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Q: What's the result of filtering an empty list?
A: The empty list.
Q: If the head of a nonempty list satisfies the predicate, what should
filter return? A: The result of consing the head onto the result of filtering the tail. Q: If the head of a nonempty list does not satisfy the predicate, what should filter return? A: The result of filtering the tail. |
(define filter
(lambda (ok? lst)
(cond ((null? lst) '())
((ok? (car lst))
(cons (car lst)
(filter ok? (cdr lst))))
(else (filter ok? (cdr lst))))))
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