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Source code for networkx.generators.classic

#    Copyright (C) 2004-2019 by
#    Aric Hagberg <hagberg@lanl.gov>
#    Dan Schult <dschult@colgate.edu>
#    Pieter Swart <swart@lanl.gov>
#
# Authors: Aric Hagberg (hagberg@lanl.gov)
#          Pieter Swart (swart@lanl.gov)
"""Generators for some classic graphs.

The typical graph generator is called as follows:

>>> G = nx.complete_graph(100)

returning the complete graph on n nodes labeled 0, .., 99
as a simple graph. Except for empty_graph, all the generators
in this module return a Graph class (i.e. a simple, undirected graph).

"""
from __future__ import division

import itertools

import networkx as nx
from networkx.classes import Graph
from networkx.exception import NetworkXError
from networkx.utils import accumulate
from networkx.utils import nodes_or_number
from networkx.utils import pairwise

__all__ = ['balanced_tree',
'barbell_graph',
'complete_graph',
'complete_multipartite_graph',
'circulant_graph',
'cycle_graph',
'dorogovtsev_goltsev_mendes_graph',
'empty_graph',
'full_rary_tree',
'lollipop_graph',
'null_graph',
'path_graph',
'star_graph',
'trivial_graph',
'turan_graph',
'wheel_graph']

# -------------------------------------------------------------------
#   Some Classic Graphs
# -------------------------------------------------------------------

def _tree_edges(n, r):
if n == 0:
return
# helper function for trees
# yields edges in rooted tree at 0 with n nodes and branching ratio r
nodes = iter(range(n))
parents = [next(nodes)]  # stack of max length r
while parents:
source = parents.pop(0)
for i in range(r):
try:
target = next(nodes)
parents.append(target)
yield source, target
except StopIteration:
break

[docs]def full_rary_tree(r, n, create_using=None):
"""Creates a full r-ary tree of n vertices.

Sometimes called a k-ary, n-ary, or m-ary tree.
"... all non-leaf vertices have exactly r children and all levels
are full except for some rightmost position of the bottom level
(if a leaf at the bottom level is missing, then so are all of the
leaves to its right." [1]_

Parameters
----------
r : int
branching factor of the tree
n : int
Number of nodes in the tree
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Returns
-------
G : networkx Graph
An r-ary tree with n nodes

References
----------
.. [1] An introduction to data structures and algorithms,
James Andrew Storer,  Birkhauser Boston 2001, (page 225).
"""
G = empty_graph(n, create_using)
return G

[docs]def balanced_tree(r, h, create_using=None):
"""Returns the perfectly balanced r-ary tree of height h.

Parameters
----------
r : int
Branching factor of the tree; each node will have r
children.

h : int
Height of the tree.

create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Returns
-------
G : NetworkX graph
A balanced r-ary tree of height h.

Notes
-----
This is the rooted tree where all leaves are at distance h from
the root. The root has degree r and all other internal nodes
have degree r + 1.

Node labels are integers, starting from zero.

A balanced tree is also known as a *complete r-ary tree*.

"""
# The number of nodes in the balanced tree is 1 + r + ... + r^h,
# which is computed by using the closed-form formula for a geometric
# sum with ratio r. In the special case that r is 1, the number
# of nodes is simply h + 1 (since the tree is actually a path
# graph).
if r == 1:
n = h + 1
else:
# This must be an integer if both r and h are integers. If
# they are not, we force integer division anyway.
n = (1 - r ** (h + 1)) // (1 - r)
return full_rary_tree(r, n, create_using=create_using)

[docs]def barbell_graph(m1, m2, create_using=None):
"""Returns the Barbell Graph: two complete graphs connected by a path.

For $m1 > 1$ and $m2 >= 0$.

Two identical complete graphs $K_{m1}$ form the left and right bells,
and are connected by a path $P_{m2}$.

The 2*m1+m2  nodes are numbered
0, ..., m1-1 for the left barbell,
m1, ..., m1+m2-1 for the path,
and m1+m2, ..., 2*m1+m2-1 for the right barbell.

The 3 subgraphs are joined via the edges (m1-1, m1) and
(m1+m2-1, m1+m2). If m2=0, this is merely two complete
graphs joined together.

This graph is an extremal example in David Aldous
and Jim Fill's e-text on Random Walks on Graphs.

"""
if m1 < 2:
raise NetworkXError(
"Invalid graph description, m1 should be >=2")
if m2 < 0:
raise NetworkXError(
"Invalid graph description, m2 should be >=0")

# left barbell
G = complete_graph(m1, create_using)
if G.is_directed():
raise NetworkXError("Directed Graph not supported")

# connecting path
G.add_nodes_from(range(m1, m1 + m2 - 1))
if m2 > 1:
G.add_edges_from(pairwise(range(m1, m1 + m2)))
# right barbell
G.add_edges_from((u, v) for u in range(m1 + m2, 2 * m1 + m2)
for v in range(u + 1, 2 * m1 + m2))
# connect it up
G.add_edge(m1 - 1, m1)
if m2 > 0:
G.add_edge(m1 + m2 - 1, m1 + m2)
return G

[docs]@nodes_or_number(0)
def complete_graph(n, create_using=None):
""" Return the complete graph K_n with n nodes.

