added expression evaluator

Author: hemangsk

Data Structures/expressionEvaluator.py

@@ -0,0 +1,52 @@
+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 top(self):
+        return self.stack[self.size() - 1]
+
+    def size(self):
+        return len(self.stack)
+
+def evaluator(string):
+    length = len(string)
+    operator_stack = Stack()
+    operand_stack = Stack()
+
+    for x in range(0, length):
+        item = string[x]
+        print item
+        if(item not in "()+-*/"):
+            operand_stack.push(item)
+
+
+        elif item == ')':
+            operand1 = int(operand_stack.pop())
+            operand2 = int(operand_stack.pop())
+            operator = operator_stack.pop()
+
+            if operator == '+':
+                operand_stack.push(operand1 + operand2)
+            elif operator == '-':
+                operand_stack.push(operand2 - operand1)
+            elif operator == '*':
+                operand_stack.push(operand2 * operand1)
+            elif operator == '/':
+                operand_stack.push(operand2 / operand1)
+
+        elif item in "+-/*":
+            operator_stack.push(item)
+
+    return operand_stack.pop()
+
+print(evaluator("(2+3)"))