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Key to CSE341/413 Final, Spring 2026 handout #13
1. Scheme expressions.
Part A:
(define a 2)
(define b 3)
(define c 5)
(let ((b (* a c))
(a (* b c)))
(list a b c))
'(15 10 5)
Part B:
(define a 2)
(define b 3)
(define c 5)
(let* ((b (* a b))
(a (* a b)))
(list a b c))
'(12 6 5)
Part C:
(define x 2)
(define (f n) (* x y n))
(define y 3)
(define a (f 7))
(set! x 5)
(define b (f 2))
(define c a)
(set! a 13)
(list a b c)
'(13 30 42)
Part D:
(define a '(1 2 3))
(define b (cons 1 (cdr a)))
(define c (append '(0) b))
(list c (eq? a b) (eq? (cdr a) (cdr b)) (eq? (cdr c) b))
'((0 1 2 3) #f #t #t)
2. Two possible solutions appear below.
(define (sum-positives lst)
(cond [(null? lst) 0]
[(> (car lst) 0) (+ (car lst) (sum-positives (cdr lst)))]
[else (sum-positives (cdr lst))]))
(define (sum-positives lst)
(foldl + 0 (filter (lambda (n) (> n 0)) lst)))
3. One possible solution appears below.
(define (prefix-apply f pre lst)
(if (null? lst)
'()
(cons pre (cons (f (car lst)) (prefix-apply f pre (cdr lst))))))
4. One possible solution appears below.
(define (average lst)
(define (helper sum rest)
(if (null? rest)
(/ sum (length lst))
(helper (+ sum (car rest)) (cdr rest))))
(if (null? lst)
(error "empty list")
(helper 0 lst)))
5. Two possible solutions appear below.
(define (join lst separator)
(define (helper result rest)
(if (null? rest)
result
(helper (string-append result separator
(number->string (car rest))) (cdr rest))))
(if (pair? lst)
(helper (number->string (car lst)) (cdr lst))
""))
(define (join lst separator)
(if (pair? lst)
(apply string-append (cdr (prefix-apply number->string separator lst)))
""))
6. One possible solution appears below.
(define (sublist lst start stop)
(define (helper index current)
(cond [(= index stop) '()]
[(null? current) (error "illegal sublist")]
[(>= index start) (cons (car current) (
helper (+ index 1) (cdr current)))]
[else (helper (+ index 1) (cdr current))]))
(if (or (> start stop) (< start 0) (< stop 0))
(error "illegal sublist")
(helper 0 lst)))
7. One possible solution appears below.
(define (convert data)
(cond [(not (list? data)) data]
[(< (length data) 2) (error "illegal expression")]
[else (let ([operator (car data)]
[first (convert2 (cadr data))])
(cons first (prefix-apply convert2 operator
(cddr data))))]))
8. Ruby expressions.
Part A:
y.each {|s| puts s + " " + s}
foo foo
bar bar
baz baz
mumble mumble
Part B:
y.find_all {|s| s.length == 3}
["foo", "bar", "baz"]
Part C:
z = []
for n in x do
z.push -n
end
[2, 1, 0, -1, -2]
91. Two possible solutions appear below.
class Integer
def first
return self.abs.to_s[0].to_i
end
end
class Integer
def first
result = self.abs
result /= 10 while result >= 10
return result
end
end
10. One possible solution appears below.
class Array
def partition
part1 = []
part2 = []
for n in self do
if yield(n) then
part1 << n
else
part2 << n
end
end
return [part1, part2]
end
end
11. One possible solution appears below.
def anagrams(word, dictionary)
result = []
for other in dictionary do
copy1 = word.downcase.split("").sort
copy2 = other.downcase.split("").sort
result << other if (copy1 == copy2)
end
return result
end
12. One possible solution appears below.
class Array
def analyze
count = Hash.new(0)
self.each {|n| count[n.first] += 1}
for n in 1..9 do
pct = 100.0 * count[n] / self.length
print n, " = ", count[n], " (", pct.round, "%)\n"
end
end
end
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