def find_model_for(self, sentence, number_of_flips,
probability_of_random_walk):
model = Model()
cnf_sentence = CNFTransformer().transform(sentence)
clauses = CNFClauseGatherer().collect(cnf_sentence)
symbols = SymbolsCollector().collect_symbols(sentence)
# model <- a random assignment of true/false to the symbols in clauses
for symbol in symbols:
model = model.extend(symbol, randbool())
# for i = 1 to max_flips do
for i in range(number_of_flips):
# if model satisfies clauses then return model
if self._all_clauses_satisfied(clauses, model):
return model
# clause <- a randomly selected clause from clauses that is false in model
symbols_list = list(
self._get_symbols_of_randomly_selected_false_clause(
clauses, model))
# with probability p flip the value in model of a randomly selected symbol from clause
if random.random() >= probability_of_random_walk:
symbol = select_randomly_from_list(symbols_list)
# else flip whichever symbol in clause maximizes the number of satisfied clauses
else:
symbol = self._get_symbol_whose_flip_maximises_satisfied_clauses(
clauses, model, symbols_list)
model.flip(symbol)
return None
def test_dpll_when_all_clauses_true(self):
model = Model()
model = model.extend("A", True).extend("B", True)
sentence = PLParser().parse("(A OR B) AND (A OR B)")
result = DPLL().dpll_satisfiable(sentence, model)
self.assertTrue(result)
def find_model_for(self, sentence, number_of_flips, probability_of_random_walk):
model = Model()
cnf_sentence = CNFTransformer().transform(sentence)
clauses = CNFClauseGatherer().collect(cnf_sentence)
symbols = SymbolsCollector().collect_symbols(sentence)
# model <- a random assignment of true/false to the symbols in clauses
for symbol in symbols:
model = model.extend(symbol, randbool())
# for i = 1 to max_flips do
for i in range(number_of_flips):
# if model satisfies clauses then return model
if self._all_clauses_satisfied(clauses, model):
return model
# clause <- a randomly selected clause from clauses that is false in model
symbols_list = list(self._get_symbols_of_randomly_selected_false_clause(clauses, model))
# with probability p flip the value in model of a randomly selected symbol from clause
if random.random() >= probability_of_random_walk:
symbol = select_randomly_from_list(symbols_list)
# else flip whichever symbol in clause maximizes the number of satisfied clauses
else:
symbol = self._get_symbol_whose_flip_maximises_satisfied_clauses(clauses, model, symbols_list)
model.flip(symbol)
return None
def test_dpll_when_all_clauses_true(self):
model = Model()
model = model.extend("A", True).extend("B", True)
sentence = PLParser().parse("(A OR B) AND (A OR B)")
result = DPLL().dpll_satisfiable(sentence, model)
self.assertTrue(result)
def dpll_satisfiable(self, sentence, model=Model()):
cnf_sentence = CNFTransformer().transform(sentence)
# clauses <- the set of clauses in the CNF representation of s
clauses = CNFClauseGatherer().collect(cnf_sentence)
# symbols <- a list of the proposition symbols in s
symbols = SymbolsCollector().collect_symbols(sentence)
# return DPLL(clauses, symbols, [])
return self._dpll(clauses, symbols, model)
def test_not(self):
expression = "NOT A"
parser = PLParser()
root_term = parser.parse(expression)
m = Model()
m = m.extend("A", False)
self.assertTrue(m.is_true(root_term))
m.clear()
m = m.extend("A", True)
self.assertFalse(m.is_true(root_term))
def tt_entails(self, knowledge_base, alpha):
kb_sentence = knowledge_base.as_sentence()
query_sentence = PLParser().parse(alpha)
collector = SymbolsCollector()
kb_symbols = collector.collect_symbols(kb_sentence)
query_symbols = collector.collect_symbols(query_sentence)
# symbols <- a list of proposition symbols in KB and alpha
symbols_list = list(kb_symbols.union(query_symbols))
# return TT-Check-All(KB, alpha, symbols, [])
return self.tt_check_all(kb_sentence, query_sentence, symbols_list,
Model())
def test_not(self):
expression = "NOT A"
parser = PLParser()
root_term = parser.parse(expression)
m = Model()
m = m.extend("A", False)
self.assertTrue(m.is_true(root_term))
m.clear()
m = m.extend("A", True)
self.assertFalse(m.is_true(root_term))
def test_biconditional(self):
expression = "A <=> B"
parser = PLParser()
root_term = parser.parse(expression)
m = Model()
m = m.extend("A", True).extend("B", True)
self.assertTrue(m.is_true(root_term))
m.clear()
m = m.extend("A", True).extend("B", False)
self.assertFalse(m.is_true(root_term))
m.clear()
m = m.extend("A", False).extend("B", True)
self.assertFalse(m.is_true(root_term))
m.clear()
m = m.extend("A", False).extend("B", False)
self.assertTrue(m.is_true(root_term))
m.clear()
def test_biconditional(self):
expression = "A <=> B"
parser = PLParser()
root_term = parser.parse(expression)
m = Model()
m = m.extend("A", True).extend("B", True)
self.assertTrue(m.is_true(root_term))
m.clear()
m = m.extend("A", True).extend("B", False)
self.assertFalse(m.is_true(root_term))
m.clear()
m = m.extend("A", False).extend("B", True)
self.assertFalse(m.is_true(root_term))
m.clear()
m = m.extend("A", False).extend("B", False)
self.assertTrue(m.is_true(root_term))
m.clear()
def test_dpll_return_false_with_one_false_in_model(self):
model = Model().extend("A", True).extend("B", False)
sentence = PLParser().parse("(A OR B) AND (A => B)")
result = DPLL().dpll_satisfiable(sentence, model)
self.assertFalse(result)
