问题
Real-world problem:
I have data on directors across many firms, but sometimes "John Smith, director of XYZ" and "John Smith, director of ABC" are the same person, sometimes they're not. Also "John J. Smith, director of XYZ" and "John Smith, director of ABC" might be the same person, or might not be. Often examination of additional information (e.g., comparison of biographical data on "John Smith, director of XYZ" and "John Smith, director of ABC") makes it possible to resolve whether two observations are the same person or not.
Conceptual version of the problem:
In that spirit, am collecting data that will identify matching pairs. For example, suppose I have the following matching pairs: {(a, b), (b, c), (c, d), (d, e), (f, g)}. I want to use the transitivity property of the relation "is the same person as" to generate "connected components" of {{a, b, c, d, e}, {f, g}}. That is {a, b, c, d, e} is one person and {f, g} is another. (An earlier version of the question referred to "cliques", which are apparently something else; this would explain why find_cliques in networkx was giving the "wrong" results (for my purposes).
The following Python code does the job. But I wonder: is there a better (less computationally costly) approach (e.g., using standard or available libraries)?
There are examples here and there that seem related (e.g., Cliques in python), but these are incomplete, so I am not sure what libraries they are referring to or how to set up my data to use them.
Sample Python 2 code:
def get_cliques(pairs):
from sets import Set
set_list = [Set(pairs[0])]
for pair in pairs[1:]:
matched=False
for set in set_list:
if pair[0] in set or pair[1] in set:
set.update(pair)
matched=True
break
if not matched:
set_list.append(Set(pair))
return set_list
pairs = [('a', 'b'), ('b', 'c'), ('c', 'd'), ('d', 'e'), ('f', 'g')]
print(get_cliques(pairs))
This produces the desired output: [Set(['a', 'c', 'b', 'e', 'd']), Set(['g', 'f'])].
Sample Python 3 code:
This produces [set(['a', 'c', 'b', 'e', 'd']), set(['g', 'f'])]):
def get_cliques(pairs):
set_list = [set(pairs[0])]
for pair in pairs[1:]:
matched=False
for a_set in set_list:
if pair[0] in a_set or pair[1] in a_set:
a_set.update(pair)
matched=True
break
if not matched:
set_list.append(set(pair))
return set_list
pairs = [('a', 'b'), ('b', 'c'), ('c', 'd'), ('d', 'e'), ('f', 'g')]
print(get_cliques(pairs))
回答1:
With networkX:
import networkx as nx
G1=nx.Graph()
G1.add_edges_from([("a","b"),("b","c"),("c","d"),("d","e"),("f","g")])
sorted(nx.connected_components(G1), key = len, reverse=True)
giving:
[['a', 'd', 'e', 'b', 'c'], ['f', 'g']]
You have to check the fastest algorithm now ...
OP:
This works great! I have this in my PostgreSQL database now. Just organize pairs into a two-column table, then use array_agg() to pass to PL/Python function get_connected(). Thanks.
CREATE OR REPLACE FUNCTION get_connected(
lhs text[],
rhs text[])
RETURNS SETOF text[] AS
$BODY$
pairs = zip(lhs, rhs)
import networkx as nx
G=nx.Graph()
G.add_edges_from(pairs)
return sorted(nx.connected_components(G), key = len, reverse=True)
$BODY$ LANGUAGE plpythonu;
(Note: I edited answer, as I thought showing this step might be helpful addendum, but too long for a comment.)
回答2:
I don't believe (correct me if I'm wrong) that this is directly related to the largest clique problem. The definition of cliques (wikipedia) says that a clique "in an undirected graph is a subset of its vertices such that every two vertices in the subset are connected by an edge". In this case, we want to find which nodes can reach eachother (even indirectly).
