Question

In Java language 

Write a static method called "evaluate" that takes a string as a parameter. The string will contain a postfix expression, consisting only of integer operands and the arithmetic operators +, -, *, and / (representing addition, subtraction, multiplication, and division respectively). All operations should be performed as integer operations. You may assume that the input string contains a properly-formed postfix expression. The method should return the integer that the expression evaluates to. The method MUST use a stack to perform the evaluation. Specifically, you should use the Stack class from the java.util package.

Also, in the same file, write a program that prompts the user to enter a postfix expression. Your program should evaluate the expression and output the result. Your program should call the "evaluate" method described above. Here are some examples:

     Postfix expression: 1 2 +     Result: 3     Postfix expression: 1 2 + 3 4 - *     Result: -3     Postfix expression: 3 4 5 + + 1 -     Result: 11

Answer 1

code:

import java.util.*;
class PostfixEvaluator
{
   static int evaluate(String exp)
   {
       Stack s=new Stack<>(); //create a stack of integers
       for(int i=0;i        {
           char c=exp.charAt(i);
           if(Character.isDigit(c))// If the scanned character is an operand (number here), push it to the stack.
           s.push(c - '0');
           else
           {       
               int v1 = s.pop();
               int v2 = s.pop();
               // If the scanned character is an operator, pop two elements from stack apply the operator
               if(c=='+')
                   s.push(v2+v1);
               if(c=='-')
                   s.push(v2- v1);
               if(c=='/')
                   s.push(v2/v1);
               if(c=='*')
                   s.push(v2*v1);
           }
       }
       return s.pop();   //returning the result
   }
   public static void main(String[] args) //main method
   {
       String exp;
       Scanner sc= new Scanner(System.in);//scanner class to read the input
       System.out.print("Postfix expression: ");
       exp=sc.next();
       System.out.println("Result: "+evaluate(exp));
   }
}

output: