postfix evaluator

Author: hemangsk

Data Structures/stack/postfixEvaluator.py

@@ -0,0 +1,80 @@
+class Stack:
+    def __init__(self):
+        self.stack = []
+    def push(self, item):
+        self.stack.append(item)
+    def pop(self):
+        return self.stack.pop()
+    def isEmpty(self):
+        return self.stack == []
+    def peek(self):
+        return self.stack[self.size() - 1 ]
+    def size(self):
+        return len(self.stack)
+
+def infix2postFix(string):
+    opStack = Stack()
+    result = ""
+    expression = string.split(" ")
+
+    for x in expression:
+        if x == "(":
+            opStack.push(x)
+        elif x == ")":
+            if opStack.peek() == "(":
+                opStack.pop()
+            else:
+                while(opStack.peek() != "("):
+                    result+=opStack.pop() + " "
+                opStack.pop()
+        elif x in "abcdefghijklmnopqrstuvwxyz" or x in "1234567890":
+            result += x + " "
+
+        elif x in "+-/*" and opStack.size() > 0:
+            if opStack.peek() in "*/+-":
+                while(prec[opStack.peek()] >= prec[x]):
+                    result+=opStack.pop() + " "
+            opStack.push(x)
+
+        elif x in "+-/*" and opStack.size() == 0:
+            opStack.push(x)
+
+    stackLeft = opStack.size()
+    while(stackLeft > 0):
+        stackLeft -=1
+        result += opStack.pop() + " "
+
+    return result[:-1]
+
+
+def evaluatePostFix(expression):
+    listExpression = expression.split(" ")
+    operandStack = Stack()
+    operatorStack = Stack()
+
+    for x in listExpression:
+        if x in "1234567890":
+            operandStack.push(x)
+        elif x in "*/+-":
+            operatorStack.push(x)
+
+
+    while(operatorStack.size() > 0):
+        a = int(operandStack.pop())
+        b = int(operandStack.pop())
+
+        sym = operatorStack.pop()
+
+        if sym == "+":
+            operandStack.push(a+b)
+        elif sym == "-":
+            operandStack.push(a-b)
+        elif sym == "/":
+            operandStack.push(a/b)
+        elif sym == "*":
+            operandStack.push(a*b)
+
+    return operandStack.pop()
+
+exp = raw_input("Enter = ")
+print(evaluatePostFix(infix2postFix(exp)))