Parameters
----------
n : int or iterable container of nodes
If n is an integer, nodes are from range(n).
If n is a container of nodes, those nodes appear in the graph.
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Examples
--------
>>> G = nx.complete_graph(9)
>>> len(G)
9
>>> G.size()
36
>>> G = nx.complete_graph(range(11, 14))
>>> list(G.nodes())
[11, 12, 13]
>>> G = nx.complete_graph(4, nx.DiGraph())
>>> G.is_directed()
True

"""
n_name, nodes = n
G = empty_graph(n_name, create_using)
if len(nodes) > 1:
if G.is_directed():
edges = itertools.permutations(nodes, 2)
else:
edges = itertools.combinations(nodes, 2)
return G

"""Returns the circular ladder graph $CL_n$ of length n.

$CL_n$ consists of two concentric n-cycles in which
each of the n pairs of concentric nodes are joined by an edge.

Node labels are the integers 0 to n-1

"""
G = ladder_graph(n, create_using)
G.add_edge(0, n - 1)
G.add_edge(n, 2 * n - 1)
return G

[docs]def circulant_graph(n, offsets, create_using=None):
"""Generates the circulant graph $Ci_n(x_1, x_2, ..., x_m)$ with $n$ vertices.

Returns
-------
The graph $Ci_n(x_1, ..., x_m)$ consisting of $n$ vertices $0, ..., n-1$ such
that the vertex with label $i$ is connected to the vertices labelled $(i + x)$
and $(i - x)$, for all $x$ in $x_1$ up to $x_m$, with the indices taken modulo $n$.

Parameters
----------
n : integer
The number of vertices the generated graph is to contain.
offsets : list of integers
A list of vertex offsets, $x_1$ up to $x_m$, as described above.
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Examples
--------
Many well-known graph families are subfamilies of the circulant graphs;
for example, to generate the cycle graph on n points, we connect every
vertex to every other at offset plus or minus one. For n = 10,

>>> import networkx
>>> G = networkx.generators.classic.circulant_graph(10, [1])
>>> edges = [
...     (0, 9), (0, 1), (1, 2), (2, 3), (3, 4),
...     (4, 5), (5, 6), (6, 7), (7, 8), (8, 9)]
...
>>> sorted(edges) == sorted(G.edges())
True

Similarly, we can generate the complete graph on 5 points with the set of
offsets [1, 2]:

>>> G = networkx.generators.classic.circulant_graph(5, [1, 2])
>>> edges = [
...     (0, 1), (0, 2), (0, 3), (0, 4), (1, 2),
...     (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]
...
>>> sorted(edges) == sorted(G.edges())
True

"""
G = empty_graph(n, create_using)
for i in range(n):
for j in offsets:
G.add_edge(i, (i - j) % n)
G.add_edge(i, (i + j) % n)
return G

[docs]@nodes_or_number(0)
def cycle_graph(n, create_using=None):
"""Returns the cycle graph $C_n$ of cyclically connected nodes.

$C_n$ is a path with its two end-nodes connected.

Parameters
----------
n : int or iterable container of nodes
If n is an integer, nodes are from range(n).
If n is a container of nodes, those nodes appear in the graph.
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Notes
-----
If create_using is directed, the direction is in increasing order.

"""
n_orig, nodes = n
G = empty_graph(nodes, create_using)
return G

[docs]def dorogovtsev_goltsev_mendes_graph(n, create_using=None):
"""Returns the hierarchically constructed Dorogovtsev-Goltsev-Mendes graph.

n is the generation.
See: arXiv:/cond-mat/0112143 by Dorogovtsev, Goltsev and Mendes.

"""
G = empty_graph(0, create_using)
if G.is_directed():
raise NetworkXError("Directed Graph not supported")
if G.is_multigraph():
raise NetworkXError("Multigraph not supported")

if n == 0:
return G
new_node = 2         # next node to be added
for i in range(1, n + 1):  # iterate over number of generations.
last_generation_edges = list(G.edges())
number_of_edges_in_last_generation = len(last_generation_edges)
for j in range(0, number_of_edges_in_last_generation):
new_node += 1
return G

[docs]@nodes_or_number(0)
def empty_graph(n=0, create_using=None, default=nx.Graph):
"""Returns the empty graph with n nodes and zero edges.

