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This documents an unmaintained version of NetworkX. Please upgrade to a maintained version and see the current NetworkX documentation.

# Source code for networkx.algorithms.link_analysis.pagerank_alg

"""PageRank analysis of graph structure. """
#    Copyright (C) 2004-2013 by
#    Aric Hagberg <hagberg@lanl.gov>
#    Dan Schult <dschult@colgate.edu>
#    Pieter Swart <swart@lanl.gov>
#    NetworkX:http://networkx.lanl.gov/
import networkx as nx
from networkx.exception import NetworkXError
from networkx.utils import not_implemented_for
__author__ = """\n""".join(["Aric Hagberg <aric.hagberg@gmail.com>",
"Brandon Liu <brandon.k.liu@gmail.com"])
__all__ = ['pagerank', 'pagerank_numpy', 'pagerank_scipy', 'google_matrix']

@not_implemented_for('multigraph')
[docs]def pagerank(G, alpha=0.85, personalization=None,
max_iter=100, tol=1.0e-6, nstart=None, weight='weight',
dangling=None):
"""Return the PageRank of the nodes in the graph.

PageRank computes a ranking of the nodes in the graph G based on
the structure of the incoming links. It was originally designed as
an algorithm to rank web pages.

Parameters
-----------
G : graph
A NetworkX graph.  Undirected graphs will be converted to a directed
graph with two directed edges for each undirected edge.

alpha : float, optional
Damping parameter for PageRank, default=0.85.

personalization: dict, optional
The "personalization vector" consisting of a dictionary with a
key for every graph node and nonzero personalization value for each node.
By default, a uniform distribution is used.

max_iter : integer, optional
Maximum number of iterations in power method eigenvalue solver.

tol : float, optional
Error tolerance used to check convergence in power method solver.

nstart : dictionary, optional
Starting value of PageRank iteration for each node.

weight : key, optional
Edge data key to use as weight.  If None weights are set to 1.

dangling: dict, optional
The outedges to be assigned to any "dangling" nodes, i.e., nodes without
any outedges. The dict key is the node the outedge points to and the dict
value is the weight of that outedge. By default, dangling nodes are given
outedges according to the personalization vector (uniform if not
specified). This must be selected to result in an irreducible transition
matrix (see notes under google_matrix). It may be common to have the
dangling dict to be the same as the personalization dict.

Returns
-------
pagerank : dictionary
Dictionary of nodes with PageRank as value

Examples
--------
>>> G = nx.DiGraph(nx.path_graph(4))
>>> pr = nx.pagerank(G, alpha=0.9)

Notes
-----
The eigenvector calculation is done by the power iteration method
and has no guarantee of convergence.  The iteration will stop
after max_iter iterations or an error tolerance of
number_of_nodes(G)*tol has been reached.

The PageRank algorithm was designed for directed graphs but this
algorithm does not check if the input graph is directed and will
execute on undirected graphs by converting each edge in the
directed graph to two edges.

--------

References
----------
.. [1] A. Langville and C. Meyer,
"A survey of eigenvector methods of web information retrieval."
http://citeseer.ist.psu.edu/713792.html
.. [2] Page, Lawrence; Brin, Sergey; Motwani, Rajeev and Winograd, Terry,
The PageRank citation ranking: Bringing order to the Web. 1999
http://dbpubs.stanford.edu:8090/pub/showDoc.Fulltext?lang=en&doc=1999-66&format=pdf
"""
if len(G) == 0:
return {}

if not G.is_directed():
D = G.to_directed()
else:
D = G

# Create a copy in (right) stochastic form
W = nx.stochastic_graph(D, weight=weight)
N = W.number_of_nodes()

# Choose fixed starting vector if not given
if nstart is None:
x = dict.fromkeys(W, 1.0 / N)
else:
# Normalized nstart vector
s = float(sum(nstart.values()))
x = dict((k, v / s) for k, v in nstart.items())

