Question

Python

The final value of the evaluation of the postfix expression 6 2 + 5 * 8 4 / - is ----------

Answer 1

Algorithm for Postfix Evaluation

• We create a stack to store values
• Read the expression in a loop and do the following
  • If itemis an operand (number) then push it in the stack we created
  • If item is an operator, pop operands for the operator from the stack, evaluate them and push the evaluated value back into the stack.
• In the end then number in the stack is the final answer

Progarm code in Python

Program Sample Output Screenshot

Program Code to copy

# Class to evaluate the expression
class PostfixEvaluation:

    # Constructor to initialize the class variables
    def __init__(self):
        self.top = -1
        # declare a stack
        self.array = []

    # check if the stack is empty
    def isEmpty(self):
        return True if self.top == -1 else False

    # Return item on stack  top
    def peek(self):
        return self.array[-1]

    # Pop item from the stack
    def pop(self):
        if not self.isEmpty():
            self.top -= 1
            return self.array.pop()
        else:
            return "$"

    # Push item into the stack
    def push(self, op):
        self.top += 1
        self.array.append(op)

    # This function will evaluate the postfix expression

    def evaluatePostfixExpression(self, expression):

        # Iterate over items in the expression array
        for item in expression:

            # push in the stack if it is an operand
            if item.isdigit():
                self.push(int(item))

            # if this is an operator then pop operands from stack and
            # evaluate and push evaluated value back in stack
            else:
                val1 = str(self.pop())
                val2 = str(self.pop())
                self.push(int(eval(val2 + item + val1)))

        return self.pop()


# Driver program to test above function
expression = "6 2 + 5 * 8 4 / -"
evaluater = PostfixEvaluation()
value = evaluater.evaluatePostfixExpression(expression.split())
print("The final value of the evaluation of the postfix expression %s is  %d" %
      (expression, value))