// File: EvaluatePostfixEvaluator.java

// Evaluates an arithmetic expression in postfix notation.
// (Also serves as test class for a stack class - in this case, Stack2.java)

// Demonstrates stack operations push and pop

import java.util.Scanner;


/**
 * Evaluates arithmetic expressions in postfix notation. 
 */
public class PostfixEvaluator {

   private Stack2<Double> stack;
   // create String containing all valid aritmetic operators
   private static final String OPERATORS = "$*/+-";    // "$" = exponentiation

   /**
    * Create a PostfixEvaluator object
    */
   public PostfixEvaluator() 
   {
      stack = new Stack2<Double>();
   }
   
   /**
    * Evaluate a postfix arithmetic expression
    * @param postfix the expression to be evaluated
    * @return the result
    */
   public double evaluate(String postfix)
   {
      Scanner scan = new Scanner(postfix) ;    // to scan postfix string

      // extract and process each symbol of postfix string

      while ( scan.hasNext() )                 // while more symbols in string
      {
         String symbol = scan.next();          // get next symbol

         if (!OPERATORS.contains(symbol))      // operand, not operator
         {
            // convert to double and place on stack
            double number = Double.parseDouble(symbol);
            stack.push(number);
         } 
         else                                  // must be operator
         {
            // pop stack twice, execute operation, and push result

            double opnd2 = stack.pop();        // right-hand operand
            double opnd1 = stack.pop();        // left-hand operand

            if (symbol.equals("$")) 
               stack.push(Math.pow(opnd1, opnd2));
            else if (symbol.equals("*")) 
               stack.push(opnd1 * opnd2);
            else if (symbol.equals("/")) 
               stack.push(opnd1 / opnd2);
            else if (symbol.equals("+")) 
               stack.push(opnd1 + opnd2);
            else // must be "-"
            
               stack.push(opnd1 - opnd2);
         }
      }
      // loop postcondition: all symbols of current postfix expression
      // have been processed

      // final result is on top of stack - pop and return
      return stack.pop();
   }
}