Download Association-Key Car - The Structure and Interpretation of Computer Program - Exams and more Exams Computer Engineering and Programming in PDF only on Docsity!
CS 61A Midterm #2 - October 5, 1998
Your name
login: cs61a-
Discussion section number
TA's name
This exam is worth 20 points, or about 13% of your total course grade. The exam contains four substantive questions, plus the following:
Question 0 (1 point): Fill out this front page correctly and put your name and login correctly at the top of each of the following pages.
This booklet contains eight numbered pages including the cover page. Put all answers on these pages, please; don't hand in stray pieces of paper. This is an open book exam.
When writing procedures, don't put in error checks. Assume that you will be given arguments of the correct type.
Our expectation is that many of you will not complete one or two of these questions. If you find one question especially difficult, leave it for later; start with the ones you find easier.
READ AND SIGN THIS:
I certify that my answers to this exam are all my own work, and that I have not discussed the exam questions or answers with anyone prior to taking this exam.
If I am taking this exam early, I certify that I shall not discuss the exam questions or answers with anyone until after the scheduled exam time.
Question 1 (4 points):
What will Scheme print in response to the following expressions? If an expression produces an error message, you may just say error''; you don't have to provide the exact text of the message. If the value of an expression is a procedure, just sayprocedure''; you don't have to show the form in which Scheme prints procedures. Also, draw a box and pointer diagram of the value produced by each expression.
(append (list 'a 'b) '(c d))
(cons (list 'a 'b) (cons 'c 'd))
(list (list 'a 'b) (append '(c) '(d)))
(cdar '((1 2) (3 4)))
Your name login cs61a-
Question 2 (5 points):
An association list is a list of pairs, each of which associates a name (the key ) with a value. For example, the association list
((a. 3) (b. 7) (c. foo))
associates the key A with the value 3 , the key B with the value 7 , and the key C with the value FOO.
Here are a constructor and two selectors for an association abstract data type:
(define make-association cons) (define association-key car) (define association-value cdr)
Note that this is an ADT for an association, not for an association list, which is just a sequence of associations.
(a) Scheme provides the procedure assoc for looking up a key in an association list. It returns the entire association, not just the value. Here is an implementation of assoc:
(define (assoc key a-list) (cond ((null? a-list) #f) ((equal? key (caar a-list)) (car a-list)) (else (assoc key (cdr a-list)))))
(Both caar and car in the next-to-last line above are correct!)
Rewrite assoc to use the association ADT defined above. Don't make any unnecessary changes; your program should look much like the one above, but with some procedure calls renamed.
This question continues on the next page.
Question 2 continued
(b) We are given an association list in which the keys are names of musical groups and the values are sentences of names of the group members, like this:
((Beatles. (John Paul George Ringo)) ((Buffalo Springfield). (Steve Neil Ritchie Dewey Bruce)) (Who. (Pete Alec Roger Keith))
What follows is a procedure to make a ``backwards'' association list in which the keys are the musicians and the values are their groups, like this example:
((John. Beatles) (Paul. Beatles) (George. Beatles) (Ringo. Beatles) (Steve. (Buffalo Springfield)) (Neil. (Buffalo Springfield)) (Richie. (Buffalo Springfield)) (Dewey. (Buffalo Springfield)) (Bruce. (Buffalo Springfield)) (Pete. Who) (Alec. Who)
(sq-ft. 24)
If two numbers with different units are multiplied, make a new unit name by putting a hyphen between the two given unit names, like this:
(times (attach-tag 'ft 3) (attach-tag 'sec 7)) (ft-sec. 21)
Continued on next page.
Your name login cs61a-
Question 3 continued.
(b) Now extend your program to recognize known unit names for products of known units, using data-directed programming. These product units will be represented by table entries such as these:
(put 'volt 'amp 'watt) (put 'dyne 'cm 'erg)
If the two arguments to times are of the same type, the program should work as in part (a). If the two arguments are of different types, look them up with get. (Don't forget that the two arguments may not be in the same order as the types in the table entry. That is,
(times (attach-tag 'volt 2) (attach-tag 'amp 6)) (times (attach-tag 'amp 6) (attach-tag 'volt 2))
should both work with the volt-amp-watt table entry above.) If you find another unit, as in the watt example, use that unit for the result, so that 3 volts times 4 amps gives a result of 12 watts. If you find no table entry at all for the given units, create a new unit with the two given units separated by a hyphen, as in part (a).
Question 4 (5 points):
The following definition comes from page 116 of SICP :
(define (accumulate op initial sequence) (if (null? sequence) initial (op (car sequence) (accumulate op initial (cdr sequence)))))
In this problem you're going to extend the idea of accumulation from sequences to ``deep lists'': lists of lists of lists to arbitrary depth. Write deep-accumulate, a function of three arguments: a two-argument function, an initial value, and a list structure. It should work like this:
(deep-accumulate + 0 '(3 (4 (5) ((6) 7) 8) (9 10))) 52
Posted by HKN (Electrical Engineering and Computer Science Honor Society)
University of California at Berkeley
If you have any questions about these online exams
please contact examfile@hkn.eecs.berkeley.edu.
Posted by HKN (Electrical Engineering and Computer Science Honor Society) University of California at Berkeley If y 5
