import java.util.Stack;
import java.util.StringTokenizer;
import java.util.Scanner;

public class PostFixCalculator {

	protected Stack<String> stack;
	protected Scanner scan;
	protected StringTokenizer tokenizer;
	
	public PostFixCalculator()
	{
		stack = new Stack<String>();
		scan = new Scanner(System.in);
		tokenizer = null;
	}
	
	public void greeting()
	{
		System.out.print("Hello, I am a post-fix calculator. Give me expressions in"
				+ "\npost-fix notation and I will compute the results.");
	}
	
	public boolean enterExpression()
	{
		System.out.print("\nEnter an expression (return to exit): ");
		tokenizer = new StringTokenizer(scan.nextLine());
		if (tokenizer.hasMoreTokens())
			return true;
		return false;
	}
	
	public double getResult()
	{
		stack.clear();
		while(tokenizer.hasMoreTokens())
		{
			stack.push(tokenizer.nextToken());
		}
		
		return getExpr();
	}
	
	private double getExpr()
	{
		String op = stack.pop();
		double a;
		double b;
		
		if (op.equals("+"))
		{
			a = getExpr();
			b = getExpr();
			return a + b;
		}
		else if (op.equals("*"))
		{
			a = getExpr();
			b = getExpr();
			return a * b;
		}
		else if (op.equals("-"))
		{
			a = getExpr();
			b = getExpr();
			return b - a;
		}
		else 
			return Double.valueOf(op);
	}
	
	public static void main(String[] args)
	{
		 
		PostFixCalculator pfc = new PostFixCalculator();
		
		pfc.greeting();
		
		while (pfc.enterExpression())
		{
			System.out.print("The result is " + pfc.getResult() + ".\n");
		}
		
		System.out.print("Goodbye.");	
		
		
	}
}