Parameters
----------
n : int or iterable container of nodes (default = 0)
If n is an integer, nodes are from range(n).
If n is a container of nodes, those nodes appear in the graph.
create_using : Graph Instance, Constructor or None
Indicator of type of graph to return.
If a Graph-type instance, then clear and use it.
If None, use the default constructor.
If a constructor, call it to create an empty graph.
default : Graph constructor (optional, default = nx.Graph)
The constructor to use if create_using is None.
If None, then nx.Graph is used.
This is used when passing an unknown create_using value
through your home-grown function to empty_graph and
you want a default constructor other than nx.Graph.

Examples
--------
>>> G = nx.empty_graph(10)
>>> G.number_of_nodes()
10
>>> G.number_of_edges()
0
>>> G = nx.empty_graph("ABC")
>>> G.number_of_nodes()
3
>>> sorted(G)
['A', 'B', 'C']

Notes
-----
The variable create_using should be a Graph Constructor or a
"graph"-like object. Constructors, e.g. nx.Graph or nx.MultiGraph
will be used to create the returned graph. "graph"-like objects
will be cleared (nodes and edges will be removed) and refitted as
an empty "graph" with nodes specified in n. This capability
is useful for specifying the class-nature of the resulting empty
"graph" (i.e. Graph, DiGraph, MyWeirdGraphClass, etc.).

The variable create_using has three main uses:
Firstly, the variable create_using can be used to create an
empty digraph, multigraph, etc.  For example,

>>> n = 10
>>> G = nx.empty_graph(n, create_using=nx.DiGraph)

will create an empty digraph on n nodes.

Secondly, one can pass an existing graph (digraph, multigraph,
etc.) via create_using. For example, if G is an existing graph
(resp. digraph, multigraph, etc.), then empty_graph(n, create_using=G)
will empty G (i.e. delete all nodes and edges using G.clear())
and then add n nodes and zero edges, and return the modified graph.

Thirdly, when constructing your home-grown graph creation function
you can use empty_graph to construct the graph by passing a user
defined create_using to empty_graph. In this case, if you want the
default constructor to be other than nx.Graph, specify default.

>>> def mygraph(n, create_using=None):
...     G = nx.empty_graph(n, create_using, nx.MultiGraph)
...     G.add_edges_from([(0, 1), (0, 1)])
...     return G
>>> G = mygraph(3)
>>> G.is_multigraph()
True
>>> G = mygraph(3, nx.Graph)
>>> G.is_multigraph()
False

"""
if create_using is None:
G = default()
# create_using is a NetworkX style Graph
create_using.clear()
G = create_using
else:
# try create_using as constructor
G = create_using()

n_name, nodes = n
return G

"""Returns the Ladder graph of length n.

This is two paths of n nodes, with
each pair connected by a single edge.

Node labels are the integers 0 to 2*n - 1.

"""
G = empty_graph(2 * n, create_using)
if G.is_directed():
raise NetworkXError("Directed Graph not supported")
G.add_edges_from(pairwise(range(n, 2 * n)))
G.add_edges_from((v, v + n) for v in range(n))
return G

[docs]@nodes_or_number([0, 1])
def lollipop_graph(m, n, create_using=None):
"""Returns the Lollipop Graph; K_m connected to P_n.

This is the Barbell Graph without the right barbell.

Parameters
----------
m, n : int or iterable container of nodes (default = 0)
If an integer, nodes are from range(m) and range(m,m+n).
If a container, the entries are the coordinate of the node.

The nodes for m appear in the complete graph $K_m$ and the nodes
for n appear in the path $P_n$
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Notes
-----
The 2 subgraphs are joined via an edge (m-1, m).
If n=0, this is merely a complete graph.

(This graph is an extremal example in David Aldous and Jim
Fill's etext on Random Walks on Graphs.)

"""
m, m_nodes = m
n, n_nodes = n
M = len(m_nodes)
N = len(n_nodes)
if isinstance(m, int):
n_nodes = [len(m_nodes) + i for i in n_nodes]
if M < 2:
raise NetworkXError(
"Invalid graph description, m should be >=2")
if N < 0:
raise NetworkXError(
"Invalid graph description, n should be >=0")

# the ball
G = complete_graph(m_nodes, create_using)
if G.is_directed():
raise NetworkXError("Directed Graph not supported")
# the stick
if N > 1:
# connect ball to stick
if M > 0 and N > 0:
return G

[docs]def null_graph(create_using=None):
"""Returns the Null graph with no nodes or edges.

See empty_graph for the use of create_using.

"""
G = empty_graph(0, create_using)
return G

[docs]@nodes_or_number(0)
def path_graph(n, create_using=None):
"""Returns the Path graph P_n of linearly connected nodes.

Parameters
----------
n : int or iterable
If an integer, node labels are 0 to n with center 0.
If an iterable of nodes, the center is the first.
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

"""
n_name, nodes = n
G = empty_graph(nodes, create_using)
return G

[docs]@nodes_or_number(0)
def star_graph(n, create_using=None):
""" Return the star graph

The star graph consists of one center node connected to n outer nodes.

