EVALUATE POSTFIX EXPRESSION

EVALUATE POSTFIX EXPRESSION

#2. W.a.p. to evaluate any postfix expression
class Evaluate:
    #Consider to evaluate the class variable
    def __init__(self,capacity):
        self.top=-1;        
        self.capacity=capacity;        #This array is used a stack
        self.array=[]
    #check if the stack is empty
    def isEmpty(self):
        return True if self.top==-1 else False

    #Return the value of the top of the stack
    def peek(self):        
        return self.array[-1]

    #Pop the element from the stack
    def pop(self):
        if not self.isEmpty():
            self.top-=1;            
            return self.array.pop()
        else:            
            return "$"
    #Push the element to the stack
    def push(self,op):
        self.top+=1
        self.array.append(op)

#The main function that converts given infix expression
#to postfix expression

    def evaluatePost(self,exp):
        #Iterate over the expression for conversion
        for i in exp:
            #If the scanned character is an operand
            #(number here ) push it to the stack
            if i.isdigit():
                self.push(i)
            #If the scanned character is an operator
            #pop two elements from stack and apply it
            else:
                val1=self.pop()
                val2=self.pop()

                self.push(str(eval(val2+i+val1)))
        return int(self.pop())    
#Driver program to test above function
exp="231*+9-"
obj=Evaluate(len(exp))
print("postfix evaluation: %d" %(obj.evaluatePost(exp)))