This is a solution to the problem.
import java.util.Queue;
import java.util.LinkedList;
import java.util.Stack;
import java.util.ArrayList;
class IterativeEvaluator2
{
private ExpressionScanner expression;
public IterativeEvaluator2 (String expression)
{
this.expression = new ExpressionScanner(expression);
}
public double evaluate(Queue<Double> operandqueue)
{
// write your code here to create an explicit context stack
Stack<Queue> temp_stack = new Stack<Queue>();
char operator = ' ';
double operand = 0.0;
int a;
// write your code here to evaluate the LISP expression iteratively
// you will need to use an explicit stack to push and pop context objects
while ( expression.hasNextOperator() || expression.hasNextOperand() )
{
// Get the open bracket
if ( expression.hasNextOperator())
{
operator = expression.nextOperator() ;
if (operator == '(')
{
temp_stack.push(operandqueue);
operandqueue = new LinkedList<Double>();
}
// push the list into the stack after the closing bracket appears
else if (operator == ')')
{
if (operandqueue.size() > 1 ) {
//System.out.println("new opqueue");
//System.out.println(operandqueue);
operand = calculate(operandqueue);
System.out.println(operand);
if (temp_stack.peek() != null)
{
operandqueue = temp_stack.pop();
//System.out.println(operandqueue);
operandqueue.offer(operand);
}
}
else if (operandqueue.size() == 1)
operandqueue = temp_stack.pop();
}
// if it is another operator then it must be +,-,/,*
else
{
operandqueue.offer( (double) operator );
}
}
// else it is an operand so just put it in the queue
else
{
a= expression.nextOperand() ;
//System.out.println(a);
operandqueue.offer( (double) a );
}
}
return operand;
}
private double calculate(Queue<Double> some_queue)
{
char operator = (char) some_queue.remove().intValue();
//System.out.println(operator);
double operand1 = 0;
double operand2;
switch(operator){
case '+' : while( !some_queue.isEmpty() )
{
operand2 = some_queue.remove();
operand1 = operand1 + operand2;
}
break;
case '-' : operand1 = some_queue.remove();
//checks for negative numbers
if (some_queue.isEmpty() ){
operand1 = 0 - operand1;
}
else{
while ( !some_queue.isEmpty() )
{
operand2 = some_queue.remove();
operand1 = operand1 - operand2;
System.out.println(operand1);
}
}
break;
case '*' : if (!some_queue.isEmpty() )
{
operand1 = 1;
while ( !some_queue.isEmpty() )
{
operand2 = some_queue.remove();
operand1 = operand1*operand2;
}
}
break;
case '/' : operand1 = some_queue.remove();
if (some_queue.isEmpty() )
operand1 = 1/operand1 ;
else{
while (!some_queue.isEmpty() )
{
operand2 = some_queue.remove();
operand1 = operand1/operand2; }
}
break;
}
return operand1;
}
public static void main(String [] args)
{
String s =
"(+\t(- 6)\n\t(/\t(+ 3)\n\t\t(- \t(+ 1 1)\n\t\t\t3\n\t\t\t1)\n\t\t(*))\n\t(* 2 3 4))"; // = 16.5
IterativeEvaluator2 myEvaluator = new IterativeEvaluator2(s);
System.out.println("Evaluating LISP Expression:\n" + s);
System.out.println("Value is: " + myEvaluator.evaluate(null));
}
} /* 201340 */