Question

Here's the problem that I have to solve:

Write a Java program that uses a Stack data structure to evaluate postfix expressions. Your program takes a postfix expression as an input,for example:3 42.3+ 5.25* ,from the user and calculates/display the result of the expression.

What I'm having trouble is getting it to reading and calculating it as postfix

Here's my code:

import java.util.Scanner;
public class Assignment2 {
    public static void main(String[] args)
    {

        Scanner scan = new Scanner(System.in);
      
        myStack<Double> stack = new myStack<Double>();
      
        System.out.println("Please enter a postfix expression: ");
      
        String input = scan.nextLine();
      
        String[] values = input.split(" ");
     
        for(String terms : values)
        {
            if(terms.equals("+") || terms.equals("-") || terms.equals("*") || terms.equals("/") || terms.equals("^") )
            {
            Double var2 = stack.pop();
            Double var1 = stack.pop();
            switch(terms)
                    {
                    case "+":
                     
                        stack.push(var1 + var2);
                       break;
                      
                    case "-":
                      
                        stack.push(var1 - var2);
                        break;
                    case "*":
                      
                        stack.push(var1 * var2);
                        break;
                    case "/":
                      
                        stack.push(var1 / var2);
                        break;
                        //
                    case "^":
                        stack.push(var1 * var1);
                    default:
                        break;
                  
                }          
            }
          
        }
         double value = Double.parseDouble(values[2]);
         System.out.println();
    }
}

A separate class called MyStack:

import java.util.Stack;
public class MyStack<T> implements StackInterface<T> {

    Stack<T> mystack;
  
    public MyStack()
            {
                mystack = new Stack<>();
            }
  
    @Override
    public void push(T newEntry)
    {
        mystack.push(newEntry);
    }
  
    @Override
    public T peek()
    {
        return mystack.peek();
    }
  
    @Override
    public T pop()
    {
        return mystack.pop();
    }
  
    @Override
    public boolean isEmpty()
    {
        return mystack.isEmpty();
    }
  
    @Override
    public void clear()
    {
        mystack.clear();
    }
}

Stack Interface class:
    public interface StackInterface<T>
    {
        //@param
        public void push(T newEntry);
        //@return
        public T pop();
        //@return
        public T peek();
        //@return
        public boolean isEmpty();
      
        public void clear();
    }

Answer 1

I have changed the code only in Assignment2.java class:

code:

import java.util.ArrayList;
import java.util.Scanner;
public class Assignment2 {
public static void main(String[] args)
{

Scanner scan = new Scanner(System.in);
  
MyStack<Double> stack = new MyStack<Double>();
  
System.out.println("Please enter a postfix expression: ");
  
String input = scan.nextLine();
String[] values = input.split("[ |+|*]");
for(String s:values) {
   if(!s.matches("\s*")) {
       stack.push(Double.parseDouble(s));
       //System.out.println(s);
}
}
ArrayList<String> sign=new ArrayList<String>();
for(int i=0;i<input.length();i++) {
   if (input.charAt(i) == '+' || input.charAt(i) == '-' || input.charAt(i) == '*' || input.charAt(i) == '/' || input.charAt(i) == '^') {
       sign.add(String.valueOf(input.charAt(i)));
   }
}
  
for(String terms : sign)
{
   if(!terms.matches("\s*")) {
   //   System.out.println(terms);
if(terms.equals("+") || terms.equals("-") || terms.equals("*") || terms.equals("/") || terms.equals("^") )
{
Double var2 = stack.pop();
Double var1 = stack.pop();
switch(terms)
{
case "+":

stack.push(var1 + var2);
break;
  
case "-":
  
stack.push(var1 - var2);
break;
case "*":
  
stack.push(var1 * var2);
break;
case "/":
  
stack.push(var1 / var2);
break;
//
case "^":
stack.push(var1 * var1);
default:
break;
  
}
}}
  
       } /*
           * double value = Double.parseDouble(values[2]); System.out.println();
           */
System.out.println(stack.pop());
}
}

output screnshot: