from abc import abstractmethod
from itertools import combinations
import numpy as np
from pgmpy.models import BayesianModel
[docs]class BaseEliminationOrder:
"""
Base class for finding elimination orders.
"""
def __init__(self, model):
"""
Init method for the base class of Elimination Orders.
Parameters
----------
model: BayesianModel instance
The model on which we want to compute the elimination orders.
"""
if not isinstance(model, BayesianModel):
raise ValueError("Model should be a BayesianModel instance")
self.bayesian_model = model
self.moralized_model = self.bayesian_model.moralize()
@abstractmethod
[docs] def cost(self, node):
"""
The cost function to compute the cost of elimination of each node.
This method is just a dummy and returns 0 for all the nodes. Actual cost functions
are implemented in the classes inheriting BaseEliminationOrder.
Parameters
----------
node: string, any hashable python object.
The node whose cost is to be computed.
"""
return 0
[docs] def get_elimination_order(self, nodes=None):
"""
Returns the optimal elimination order based on the cost function.
The node having the least cost is removed first.
Parameters
----------
nodes: list, tuple, set (array-like)
The variables which are to be eliminated.
Examples
--------
>>> import numpy as np
>>> from pgmpy.models import BayesianModel
>>> from pgmpy.factors.discrete import TabularCPD
>>> from pgmpy.inference.EliminationOrder import WeightedMinFill
>>> model = BayesianModel([('c', 'd'), ('d', 'g'), ('i', 'g'),
... ('i', 's'), ('s', 'j'), ('g', 'l'),
... ('l', 'j'), ('j', 'h'), ('g', 'h')])
>>> cpd_c = TabularCPD('c', 2, np.random.rand(2, 1))
>>> cpd_d = TabularCPD('d', 2, np.random.rand(2, 2),
... ['c'], [2])
>>> cpd_g = TabularCPD('g', 3, np.random.rand(3, 4),
... ['d', 'i'], [2, 2])
>>> cpd_i = TabularCPD('i', 2, np.random.rand(2, 1))
>>> cpd_s = TabularCPD('s', 2, np.random.rand(2, 2),
... ['i'], [2])
>>> cpd_j = TabularCPD('j', 2, np.random.rand(2, 4),
... ['l', 's'], [2, 2])
>>> cpd_l = TabularCPD('l', 2, np.random.rand(2, 3),
... ['g'], [3])
>>> cpd_h = TabularCPD('h', 2, np.random.rand(2, 6),
... ['g', 'j'], [3, 2])
>>> model.add_cpds(cpd_c, cpd_d, cpd_g, cpd_i, cpd_s, cpd_j,
... cpd_l, cpd_h)
>>> WeightedMinFill(model).get_elimination_order(['c', 'd', 'g', 'l', 's'])
['c', 's', 'l', 'd', 'g']
>>> WeightedMinFill(model).get_elimination_order(['c', 'd', 'g', 'l', 's'])
['c', 's', 'l', 'd', 'g']
>>> WeightedMinFill(model).get_elimination_order(['c', 'd', 'g', 'l', 's'])
['c', 's', 'l', 'd', 'g']
"""
if not nodes:
nodes = self.bayesian_model.nodes()
ordering = []
while nodes:
scores = {node: self.cost(node) for node in nodes}
min_score_node = min(scores, key=scores.get)
ordering.append(min_score_node)
nodes.remove(min_score_node)
return ordering
[docs] def fill_in_edges(self, node):
"""
Return edges needed to be added to the graph if a node is removed.
Parameters
----------
node: string (any hashable python object)
Node to be removed from the graph.
"""
return combinations(self.bayesian_model.neighbors(node), 2)
class WeightedMinFill(BaseEliminationOrder):
def cost(self, node):
"""
Cost function for WeightedMinFill.
The cost of eliminating a node is the sum of weights of the edges that need to
be added to the graph due to its elimination, where a weight of an edge is the
product of the weights, domain cardinality, of its constituent vertices.
"""
edges = combinations(self.moralized_model.neighbors(node), 2)
return sum([self.bayesian_model.get_cardinality(edge[0]) *
self.bayesian_model.get_cardinality(edge[1]) for edge in edges])
class MinNeighbours(BaseEliminationOrder):
def cost(self, node):
"""
The cost of a eliminating a node is the number of neighbors it has in the
current graph.
"""
return len(self.moralized_model.neighbors(node))
class MinWeight(BaseEliminationOrder):
def cost(self, node):
"""
The cost of a eliminating a node is the product of weights, domain cardinality,
of its neighbors.
"""
return np.prod([self.bayesian_model.get_cardinality(neig_node) for neig_node in
self.moralized_model.neighbors(node)])
class MinFill(BaseEliminationOrder):
def cost(self, node):
"""
The cost of a eliminating a node is the number of edges that need to be added
(fill in edges) to the graph due to its elimination
"""
return len(list(self.fill_in_edges(node)))
