NEED HELP:To Parse Conditional Operator Within An Expression.

Question

Hi there:
I'm having some difficulties to parse some conditoional operator. My requirements is:
Constants value: int a=1,b=2,c=3
Input/Given value: 2
conditional operator expression: d=a|b|c
Expected result: d=true
Summary: I like to receive a boolean from anys expressions defined, which check against Input/Given value.Note: The expression are various from time to time, based on user's setup, the method is smart enough to return boolean based on any expression.
Let me know if you have any concerns.
Thanks a million,
selena

Answer 1

here is a simple example.
BNF changes
EXPR ::= OPERATOR
EXPR ::= OPERATION OPERANT EXPR
This is as far as I can go, please use it as a template only
because you need to take into account the precedence using ()
the logic of the eval was simply right to left so I think it is not what you want
Cheers
public interface Expression
     String OR = "|";
     String AND = "&";
     public boolean eval(int value) throws Exception;
public class ExpressionImpl implements Expression
     public String oper1;
     public String operant;
     public Expression tail;
     /* (non-Javadoc)
      * @see parser.Expression#eval()
     public boolean eval(int value) throws Exception
          int val1 = getValue(oper1);
          if (null == tail)
System.out.println(val1 + ":" + value);               
               return (val1 == value);
          else
               if (OR.equals(operant))
                    return (val1 == value || tail.eval(value));
               else if (AND.equals(operant))
                    return (val1 == value && tail.eval(value));
               else
                    throw new Exception("unsupported operant " + operant);
     private int getValue(String operator) throws Exception
          Integer temp = ((Integer)Parser.symbol_table.get(operator));
          if (null == temp)
               throw new Exception("symbol not found " + operator);
          return temp.intValue();
     public String toString()
          if (null == operant) return oper1;
          return oper1 + operant + tail.toString();
public class Parser
     public static HashMap symbol_table = new HashMap();
      * recursive parsing
     public Expression parse(String s)
          ExpressionImpl e = new ExpressionImpl();
          e.oper1 = String.valueOf(s.charAt(0));
          if (s.length() == 1)
               return e;
          else if (s.length() > 2)
               e.operant = String.valueOf(s.charAt(1));
               e.tail = parse(s.substring(2));
          else
               throw new IllegalArgumentException("invalid input " + s);
          return e;
     public static void main(String[] args) throws Exception
          Parser p = new Parser();
          Parser.symbol_table.put("a", new Integer(1));
          Parser.symbol_table.put("b", new Integer(2));
          Parser.symbol_table.put("c", new Integer(3));
          Parser.symbol_table.put("d", new Integer(4));
          Expression e = p.parse("a|b|c&d");
          System.out.println("input " + e.toString());
          System.out.println(e.eval(2));
}