class Shunter():
'''
Converts infix to postfix, storing tokens and the progress of the
computation as object data.
'''
def __init__(self, expression, **kwargs):
'''
Initialize the module.
'''
self.expr = ' '.join(expression)
self.output = []
self.operators = {
'+': lambda x,y: x + y,
'-': lambda x,y: x - y,
'/': lambda x,y: x / y,
'*': lambda x,y: x * y,
'^': lambda x,y: x ** y
}
self.tokenizer = Tokenizer(self.expr, '+-*/^')
self.precedence = {'+': 0, '-': 0, '*': 1, '/': 1, '^': 2 }
def _has_higher_precedence(self, op1, op2):
'''
Returns true if op1 has a higher precedence than op2; false otherwise.
'''
return self.precedence[op1] > self.precedence[op2]
def convert(self):
'''
Converts the given infix into postfix using the shunting-yard
algorithm.
'''
stack = []
while self.tokenizer.has_token():
token = self.tokenizer.next_token().strip()
if token.isdigit():
self.output.append(token)
elif self.operators.has_key(token):
# TODO optimize
if len(stack) == 0:
stack.append(token)
else:
while len(stack) and self._has_higher_precedence(
stack[-1], token):
self.output.append(stack.pop())
stack.append(token)
else:
print 'Parser error: invalid character "{0}"'.format(token)
sys.exit(1)
while len(stack):
print stack
self.output.append(stack.pop())
return ' '.join(self.output)
def eval(self):
'''
Evaluates an expression in postfix notation.
'''
stack = []
for token in self.output:
if token.isdigit():
stack.append(token)
elif self.operators.has_key(token):
try:
y, x = stack.pop(), stack.pop()
stack.append(self.operators[token](x, y))
except IndexError:
print 'Evaluator error: invalid postfix. Sorry.'
sys.exit(1)
else:
if len(stack) != 1:
print 'Invalid postfix.'
sys.exit(1)
else:
return stack.pop()
