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"""
Laplacian of a compressed-sparse graph
"""
# Authors: Aric Hagberg <hagberg@lanl.gov>
# Gael Varoquaux <gael.varoquaux@normalesup.org>
# Jake Vanderplas <vanderplas@astro.washington.edu>
# License: BSD
from __future__ import division, print_function, absolute_import
import numpy as np
from scipy.sparse import isspmatrix, coo_matrix
###############################################################################
# Graph laplacian
def laplacian(csgraph, normed=False, return_diag=False):
""" Return the Laplacian matrix of a directed graph.
For non-symmetric graphs the out-degree is used in the computation.
Parameters
----------
csgraph : array_like or sparse matrix, 2 dimensions
compressed-sparse graph, with shape (N, N).
normed : bool, optional
If True, then compute normalized Laplacian.
return_diag : bool, optional
If True, then return diagonal as well as laplacian.
Returns
-------
lap : ndarray
The N x N laplacian matrix of graph.
diag : ndarray
The length-N diagonal of the laplacian matrix.
diag is returned only if return_diag is True.
Notes
-----
The Laplacian matrix of a graph is sometimes referred to as the
"Kirchoff matrix" or the "admittance matrix", and is useful in many
parts of spectral graph theory. In particular, the eigen-decomposition
of the laplacian matrix can give insight into many properties of the graph.
For non-symmetric directed graphs, the laplacian is computed using the
out-degree of each node.
Examples
--------
>>> from scipy.sparse import csgraph
>>> G = np.arange(5) * np.arange(5)[:, np.newaxis]
>>> G
array([[ 0, 0, 0, 0, 0],
[ 0, 1, 2, 3, 4],
[ 0, 2, 4, 6, 8],
[ 0, 3, 6, 9, 12],
[ 0, 4, 8, 12, 16]])
>>> csgraph.laplacian(G, normed=False)
array([[ 0, 0, 0, 0, 0],
[ 0, 9, -2, -3, -4],
[ 0, -2, 16, -6, -8],
[ 0, -3, -6, 21, -12],
[ 0, -4, -8, -12, 24]])
"""
if csgraph.ndim != 2 or csgraph.shape[0] != csgraph.shape[1]:
raise ValueError('csgraph must be a square matrix or array')
if normed and (np.issubdtype(csgraph.dtype, np.int)
or np.issubdtype(csgraph.dtype, np.uint)):
csgraph = csgraph.astype(np.float)
if isspmatrix(csgraph):
return _laplacian_sparse(csgraph, normed=normed,
return_diag=return_diag)
else:
return _laplacian_dense(csgraph, normed=normed,
return_diag=return_diag)
def _laplacian_sparse(graph, normed=False, return_diag=False):
n_nodes = graph.shape[0]
if not graph.format == 'coo':
lap = (-graph).tocoo()
else:
lap = -graph.copy()
diag_mask = (lap.row == lap.col)
if not diag_mask.sum() == n_nodes:
# The sparsity pattern of the matrix has holes on the diagonal,
# we need to fix that
diag_idx = lap.row[diag_mask]
diagonal_holes = list(set(range(n_nodes)).difference(
diag_idx))
new_data = np.concatenate([lap.data, np.ones(len(diagonal_holes))])
new_row = np.concatenate([lap.row, diagonal_holes])
new_col = np.concatenate([lap.col, diagonal_holes])
lap = coo_matrix((new_data, (new_row, new_col)), shape=lap.shape)
diag_mask = (lap.row == lap.col)
lap.data[diag_mask] = 0
w = -np.asarray(lap.sum(axis=1)).squeeze()
if normed:
w = np.sqrt(w)
w_zeros = (w == 0)
w[w_zeros] = 1
lap.data /= w[lap.row]
lap.data /= w[lap.col]
lap.data[diag_mask] = (1 - w_zeros[lap.row[diag_mask]]).astype(lap.data.dtype)
else:
lap.data[diag_mask] = w[lap.row[diag_mask]]
if return_diag:
return lap, w
return lap
def _laplacian_dense(graph, normed=False, return_diag=False):
n_nodes = graph.shape[0]
lap = -np.asarray(graph) # minus sign leads to a copy
# set diagonal to zero
lap.flat[::n_nodes + 1] = 0
w = -lap.sum(axis=0)
if normed:
w = np.sqrt(w)
w_zeros = (w == 0)
w[w_zeros] = 1
lap /= w
lap /= w[:, np.newaxis]
lap.flat[::n_nodes + 1] = 1 - w_zeros
else:
lap.flat[::n_nodes + 1] = w
if return_diag:
return lap, w
return lap
|