if personalization is None:
# Assign uniform personalization vector if not given
p = dict.fromkeys(W, 1.0 / N)
else:
missing = set(G) - set(personalization)
if missing:
raise NetworkXError('Personalization dictionary '
'must have a value for every node. '
'Missing nodes %s' % missing)
s = float(sum(personalization.values()))
p = dict((k, v / s) for k, v in personalization.items())

if dangling is None:
# Use personalization vector if dangling vector not specified
dangling_weights = p
else:
missing = set(G) - set(dangling)
if missing:
raise NetworkXError('Dangling node dictionary '
'must have a value for every node. '
'Missing nodes %s' % missing)
s = float(sum(dangling.values()))
dangling_weights = dict((k, v/s) for k, v in dangling.items())
dangling_nodes = [n for n in W if W.out_degree(n, weight=weight) == 0.0]

# power iteration: make up to max_iter iterations
for _ in range(max_iter):
xlast = x
x = dict.fromkeys(xlast.keys(), 0)
danglesum = alpha * sum(xlast[n] for n in dangling_nodes)
for n in x:
# this matrix multiply looks odd because it is
# doing a left multiply x^T=xlast^T*W
for nbr in W[n]:
x[nbr] += alpha * xlast[n] * W[n][nbr][weight]
x[n] += danglesum * dangling_weights[n] + (1.0 - alpha) * p[n]
# check convergence, l1 norm
err = sum([abs(x[n] - xlast[n]) for n in x])
if err < N*tol:
return x
raise NetworkXError('pagerank: power iteration failed to converge '
'in %d iterations.' % max_iter)

[docs]def google_matrix(G, alpha=0.85, personalization=None,
nodelist=None, weight='weight', dangling=None):
"""Return the Google matrix of the graph.

Parameters
-----------
G : graph
A NetworkX graph.  Undirected graphs will be converted to a directed
graph with two directed edges for each undirected edge.

alpha : float
The damping factor.

personalization: dict, optional
The "personalization vector" consisting of a dictionary with a
key for every graph node and nonzero personalization value for each node.
By default, a uniform distribution is used.

nodelist : list, optional
The rows and columns are ordered according to the nodes in nodelist.
If nodelist is None, then the ordering is produced by G.nodes().

weight : key, optional
Edge data key to use as weight.  If None weights are set to 1.

dangling: dict, optional
The outedges to be assigned to any "dangling" nodes, i.e., nodes without
any outedges. The dict key is the node the outedge points to and the dict
value is the weight of that outedge. By default, dangling nodes are given
outedges according to the personalization vector (uniform if not
specified) This must be selected to result in an irreducible transition
matrix (see notes below). It may be common to have the dangling dict to
be the same as the personalization dict.

Returns
-------
A : NumPy matrix
Google matrix of the graph

Notes
-----
The matrix returned represents the transition matrix that describes the
Markov chain used in PageRank. For PageRank to converge to a unique
solution (i.e., a unique stationary distribution in a Markov chain), the
transition matrix must be irreducible. In other words, it must be that
there exists a path between every pair of nodes in the graph, or else there
is the potential of "rank sinks."

This implementation works with Multi(Di)Graphs. For multigraphs the
weight between two nodes is set to be the sum of all edge weights
between those nodes.

--------
pagerank, pagerank_numpy, pagerank_scipy
"""
import numpy as np

if nodelist is None:
nodelist = G.nodes()

M = nx.to_numpy_matrix(G, nodelist=nodelist, weight=weight)
N = len(G)
if N == 0:
return M

# Personalization vector
if personalization is None:
p = np.repeat(1.0 / N, N)
else:
missing = set(nodelist) - set(personalization)
if missing:
raise NetworkXError('Personalization vector dictionary '
'must have a value for every node. '
'Missing nodes %s' % missing)
p = np.array([personalization[n] for n in nodelist], dtype=float)
p /= p.sum()

