Question

Python Issue

Postfix notation (also known as Reverse Polish Notation or RPN in short) is a mathematical notation in which operators follow all of its operands. It is different from infix notation in which operators are placed between its operands.

The algorithm to evaluate any postfix expression is based on stack and is pretty simple:

Initialize empty stack

For every token in the postfix expression (scanned from left to right):

If the token is an operand (number), push it on the stack

Otherwise, if the token is an operator (or function):

Check if the stack contains the sufficient number of values (usually two) for given operator

If there are not enough values, finish the algorithm with an error

Pop the appropriate number of values from the stack

Evaluate the operator using the popped values and push the single result on the stack

Return the final result of the calculation

Implement a function def eval_postfix(expression_str) that evaluates and returns the result of a postfix expression. The postfix expression to be evaluated is passed to the function as a string parameter. Each component of the postfix expression string is separated by a single space. Your function needs to support the following operations: "+", "-", "*", "/", "%" and "**". Your function should also work with values consisting of more than one digit.

For example:

Test	Result
print(eval_postfix("2 3 * 4 +"))	10
print(eval_postfix("2 3 4 + *"))	14

Answer 1

def evaluate_postfix(input_string):

    stack = list()                                      # initializing the empty stack

    for i in input_string:

        result = 0;                                     #result variable for storing the result

        if i in "0123456789":                            #checking whether input symbol is number or not

            stack.append(int(i))

        elif stack:                                     #checking whether stack is not empty

            operand1=stack.pop()                        # removing the top value of the stack

            operand2=stack.pop()                         #removing the next top value of the stack

            if i == "+":

                result = operand1+operand2

            elif i == "-":

                result = operand2 - operand1

            elif i == "*":

                result = operand1 * operand2

            elif i == "/":

                result = operand2 / operand1

            elif i == "^":                              #taking raise power as ^ operator

                result = operand2 ** operand1

            elif i == "%":

                result= operand2 % operand1

        if result!=0:                                   ##checking result is zero or not

            stack.append(result)                        ## if any result we are ushing it into stcak

    return stack.pop()                                   ## returning the final stack value

input_string=raw_input("enter the postfix string\n");   #taking the expression string as input

final_val=evaluate_postfix(input_string)                #calling the function

print final_val                                         #printing the final result