Coverage for local/lib/python2.7/site-packages/sage/logic/booleval.py : 81%

r""" 

Evaluation of Boolean Formulas 

AUTHORS: 

- Chris Gorecki (2006): initial version 

- Paul Scurek (2013-08-05): updated docstring formatting 

EXAMPLES: 

We can assign values to the variables and evaluate a formula:: 

sage: import sage.logic.booleval as booleval 

sage: t = ['|', ['&', 'a', 'b'], ['&', 'a', 'c']] 

sage: d = {'a' : True, 'b' : False, 'c' : True} 

sage: booleval.eval_formula(t, d) 

True 

We can change our assignment of values by modifying the dictionary:: 

sage: d['a'] = False 

sage: booleval.eval_formula(t, d) 

False 

""" 

from __future__ import absolute_import 

#***************************************************************************** 

# Copyright (C) 2006 Chris Gorecki <chris.k.gorecki@gmail.com> 

# Copyright (C) 2013 Paul Scurek <scurek86@gmail.com> 

# 

# Distributed under the terms of the GNU General Public License (GPL) 

# as published by the Free Software Foundation; either version 2 of 

# the License, or (at your option) any later version. 

# http://www.gnu.org/licenses/ 

#***************************************************************************** 

from . import logicparser 

# dictionary containing variable keys and boolean values 

__vars = {} 

def eval_formula(tree, vdict): 

r""" 

Evaluate the tree and return a boolean value. 

INPUT: 

- ``tree`` -- a list of three elements corresponding to a branch of a 

parse tree 

- ``vdict`` -- a dictionary containing variable keys and boolean values 

OUTPUT: 

The result of the evaluation as a boolean value. 

EXAMPLES: 

This example illustrates evaluating a boolean formula:: 

sage: import sage.logic.booleval as booleval 

sage: t = ['|', ['&', 'a', 'b'], ['&', 'a', 'c']] 

sage: d = {'a' : True, 'b' : False, 'c' : True} 

sage: booleval.eval_formula(t, d) 

True 

:: 

sage: d['a'] = False 

sage: booleval.eval_formula(t, d) 

False 

""" 

global __vars 

__vars = vdict 

b = logicparser.apply_func(tree, eval_f) 

return b 

def eval_f(tree): 

r""" 

Evaluate the tree. 

INPUT: 

- ``tree`` -- a list of three elements corresponding to a branch of a 

parse tree 

OUTPUT: 

The result of the evaluation as a boolean value. 

EXAMPLES: 

This example illustrates how to evaluate a parse tree:: 

sage: import sage.logic.booleval as booleval 

sage: booleval.eval_f(['&', True, False]) 

False 

sage: booleval.eval_f(['^', True, True]) 

False 

sage: booleval.eval_f(['|', False, True]) 

True 

""" 

return eval_op(tree[0], tree[1], tree[2]) 

def eval_op(op, lv, rv): 

r""" 

Evaluate ``lv`` and ``rv`` according to the operator ``op``. 

INPUT: 

- ``op`` -- a string or character representing a boolean operator 

- ``lv`` -- a boolean or variable 

- ``rv`` -- a boolean or variable 

OUTPUT: 

The evaluation of ``lv op rv`` as a boolean value. 

EXAMPLES: 

We can evaluate an operator given the values on either side:: 

sage: import sage.logic.booleval as booleval 

sage: booleval.eval_op('&', True, False) 

False 

sage: booleval.eval_op('^', True, True) 

False 

sage: booleval.eval_op('|', False, True) 

True 

""" 

lval = rval = None 

if lv is False: 

lval = False 

elif lv is True: 

lval = True 

elif lv is not None: 

lval = __vars[lv] 

if rv is False: 

rval = False 

elif rv is True: 

rval = True 

elif rv is not None: 

rval = __vars[rv] 

if op == '~': 

return not lval 

elif op == '&': 

return lval and rval 

elif op == '|': 

return lval or rval 

elif op == '^': 

return lval ^ rval 

elif op == '->': 

return (not lval) or rval 

elif op == '<->': 

return (not lval or rval) and (not rval or lval) 

else: # one variable 

return __vars[op]