# Dangling nodes
if dangling is None:
dangling_weights = p
else:
missing = set(nodelist) - set(dangling)
if missing:
raise NetworkXError('Dangling node dictionary '
'must have a value for every node. '
'Missing nodes %s' % missing)
# Convert the dangling dictionary into an array in nodelist order
dangling_weights = np.array([dangling[n] for n in nodelist],
dtype=float)
dangling_weights /= dangling_weights.sum()
dangling_nodes = np.where(M.sum(axis=1) == 0)[0]

# Assign dangling_weights to any dangling nodes (nodes with no out links)
for node in dangling_nodes:
M[node] = dangling_weights

M /= M.sum(axis=1)  # Normalize rows to sum to 1

return alpha * M + (1 - alpha) * np.outer(np.ones(N), p)

[docs]def pagerank_numpy(G, alpha=0.85, personalization=None, weight='weight',
dangling=None):
"""Return the PageRank of the nodes in the graph.

PageRank computes a ranking of the nodes in the graph G based on
the structure of the incoming links. It was originally designed as
an algorithm to rank web pages.

Parameters
-----------
G : graph
A NetworkX graph.  Undirected graphs will be converted to a directed
graph with two directed edges for each undirected edge.

alpha : float, optional
Damping parameter for PageRank, default=0.85.

personalization: dict, optional
The "personalization vector" consisting of a dictionary with a
key for every graph node and nonzero personalization value for each
node. By default, a uniform distribution is used.

weight : key, optional
Edge data key to use as weight.  If None weights are set to 1.

dangling: dict, optional
The outedges to be assigned to any "dangling" nodes, i.e., nodes without
any outedges. The dict key is the node the outedge points to and the dict
value is the weight of that outedge. By default, dangling nodes are given
outedges according to the personalization vector (uniform if not
specified) This must be selected to result in an irreducible transition
matrix (see notes under google_matrix). It may be common to have the
dangling dict to be the same as the personalization dict.

Returns
-------
pagerank : dictionary
Dictionary of nodes with PageRank as value.

Examples
--------
>>> G = nx.DiGraph(nx.path_graph(4))
>>> pr = nx.pagerank_numpy(G, alpha=0.9)

Notes
-----
The eigenvector calculation uses NumPy's interface to the LAPACK
eigenvalue solvers.  This will be the fastest and most accurate
for small graphs.

This implementation works with Multi(Di)Graphs. For multigraphs the
weight between two nodes is set to be the sum of all edge weights
between those nodes.

--------

References
----------
.. [1] A. Langville and C. Meyer,
"A survey of eigenvector methods of web information retrieval."
http://citeseer.ist.psu.edu/713792.html
.. [2] Page, Lawrence; Brin, Sergey; Motwani, Rajeev and Winograd, Terry,
The PageRank citation ranking: Bringing order to the Web. 1999
http://dbpubs.stanford.edu:8090/pub/showDoc.Fulltext?lang=en&doc=1999-66&format=pdf
"""
import numpy as np
if len(G) == 0:
return {}
M = google_matrix(G, alpha, personalization=personalization,
weight=weight, dangling=dangling)
# use numpy LAPACK solver
eigenvalues, eigenvectors = np.linalg.eig(M.T)
ind = eigenvalues.argsort()
# eigenvector of largest eigenvalue at ind[-1], normalized
largest = np.array(eigenvectors[:, ind[-1]]).flatten().real
norm = float(largest.sum())
return dict(zip(G, map(float, largest / norm)))

[docs]def pagerank_scipy(G, alpha=0.85, personalization=None,
max_iter=100, tol=1.0e-6, weight='weight',
dangling=None):
"""Return the PageRank of the nodes in the graph.

PageRank computes a ranking of the nodes in the graph G based on
the structure of the incoming links. It was originally designed as
an algorithm to rank web pages.

