail.com>'])
__all__ = ['dfs_edges', 'dfs_tree',
           'dfs_predecessors', 'dfs_successors',
           'dfs_preorder_nodes','dfs_postorder_nodes',
           'dfs_labeled_edges']

[docs]def dfs_edges(G, source=None): """Produce edges in a depth-first-search (DFS). Parameters ---------- G : NetworkX graph source : node, optional Specify starting node for depth-first search and return edges in the component reachable from source. Returns ------- edges: generator A generator of edges in the depth-first-search. Examples -------- >>> G = nx.Graph() >>> G.add_path([0,1,2]) >>> print(list(nx.dfs_edges(G,0))) [(0, 1), (1, 2)] Notes ----- Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py by D. Eppstein, July 2004. If a source is not specified then a source is chosen arbitrarily and repeatedly until all components in the graph are searched. """ if source is None: # produce edges for all components nodes = G else: # produce edges for components with source nodes = [source] visited=set() for start in nodes: if start in visited: continue visited.add(start) stack = [(start,iter(G[start]))] while stack: parent,children = stack[-1] try: child = next(children) if child not in visited: yield parent,child visited.add(child) stack.append((child,iter(G[child]))) except StopIteration: stack.pop()
[docs]def dfs_tree(G, source): """Return oriented tree constructed from a depth-first-search from source. Parameters ---------- G : NetworkX graph source : node, optional Specify starting node for depth-first search. Returns ------- T : NetworkX DiGraph An oriented tree Examples -------- >>> G = nx.Graph() >>> G.add_path([0,1,2]) >>> T = nx.dfs_tree(G,0) >>> print(T.edges()) [(0, 1), (1, 2)] """ T = nx.DiGraph() if source is None: T.add_nodes_from(G) else: T.add_node(source) T.add_edges_from(dfs_edges(G,source)) return T
[docs]def dfs_predecessors(G, source=None): """Return dictionary of predecessors in depth-first-search from source. Parameters ---------- G : NetworkX graph source : node, optional Specify starting node for depth-first search and return edges in the component reachable from source. Returns ------- pred: dict A dictionary with nodes as keys and predecessor nodes as values. Examples -------- >>> G = nx.Graph() >>> G.add_path([0,1,2]) >>> print(nx.dfs_predecessors(G,0)) {1: 0, 2: 1} Notes ----- Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py by D. Eppstein, July 2004. If a source is not specified then a source is chosen arbitrarily and repeatedly until all components in the graph are searched. """ return dict((t,s) for s,t in dfs_edges(G,source=source))
[docs]def dfs_successors(G, source=None): """Return dictionary of successors in depth-first-search from source. Parameters ---------- G : NetworkX graph source : node, optional Specify starting node for depth-first search and return edges in the component reachable from source. Returns ------- succ: dict A dictionary with nodes as keys and list of successor nodes as values. Examples -------- >>> G = nx.Graph() >>> G.add_path([0,1,2]) >>> print(nx.dfs_successors(G,0)) {0: [1], 1: [2]} Notes ----- Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py by D. Eppstein, July 2004. If a source is not specified then a source is chosen arbitrarily and repeatedly until all components in the graph are searched. """ d = defaultdict(list) for s,t in dfs_edges(G,source=source): d[s].append(t) return dict(d)
[docs]def dfs_postorder_nodes(G,source=None): """Produce nodes in a depth-first-search post-ordering starting from source. Parameters ---------- G : NetworkX graph source : node, optional Specify starting node for depth-first search and return edges in the component reachable from source. Returns ------- nodes: generator A generator of nodes in a depth-first-search post-ordering. Examples -------- >>> G = nx.Graph() >>> G.add_path([0,1,2]) >>> print(list(nx.dfs_postorder_nodes(G,0))) [2, 1, 0] Notes ----- Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py by D. Eppstein, July 2004. If a source is not specified then a source is chosen arbitrarily and repeatedly until all components in the graph are searched. """ post=(v for u,v,d in nx.dfs_labeled_edges(G,source=source) if d['dir']=='reverse') # potential modification: chain source to end of post-ordering # return chain(post,[source]) return post
[docs]def dfs_preorder_nodes(G, source=None): """Produce nodes in a depth-first-search pre-ordering starting from source. Parameters ---------- G : NetworkX graph source : node, optional Specify starting node for depth-first search and return edges in the component reachable from source. Returns ------- nodes: generator A generator of nodes in a depth-first-search pre-ordering. Examples -------- >>> G = nx.Graph() >>> G.add_path([0,1,2]) >>> print(list(nx.dfs_preorder_nodes(G,0))) [0, 1, 2] Notes ----- Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py by D. Eppstein, July 2004. If a source is not specified then a source is chosen arbitrarily and repeatedly until all components in the graph are searched. """ pre=(v for u,v,d in nx.dfs_labeled_edges(G,source=source) if d['dir']=='forward') # potential modification: chain source to beginning of pre-ordering # return chain([source],pre) return pre
[docs]def dfs_labeled_edges(G, source=None): """Produce edges in a depth-first-search (DFS) labeled by type. Parameters ---------- G : NetworkX graph source : node, optional Specify starting node for depth-first search and return edges in the component reachable from source. Returns ------- edges: generator A generator of edges in the depth-first-search labeled with 'forward', 'nontree', and 'reverse'. Examples -------- >>> G = nx.Graph() >>> G.add_path([0,1,2]) >>> edges = (list(nx.dfs_labeled_edges(G,0))) Notes ----- Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py by D. Eppstein, July 2004. If a source is not specified then a source is chosen arbitrarily and repeatedly until all components in the graph are searched. """ # Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py # by D. Eppstein, July 2004. if source is None: # produce edges for all components nodes = G else: # produce edges for components with source nodes = [source] visited = set() for start in nodes: if start in visited: continue yield start,start,{'dir':'forward'} visited.add(start) stack = [(start,iter(G[start]))] while stack: parent,children = stack[-1] try: child = next(children) if child in visited: yield parent,child,{'dir':'nontree'} else: yield parent,child,{'dir':'forward'} visited.add(child) stack.append((child,iter(G[child]))) except StopIteration: stack.pop() if stack: yield stack[-1][0],parent,{'dir':'reverse'} yield start,start,{'dir':'reverse'}