University of Virginia, Department of Computer Science
CS200: Computer Science, Spring 2002

Final Exam Out: 29 April 2002
Due: Monday, 6 May 2002, 5:00 PM (turn in your exam at Olsson 236A)

Read these directions carefully before opening the exam.

Work alone. You may not discuss these problems or anything else related to this course with anyone except for the course staff between receiving this exam and when it is due, Monday 6 May. You may send email to cs200-staff@cs.virginia.edu to ask for clarifications on what the questions mean.

Open book. You may use any books you want, lecture notes and slides, your notes, and problem sets. If you use anything other than the course books and notes, cite what you used. You may also use anything you find on the web, but you should cite your sources.

Closed Interpreter. You may not use a Scheme interpreter to develop your answers.

Time Limit. Do not spend more than three hours total on this exam where you are looking at the exam questions. I recommend looking through all the questions first, and answering the ones you can answer quickly right away. Then, take a break from the exam and return to work on the harder questions later.

This exam is intended to measure what you know, not to be a long ordeal. The time limit is not meant to put time pressure on you, but to save you from spending inordinate amounts of time on this exam and to make the exam fair for people who have lots of other finals this week. If you have spent three hours but feel you would benefit from more time, you may email me to explain how you have spent your time so far, and why you think more time would help you.

Keep track of the total time you spend on the exam using this table:

Start TimeStop TimeTime Spent
   
   
   
   

Answer well. Answer all 10 questions. Each question is worth 10 points. Your answers should fit in the space provided on this exam. If you really need more space, you may attach additional sheets (but if you need more space it is probably a sign that you are misinterpreting or misanswering the question). Full credit depends on the clarity and elegance of your answer, not just correctness. Your answers should be as short and simple as possible, but not simpler.

Procedures

1. Define a procedure increment-list that takes a list of numbers, and produces a new list of numbers with each number incremented by one. For example, (increment-list (list 2 0 0 88)) should evaluate to the list (3 1 1 89). Do not use map in your definition.











2. Use map to define increment-list as in question 1.







3. Define a procedure make-traced-procedure that can be used to observe applications of another procedure. Your make-traced-procedure should work like this:

> (define double (make-traced-procedure (lambda (x) (printf "in double~n") (+ x x))))

> (map double (list 1 2 3))

Called procedure
in double
(Result: 2)
Called procedure
in double
(Result: 4)
Called procedure
in double
(Result: 6)

(2 4 6)

Note that the traced procedure is only evaluated once (for each time it would be evaluated if it were not traced). You may use (display (list "Result: " val)) to print out the string "Result:" and the value val.













Objects and Environments

Cy D. Fect defines a procedure reverse! that uses mutation to reverse a list:
(define (next-to-last-pair lst)
  (if (null? lst) (error "Null list!")
      (if (null? (cdr lst)) (error "Single length list!")
          (if (null? (cdr (cdr lst))) lst
              (next-to-last-pair (cdr lst))))))

(define (reverse! lst)
    (if (or (null? lst) (eq? 1 (length lst)))
        #f ;; Nothing to do for empty or length 1 list (value doesn't matter)
	(let* ((nexttolast (next-to-last-pair lst))
	       (lastpair (cdr nexttolast))
	       (first (car lst)))
	  ;;; Point 0
	  (set-cdr! nexttolast null)
	  ;;; Point 1
	  (reverse! (cdr lst))
	  (set-cdr! nexttolast lastpair)
	  (set-car! lst (car lastpair))
	  ;;; Point 2
	  (set-car! lastpair first))))
Consider evaluating:

> (define ilist (list 1 2 3))

> (reverse! ilist)

> ilist

(3 2 1)

At Point 0 on the first call to reverse! the environment looks like:

The E1 environment is created by the application (reverse! ilist). The E2 environment is created by the let inside reverse! (which desugars to an application of a lambda). (The let* actually desugars into more than one lambda, and more than one environment, but for this question we combine them into one environment.)

4. Show what the environment looks like at Point 1 (after evaluating (set-cdr! nexttolast null)). To make your drawing easier, here is a template drawing. You can answer this question by only adding to this template.

5. Show what the environment looks like at Point 2. You can answer this question by only adding to this template.

Turing Machines

6. The Turing Machine for checking balanced parentheses from Lecture 34 was incorrect. In particular, it would incorrectly report balanced parentheses for input such as ((). Correct the Turing Machine so it correctly ends with 1 at the tape head if the parentheses are balanced, and 0 otherwise. You should only need to add 1 state to the Turing Machine below:









Lambda Calculus

7. Given T, F and if as defined in class:
T ≡ λ xy . x)
F ≡ λ xy . y)
if ≡ λ pca . pca
a. Define a Lambda Calculus term that behaves like not. For example, if (not T) M N should reduce to N and if (not F) M N should reduce to M.





b. Show that your definition of not works by showing the steps to reduce if (not T) M N to normal form (where M and N represent and Lambda Calculus term in normal form).


       if (not T) M N





















Models of Computation

Phine Knight suggests the modelling computation using a Phine Machine consisting of a list of numbered instructions (the Instructions), a pointer to the current instructions (the InstructionPointer), and a store that associates name and values (the Store).

A program executes by executing the instruction numbered by the InstructionPointer, and then increasing the InstructionPointer by 1. This continues unless the Instruction is HALT.

Instructions can do one of four things:

We can describe the state of a Phine Machine by listing the Instructions, InstructionPointer and Store. For example, here is a Phine Machine with four instructions and an empty store:
   ( <1: X := 0
      2: X := X + 1
      3: Y := X - 1
      4: HALT >,
     1,                     
     { } )
The model will start by executing instruction number 1, which puts <X, 0> in the store and advances the Instruction Pointer to 2. After this, the state is:
   ( <1: X := 0
      2: X := X + 1
      3: Y := X - 1
      4: HALT >,
     2,                     
     { <X, 0> } )
After the next two steps, the state will be:
   ( <1: X := 0
      2: X := X + 1
      3: Y := X - 1 
      4: HALT>,
     4,                     
     { <X, 1>, <Y, 0> } )

8. Write a BNF grammar that could be used to describe the state of a Phine Machine. A good answer will describe the smaller language possible that includes strings for describing all possible Phine Machine states.






















9. Is this Phine Machine model of computation equivalent to a Turing Machine? Argue convincingly why it is or isn't.














10. Condy Shonal suggests adding one new instruction to Knight's model:

    Instruction ::= IF Name = 0 GOTO Number
If the value of Name in the Store is 0, then this instruction sets the InstructionPointer to Number. If it is not, then this instruction does nothing and advances the InstructionPointer by one.

For example,

    ( <1: IF A = 0 GOTO 5
       2: B := B + 1
       3: A := A - 1
       4: GOTO 1
       5: HALT>,
      1,
      { <A, n>, <B, m> } )
is a program that will halt with B having the value n + m.

Condy claims her new model of computation is as powerful as a Turing Machine, but Fine Knight does not believe her. Write an informal but convincing argument that this model is as powerful as a Turing Machine.




























Post Script

Your goal on this exam is to convince me you can think like a computer scientist. If you don't feel your answers to these questions will reflect fairly on your abilities, use this space to explain why. Otherwise, you don't need to write anything for this question.







































CS 655 University of Virginia
Department of Computer Science
CS 200: Computer Science
David Evans
evans@cs.virginia.edu
Using these Materials