Parameters
-----------
G : graph
A NetworkX graph.  Undirected graphs will be converted to a directed
graph with two directed edges for each undirected edge.

alpha : float, optional
Damping parameter for PageRank, default=0.85.

personalization: dict, optional
The "personalization vector" consisting of a dictionary with a
key for every graph node and nonzero personalization value for each
node. By default, a uniform distribution is used.

max_iter : integer, optional
Maximum number of iterations in power method eigenvalue solver.

tol : float, optional
Error tolerance used to check convergence in power method solver.

weight : key, optional
Edge data key to use as weight.  If None weights are set to 1.

dangling: dict, optional
The outedges to be assigned to any "dangling" nodes, i.e., nodes without
any outedges. The dict key is the node the outedge points to and the dict
value is the weight of that outedge. By default, dangling nodes are given
outedges according to the personalization vector (uniform if not
specified) This must be selected to result in an irreducible transition
matrix (see notes under google_matrix). It may be common to have the
dangling dict to be the same as the personalization dict.

Returns
-------
pagerank : dictionary
Dictionary of nodes with PageRank as value

Examples
--------
>>> G = nx.DiGraph(nx.path_graph(4))
>>> pr = nx.pagerank_scipy(G, alpha=0.9)

Notes
-----
The eigenvector calculation uses power iteration with a SciPy
sparse matrix representation.

This implementation works with Multi(Di)Graphs. For multigraphs the
weight between two nodes is set to be the sum of all edge weights
between those nodes.

--------

References
----------
.. [1] A. Langville and C. Meyer,
"A survey of eigenvector methods of web information retrieval."
http://citeseer.ist.psu.edu/713792.html
.. [2] Page, Lawrence; Brin, Sergey; Motwani, Rajeev and Winograd, Terry,
The PageRank citation ranking: Bringing order to the Web. 1999
http://dbpubs.stanford.edu:8090/pub/showDoc.Fulltext?lang=en&doc=1999-66&format=pdf
"""
import scipy.sparse

N = len(G)
if N == 0:
return {}

nodelist = G.nodes()
M = nx.to_scipy_sparse_matrix(G, nodelist=nodelist, weight=weight,
dtype=float)
S = scipy.array(M.sum(axis=1)).flatten()
S[S != 0] = 1.0 / S[S != 0]
Q = scipy.sparse.spdiags(S.T, 0, *M.shape, format='csr')
M = Q * M

# initial vector
x = scipy.repeat(1.0 / N, N)

# Personalization vector
if personalization is None:
p = scipy.repeat(1.0 / N, N)
else:
missing = set(nodelist) - set(personalization)
if missing:
raise NetworkXError('Personalization vector dictionary '
'must have a value for every node. '
'Missing nodes %s' % missing)
p = scipy.array([personalization[n] for n in nodelist],
dtype=float)
p = p / p.sum()

# Dangling nodes
if dangling is None:
dangling_weights = p
else:
missing = set(nodelist) - set(dangling)
if missing:
raise NetworkXError('Dangling node dictionary '
'must have a value for every node. '
'Missing nodes %s' % missing)
# Convert the dangling dictionary into an array in nodelist order
dangling_weights = scipy.array([dangling[n] for n in nodelist],
dtype=float)
dangling_weights /= dangling_weights.sum()
is_dangling = scipy.where(S == 0)[0]

# power iteration: make up to max_iter iterations
for _ in range(max_iter):
xlast = x
x = alpha * (x * M + sum(x[is_dangling]) * dangling_weights) + \
(1 - alpha) * p
# check convergence, l1 norm
err = scipy.absolute(x - xlast).sum()
if err < N * tol:
return dict(zip(nodelist, map(float, x)))
raise NetworkXError('pagerank_scipy: power iteration failed to converge '
'in %d iterations.' % max_iter)

# fixture for nose tests
def setup_module(module):
from nose import SkipTest
try:
import numpy
except:
raise SkipTest("NumPy not available")
try:
import scipy
except:
raise SkipTest("SciPy not available")