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evaluator.py
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evaluator.py
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"""
Author: Leigh Stauffer
File: evaluator.py
Project 10
"""
from tokens import Token
from scanner import Scanner
from linkedstack import LinkedStack
class Evaluator(object):
"""Evaluator for postfix expressions.
Assumes that the input is a syntactically correct
sequence of tokens."""
def __init__(self, scanner):
"""Sets the initial state of the evaluator."""
self._operandStack = LinkedStack()
self._scanner = scanner
def evaluate(self):
"""Returns the value of the postfix expression."""
for currentToken in self._scanner:
if currentToken.getType() == Token.INT:
self._operandStack.push(currentToken)
elif currentToken.isOperator():
right = self._operandStack.pop()
left = self._operandStack.pop()
result = Token(self._computeValue(currentToken,
left.getValue(),
right.getValue()))
self._operandStack.push(result)
result = self._operandStack.pop()
return result.getValue();
def _computeValue(self, op, value1, value2):
"""Utility routine to compute a value."""
result = 0
theType = op.getType()
if theType == Token.PLUS:
result = value1 + value2
elif theType == Token.MINUS:
result = value1 - value2
elif theType == Token.MUL:
result = value1 * value2
elif theType == Token.REMDR:
result = value1 % value2
elif theType == Token.EXPO:
result = value1 ** value2
elif theType == Token.DIV:
if value2 == 0:
raise ZeroDivisionError("Attempt to divide by 0")
else:
result = value1 // value2
return result
def main():
"""Tester function for the evaluator."""
while True:
sourceStr = input("Enter a postfix expression: ")
if sourceStr == "": break
try:
evaluator = Evaluator(Scanner(sourceStr))
print("The value is", evaluator.evaluate())
except Exception as e:
print("Exception:", str(e))
if __name__ == '__main__':
main()