Parameters
----------
n : int or iterable
If an integer, node labels are 0 to n with center 0.
If an iterable of nodes, the center is the first.
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Notes
-----
The graph has n+1 nodes for integer n.
So star_graph(3) is the same as star_graph(range(4)).
"""
n_name, nodes = n
if isinstance(n_name, int):
nodes = nodes + [n_name]  # there should be n+1 nodes
first = nodes[0]
G = empty_graph(nodes, create_using)
if G.is_directed():
raise NetworkXError("Directed Graph not supported")
G.add_edges_from((first, v) for v in nodes[1:])
return G

[docs]def trivial_graph(create_using=None):
""" Return the Trivial graph with one node (with label 0) and no edges.

"""
G = empty_graph(1, create_using)
return G

[docs]def turan_graph(n, r):
r""" Return the Turan Graph

The Turan Graph is a complete multipartite graph on $n$ vertices
with $r$ disjoint subsets. It is the graph with the edges for any graph with
$n$ vertices and $r$ disjoint subsets.

Given $n$ and $r$, we generate a complete multipartite graph with
$r-(n \mod r)$ partitions of size $n/r$, rounded down, and
$n \mod r$ partitions of size $n/r+1$, rounded down.

Parameters
----------
n : int
The number of vertices.
r : int
The number of partitions.
Must be less than or equal to n.

Notes
-----
Must satisfy $1 <= r <= n$.
The graph has $(r-1)(n^2)/(2r)$ edges, rounded down.
"""

if not 1 <= r <= n:
raise NetworkXError("Must satisfy 1 <= r <= n")

partitions = [n // r] * (r - (n % r)) + [n // r + 1] * (n % r)
G = complete_multipartite_graph(*partitions)
return G

[docs]@nodes_or_number(0)
def wheel_graph(n, create_using=None):
""" Return the wheel graph

The wheel graph consists of a hub node connected to a cycle of (n-1) nodes.

Parameters
----------
n : int or iterable
If an integer, node labels are 0 to n with center 0.
If an iterable of nodes, the center is the first.
create_using : NetworkX graph constructor, optional (default=nx.Graph)
Graph type to create. If graph instance, then cleared before populated.

Node labels are the integers 0 to n - 1.
"""
n_name, nodes = n
if n_name == 0:
G = empty_graph(0, create_using)
return G
G = star_graph(nodes, create_using)
if len(G) > 2:
return G

[docs]def complete_multipartite_graph(*subset_sizes):
"""Returns the complete multipartite graph with the specified subset sizes.

Parameters
----------
subset_sizes : tuple of integers or tuple of node iterables
The arguments can either all be integer number of nodes or they
can all be iterables of nodes. If integers, they represent the
number of vertices in each subset of the multipartite graph.
If iterables, each is used to create the nodes for that subset.
The length of subset_sizes is the number of subsets.

Returns
-------
G : NetworkX Graph
Returns the complete multipartite graph with the specified subsets.

For each node, the node attribute 'subset' is an integer
indicating which subset contains the node.

Examples
--------
Creating a complete tripartite graph, with subsets of one, two, and three
vertices, respectively.

>>> import networkx as nx
>>> G = nx.complete_multipartite_graph(1, 2, 3)
>>> [G.nodes[u]['subset'] for u in G]
[0, 1, 1, 2, 2, 2]
>>> list(G.edges(0))
[(0, 1), (0, 2), (0, 3), (0, 4), (0, 5)]
>>> list(G.edges(2))
[(2, 0), (2, 3), (2, 4), (2, 5)]
>>> list(G.edges(4))
[(4, 0), (4, 1), (4, 2)]

>>> G = nx.complete_multipartite_graph('a', 'bc', 'def')
>>> [G.nodes[u]['subset'] for u in sorted(G)]
[0, 1, 1, 2, 2, 2]

Notes
-----
This function generalizes several other graph generator functions.

- If no subset sizes are given, this returns the null graph.
- If a single subset size n is given, this returns the empty graph on
n nodes.
- If two subset sizes m and n are given, this returns the complete
bipartite graph on m + n nodes.
- If subset sizes 1 and n are given, this returns the star graph on
n + 1 nodes.

--------
complete_bipartite_graph
"""
# The complete multipartite graph is an undirected simple graph.
G = Graph()

if len(subset_sizes) == 0:
return G

# set up subsets of nodes
try:
extents = pairwise(accumulate((0,) + subset_sizes))
subsets = [range(start, end) for start, end in extents]
except TypeError:
subsets = subset_sizes

# add nodes with subset attribute
# while checking that ints are not mixed with iterables
try:
for (i, subset) in enumerate(subsets):