Truth tables in python using sympy

Question

I'm trying to create a program, that uses sympy to take a set of variables and evaluate a symbolic logic expression over the domain of those variables. The problem is that I cannot get python to evaluate the expression after it spits out the truth table.

Here's the code:

    from sympy import *
from sympy.abc import p, q, r

def get_vars():
    vars = []
        print "Please enter the number of variables to use in the equation"
        numVars = int(raw_input())
    print "please enter each of the variables on a newline"
        for i in xrange(numVars):
        vars.append(raw_input())
    return vars

def get_expr():
    print "Please enter the expression to use"
    return str(raw_input())

def convert_to_expr(inputStr):
    return eval(inputStr)

def main():
    vars = get_vars()
    expr = get_expr()

    print("recieved input: " + str(vars) + " expr " + str(expr))

    print "Truth table for " + str(len(vars)) + "variable(s)"
    for i in enumerate(truth_table(vars, expr)):
        print i

def fixed_table(numvars):
    """
    Generate true/false permutations for the given number of variables.
    So if numvars=2
    Returns (not necessarily in this order):
        True, True
        True, False
        False, False
        False, True
    """
    if numvars is 1:
        yield [True]
        yield [False]
    else:
        for i in fixed_table(numvars-1):
            yield i + [True]
            yield i + [False]


def truth_table(vars, expr):
    """
    Takes an array of variables, vars, and displays a truth table
    for each possible value combination of vars.
    """
    for cond in fixed_table(len(vars)):
        values=dict(zip(vars,cond))
        yield cond + [eval(expr)]   

if __name__ == "__main__":
    main()

If I do the following, here's the output:

    Please enter the number of variables to use in the equation
3
please enter each of the variables on a newline
p
q
r
Please enter the expression to use
p&q&r
recieved input: ['p', 'q', 'r'] expr p&q&r
Truth table for 3variable(s)
(0, [True, True, True, And(p, q, r)])
(1, [True, True, False, And(p, q, r)])
(2, [True, False, True, And(p, q, r)])
(3, [True, False, False, And(p, q, r)])
(4, [False, True, True, And(p, q, r)])
(5, [False, True, False, And(p, q, r)])
(6, [False, False, True, And(p, q, r)])
(7, [False, False, False, And(p, q, r)])

If some software exists to perform this task, I'd really like to know about it :-)

Thanks in advance.

Answer 1

You're really close! Once you've got And(p, q, r) and your truth tables, you can use the subs method to push your values dict into the expression: i.e.

    yield cond + [eval(expr).subs(values)]

gives

p&q&r
recieved input: ['p', 'q', 'r'] expr p&q&r
Truth table for 3variable(s)
(0, [True, True, True, True])
(1, [True, True, False, False])
(2, [True, False, True, False])
(3, [True, False, False, False])
(4, [False, True, True, False])
(5, [False, True, False, False])
(6, [False, False, True, False])
(7, [False, False, False, False])

But I think there's a simpler way to do this. The sympify function already works to generate expressions from strings:

In [7]: expr = sympify("x & y | z")

In [8]: expr
Out[8]: Or(z, And(x, y))

and we can get the variables too:

In [9]: expr.free_symbols
Out[9]: set([x, z, y])

plus itertools.product can generate the values (and cartes is an alias for it in sympy):

In [12]: cartes([False, True], repeat=3)
Out[12]: <itertools.product at 0xa24889c>

In [13]: list(cartes([False, True], repeat=3))
Out[13]: 
[(False, False, False),
 (False, False, True),
 (False, True, False),
 (False, True, True),
 (True, False, False),
 (True, False, True),
 (True, True, False),
 (True, True, True)]

Combining these, which is basically just using sympify to get the expression and avoid eval, using the built-in Cartesian product, and adding .subs() to use your values dictionary, we get:

def explore():
    expr_string = raw_input("Enter an expression: ")
    expr = sympify(expr_string)
    variables = sorted(expr.free_symbols)
    for truth_values in cartes([False, True], repeat=len(variables)):
        values = dict(zip(variables, truth_values))
        print sorted(values.items()), expr.subs(values)

which gives

In [22]: explore()
Enter an expression: a & (b | c)
[(a, False), (b, False), (c, False)] False
[(a, False), (b, False), (c, True)] False
[(a, False), (b, True), (c, False)] False
[(a, False), (b, True), (c, True)] False
[(a, True), (b, False), (c, False)] False
[(a, True), (b, False), (c, True)] True
[(a, True), (b, True), (c, False)] True
[(a, True), (b, True), (c, True)] True

This is shorter than yours, but it uses exactly your approach.