I made a little sample. It builds a graph and traverses it looking for neighbors. This should be pretty efficient since each node is only traversed once when groups are formed.
from collections import defaultdict
def get_cliques(pairs):
# Build a graph using the pairs
nodes = defaultdict(lambda: [])
for a, b in pairs:
if b is not None:
nodes[a].append((b, nodes[b]))
nodes[b].append((a, nodes[a]))
else:
nodes[a] # empty list
# Add all neighbors to the same group
visited = set()
def _build_group(key, group):
if key in visited:
return
visited.add(key)
group.add(key)
for key, _ in nodes[key]:
_build_group(key, group)
groups = []
for key in nodes.keys():
if key in visited: continue
groups.append(set())
_build_group(key, groups[-1])
return groups
if __name__ == '__main__':
pairs = [
('a', 'b'), ('b', 'c'), ('b', 'd'), # a "tree"
('f', None), # no relations
('h', 'i'), ('i', 'j'), ('j', 'h') # circular
]
print get_cliques(pairs)
# Output: [set(['a', 'c', 'b', 'd']), set(['f']), set(['i', 'h', 'j'])]
If your data set is best modeled like a graph and really big, maybe a graph database such as Neo4j is appropriate?
回答3:
DSM's comment made me look for set consolidation algorithms in Python. Rosetta Code has two versions of the same algorithm. Example use (the non-recursive version):
[('a', 'b'), ('b', 'c'), ('c', 'd'), ('d', 'e'), ('f', 'g')]
# Copied from Rosetta Code
def consolidate(sets):
setlist = [s for s in sets if s]
for i, s1 in enumerate(setlist):
if s1:
for s2 in setlist[i+1:]:
intersection = s1.intersection(s2)
if intersection:
s2.update(s1)
s1.clear()
s1 = s2
return [s for s in setlist if s]
print consolidate([set(pair) for pair in pairs])
# Output: [set(['a', 'c', 'b', 'd']), set([None, 'f']), set(['i', 'h', 'j'])]
回答4:
I tried an alternate implementation using dictionaries as lookups and may have gotten a small reduction in computational latency.
# Modified to use a dictionary
from collections import defaultdict
def get_cliques2(pairs):
maxClique = 1
clique = defaultdict(int)
for (a, b) in pairs:
currentClique = max(clique[i] for i in (a,b))
if currentClique == 0:
currentClique = maxClique
maxClique += 1
clique[a] = clique[b] = currentClique
reversed = defaultdict(list)
for (k, v) in clique.iteritems(): reversed[v].append(k)
return reversed
And just to convince myself that it returns the right result (get_cliques1 here is your original Python 2 solution):
>>> from cliques import *
>>> get_cliques1(pairs) # Original Python 2 solution
[Set(['a', 'c', 'b', 'e', 'd']), Set(['g', 'f'])]
>>> get_cliques2(pairs) # Dictionary-based alternative
[['a', 'c', 'b', 'e', 'd'], ['g', 'f']]
Timing info in seconds (with 10 million repetitions):
$ python get_times.py
get_cliques: 75.1285209656
get_cliques2: 69.9816100597
For the sake of completeness and reference, this is the full listing of both cliques.py and the get_times.py timing script:
# cliques.py
# Python 2.7
from collections import defaultdict
from sets import Set # I moved your import out of the function to try to get closer to apples-apples
# Original Python 2 solution
def get_cliques1(pairs):
set_list = [Set(pairs[0])]
for pair in pairs[1:]:
matched=False
for set in set_list:
if pair[0] in set or pair[1] in set:
set.update(pair)
matched=True
break
if not matched:
set_list.append(Set(pair))
return set_list
# Modified to use a dictionary
def get_cliques2(pairs):
maxClique = 1
clique = defaultdict(int)
for (a, b) in pairs:
currentClique = max(clique[i] for i in (a,b))
if currentClique == 0:
currentClique = maxClique
maxClique += 1
clique[a] = clique[b] = currentClique
reversed = defaultdict(list)
for (k, v) in clique.iteritems(): reversed[v].append(k)
return reversed.values()
pairs = [('a', 'b'), ('b', 'c'), ('c', 'd'), ('d', 'e'), ('f', 'g')]
# get_times.py
# Python 2.7
from timeit import timeit
REPS = 10000000
print "get_cliques: " + str(timeit(
stmt='get_cliques1(pairs)', setup='from cliques import get_cliques1, pairs',
number=REPS
))
print "get_cliques2: " + str(timeit(
stmt='get_cliques2(pairs)', setup='from cliques import get_cliques2, pairs',
number=REPS
))
So at least in this contrived scenario, there is a measurable speedup. It's admittedly not groundbreaking, and I'm sure I left some performance bits on the table in my implementation, but maybe it will help get you thinking about other alternatives?
来源：https://stackoverflow.com/questions/27967093/how-to-aggregate-matching-pairs-into-connected-components-in-python