Question

Postfix notation is an unambiguous way of writing an arithmetic expression without parentheses. It is defined so that if “(exp1)op(exp2)” is a normal, fully parenthesized expression whose operation is op, the postfix version of this is “pexp1 pexp2 op”, where pexp1 is the postfix version of exp1 and pexp2 is the postfix version of exp2. The postfix version of a sin- gle number or variable is just that number or variable. For example, the postfix version of "((5+2) * (8-3))/4" is "5 2 + 8 3 - * 4 /". Write a Python program that evaluate a postfix expression non-recursively if consider only binary operators "+", "*", "/", "-".

Answer 1

Solution:

# Python program to evaluate value of a postfix expression

# Class to convert the expression

class Evaluate:

# Constructor to initialize the class variables

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 evaluatePostfix(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 (self.pop())

# Driver program to test above function

exp = "52+83-*4/"

obj = Evaluate(len(exp))

print("Value of %s is %s" %(exp, obj.evaluatePostfix(exp)))

Output:

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