Fall  2011                                Final Examination                                     Prasad

 1.   Scheme: Recursive Definition (5 + 4 / 3 + 4 + 2 pts)

Consider the list functions computed by the following Standard (R5RS) Scheme definitions. 

   (define (mystery   lis)
            (cond ((null? lis) '())
                  ((symbol? (car lis)) (cons 's (mystery (cdr lis))))

                  ((number? (car lis)) (cons 'n (mystery (cdr lis))))                                  
                  ((list? (car lis))  (cons (mystery (car lis))   
                                            (mystery (cdr lis)) ) )
                  (else    (mystery (cdr lis)) )             
            )
     )

What values are returned for the following calls: (mystery '(a -21)) and (mystery '(a (b 2) 3))? Trace the computation carefully by displaying all the calls to mystery.

Explain clearly the list function computed by the above Scheme definition.

(Additional problem for CS680 Students) Write a Scheme function eliminate that takes a nested list of symbols and numbers, and eliminates all the symbols and nested lists, return just the flat list of top-level numbers. 

(eliminate '(a (1) () (b 2) 3 4)) =returns=> (3 4)

2.  PL Concepts (5 * 3 / 3 * 5 pts)

• What is C#'s rationale for introducing "new" and "override" keywords in the context of method overidding, in contrast with Java's approach? 
  
• What is portability? How is it achieved in Java?
• What is the significance of checked exception in Java?

• Explain two benefits of supporting multi-threading in a programming language?
• What is a closure? What is its significance?

3.  Java : Polymorphism and Dynamic Binding (5 pts)

Does the following program compile without any errors? If not, correct the program "minimally" before proceeding further.

Write the output of executing the (possibly corrected) Java program indicating any run-time exceptions it may generate. (Hint: Focus on the declared type of a variable and the class associated with the object to which the variable holds a reference.)

    class P {
       String f(P x) { return ("  abc ");}

       String f(Q x) { return ("  def ");}
    }
    class Q extends P {
       String f(P x) { return ("  ghi ");}

       String f(Q x) { return ("  jkl ");}
    }
    class Final {
        public static void main(String[] args) {
            P[] arr  = {new P(), new Q(), null};

            System.out.println(arr[0].f(arr[1]));
            System.out.println(arr[1].f(arr[0]));                   
            System.out.println(arr[1].f(arr[1]));
            System.out.println(arr[1].f(arr[2]));
            System.out.println(arr[2].f(arr[2]));

        }
    }

4  Top-down Parser (4 + (3 + 4) pts)

Study the following Java program carefully and answer the following questions.

• Specify the grammar for the input language in EBNF form. (Use a pair of curly braces for Kleene Star operation.)
• What is the result of invoking the program on the exprFinal.dat file containing the following 3 expressions? Provide the abstract syntax tree constructed to justify your answer.
  

        x - y
        x - x - y