Computer Science 61A - Spring 1996 - Harvey - Final Exam

Computer Science 61A - Spring 1996 - Harvey - Final Exam -...

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Computer Science 61A - Spring 1996 - Harvey - Final Exam Answers to CS61 A Final of May 23, 1997 We repeat the questions; some have typos corrected with respect to the actual exam handed out. Question 1 (5 points): Let us represent a set S of unique expressions, atoms or pairs, as a Lisp list of those expressions. As you know, H is a subset of S if each element e of H is also an element of S. The powerset of S is the set of all subsets of S. The powerset of S includes the empty set and S. Here is a recursive way of thinking about powersets as lists: First, the powerset of () is (()). That is, it has one element, namely (). Next, assume that S has at least one element e, and let K be the rest of S. Let P be the powerset of K. The powerset of S is the union of P and a set formed by sticking e in front of each element of P. Here are two examples: The powerset of (1 2 3) is (() (1) (2) (3) (1 2) (1 3) (2 3) (1 2 3)) The powerset of (1 (2 3)) is (() (1) ((2 3)) (1 (2 3))) What is the powerset of '(1 (2 3) 4)? Answer: 1 point. no partial credit ( () (1) ((2 3)) (1 (2 3)) (4) (1 4) ((2 3) 4) (1 (2 3) 4)) all 8 elements. What is the length of the list representing the powerset of '(1 2 3 4 5)? Answer: 1 point. no partial credit. 32. (2^5 = 32) Write a procedure powerset which takes as its only argument a set S and returns the powerset of S. Answer: 3 points total (define (powerset set) (if (null? set) (list '()) ; 1 point for the base case (append (powerset (cdr set)) ; 1 point for the append (map (lambda (s) (cons (car set) s)) ;1 point for the map/lambda (powerset (cdr set)))))) Some people copied the "subsets " program. That was ok except if you called it subsets instead of powerset. Question 2 (7 points): Draw the environment diagram resulting from the following inputs to the Scheme system. What is the value of the global variable x at the end? (define x 10) (define (foo x) (define (bar y) (set! x y) x) bar) ((foo 50) 60) Answer: global env has x:10 , foo pointing to function def. 1pt Page 1
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Computer Science 61A - Spring 1996 - Harvey - Final Exam sub environment has x:50, then 60 and bar in it. 1 pt sub sub environment has y:50 in it 1 pt correct arrows 2 pt x is 10 at the end 2 pt Question 3 (5 points): Consider the following piece of code: (define foo (lambda (x) (- y x))) (define bar foo) (define y 7) (parallel-execute (lambda () (set! foo (let ((z y)) (lambda (x) (+ (bar x) z))))) (lambda () (set! y (foo 3))) ) What are the possible values of y? ANSWER: 4, 11 (2 pts.) 1 off if you had wrong answers too. The easy part is if branch 2 runs first. y is set to (- 7 3), or 4 Next foo is redefined as though we did (define foo (let ((z y)) (lambda (x) (+ (bar x) z)))) which is actually the same as (define (foo x)(+ (bar x) 4)) where we have just put the value of z, namely the current value of y, namely 4 in there. So foo is (define (foo x)(+ (- y x) 4)) (*) in which case (foo 3) will be (+ (- 4 3) 4) or 5 Another possibility is that branch 1 is executed first,
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Computer Science 61A - Spring 1996 - Harvey - Final Exam -...

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