Computer Science 61A Study Group
Student Learning Center
Leader: Joo-Rak Son
Week 15, December 6 th and 8 th
1. Object-Oriented Programming
(define (make-echo saved)
(let ((count 0))
(lambda (message)
(cond ((eq? message ‘saved) saved)
((eq? message ‘count) count)
(else (let ((result saved))
(set! saved message)
(set! count (+ count 1))
result))))))
2. Data Abstraction and Higher-Order Functions
(a)
(define (prefix-to-infix exp)
(if (pair? exp)
(list (prefix-to-infix (cadr exp))
(prefix-to-infix (car exp))
(prefix-to-infix (caddr exp)))
(list (cadr exp) (car exp) (caddr exp))))
(b)
(define (tree-reorder ordering tree)
(if (atom? tree)
tree
(map (lambda (index)
(tree-reorder ordering (list-ref tree index)))
ordering)))
3. Logic Programming
(a)
! (assq ?key ((?key . ?value) . ?tail) (?key . ?value)) . [I]
! (assq ?key (?irrelevant . ?tail) ?result) :- [II]
(assq ?key ?tail ?result) .
(b)
___ (assq bbb ((aaa . 5) (bbb . 6) (ccc . 7)) ?what)
___ (assq ?who ((aaa . 5) (bbb . 6) (ccc . 7)) ?what)
* __ (assq bbb ?which (bbb . 6))
___ (assq ?who ((aaa . 5) (bbb . 6) (ccc . 7)) (bbb . 6))
If you had the rule [I] as the first rule, then the query result would be:
(assq bbb ((bbb . 6) . ?var) (bbb . 6)) displaying the undesired variable. If you had the rule [II] as the first thing, the query result would be:
(assq bbb (?irrelevant1 (bbb . 6)) (bbb . 6))
(assq bbb (?irrelevant2 ?irrelevant1 (bbb . 6)) (bbb . 6))
(assq bbb (?irrelevant3 ?irrelevant2 ?irrelevant1 (bbb . 6)) (bbb . 6))
Not knowing when to stop.
All the other queries are valid.
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Computer Science 61A Study Group
Student Learning Center
Leader: Joo-Rak Son
Week 15, December 6 th and 8 th
4. Memoization
(a)
(define (cc amount kinds-of-coins)
(let ((table (make-table amount kinds-of-coins #f)))
(define (helper amount kinds-of-coins)
(cond ((= amount 0) 1)
((or (< amount 0) (= kinds-of-coins 0)) 0)
(else (let ((lookup (lookup-table table amount kinds-of-coins)))
(if lookup lookup
(let ((result
(helper (+ (cc (- amount (first-denomination
kinds-of-coins))
kinds-of-coins)
(cc amount (- kinds-of-coins 1)) ))))
(insert-table! table amount kinds-of-coins result)
result)))))))
(helper amount kinds-of-coins)))
(b)
(c)
Any tree-recursive function that has to look at every node of the tree, even if two equal nodes appear within it: countatoms, equal?, deep-reverse, and subsets are examples.
5. Where would we implement square-bracket?
Something else. You need to be able to parse first and then evaluate.
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