public class Calculator {
    VList varList;
    

    //constructor - should only create
    //the variable list - there is one variable list per calculator
    //should not assign any stacks here
    public Calculator () {
	varList = new VList ();
    }
    



    //assign method
    public void assign (String line) {
	
	//copy line into a character array
	char[] chars = line.toCharArray ();
	
	// get the variable name
	char var = chars [0];
	//and length
	int len = line.length ();
	
	//make it into a substring for calculate/evaluate
	String expr = line.substring (2, len);
	
	//send it to calculate to change to int
	int val = calculate (expr.toCharArray ());
	System.out.println ("Assigning: " + var + "=(" + String.valueOf (expr) + ")" + val);
	
	//insert the variable and the value into the vlist
	varList.insert (var, val);
    }
    

    //public evaluate - converts the string into a char array
    //and send it to calculate 
    public int evaluate (String expression) {
	char[] expr = expression.toCharArray ();
	int val = calculate (expr);
	System.out.println ("Calculating: " + expression + "=" + val);
	return val;
    }


    //private method to do the work of evaluate
    
    private int calculate (char[] expr) {
	
	//make a stack to evaluate the postfix expression
	CStack st = new CStack ();
	
	//number of symbols to process is the length of the
	//string
	int numSymb = expr.length;
	
	
	//loop though and process each depending on type
	for (int i = 0; i < numSymb; ++i) {
	    char sym = expr[i];

	    // if we read an operator
	    //pop and perform calculation
	    //push back onto stack
	    if (isOperator (sym)) {
		int val1 = st.pop ();
		int val2 = st.pop ();
		int val = doOperation (val1, sym, val2);
		st.push (val);
	    } 
	    //if it is a letter - it
	    //is a variable name so we get value and
	    //puch it onto the stack
	    else if (Character.isLetter (sym))
		st.push (varList.getValue (sym));

	    //if it is a number we just push the 
	    //actual int it represents onto the stack
	    else if (Character.isDigit (sym))
		st.push (Character.digit (sym, 10));
	}

	//finally whatever is leftover is our answer
	return st.pop ();
    }

    //helper method to check if something is an operator
    public boolean isOperator (char sym) {
	return ((sym == '+') ||
		(sym == '-') ||
		(sym == '*') ||
		(sym == '/'));
    }

    //do operation takes two vals and does the operation
    // its postfix, values are in reverse order that
    //is why  it is named val2 and val1 in that order
    public int doOperation (int val2, char oper, int val1) {
	int ret = 0;
	switch (oper) {
	    case '+': ret = val1 + val2;
		      break;
	    case '-':
		      ret = val1 - val2;
		      break;
	    case '*':
		      ret = val1 * val2;
		      break;
	    case '/':
		      ret = val1 / val2;
		      break;
	}
	return ret;
    }
}