减少Racket的功能？

Question

我在Racket中遇到了这个任务5天,有人知道我该如何处理它？

给定arity 2的功能和n元素列表,返回所有元素的字符串函数的评估,例如:

>(reduce + '(1 2 3 4 5 6 7 8 9 10))

55

> (reduce zip '((1 2 3) (4 5 6) (7 8 9)))
'((1 (4 7)) (2 (5 8)) (3 (6 9)))

Answer 1

干得好.

(define (reduce func list)
  (assert (not (null? list)))
  (if (null? (cdr list))
      (car list)
      (func (car list) (reduce func (cdr list)))))

测试:

> (reduce + '(1 2 3 4 5 6 7 8 9 10))
55
> (reduce zip '((1 2 3) (4 5 6) (7 8 9)))
((1 (4 7)) (2 (5 8)) (3 (6 9)))

为了完整性,zip(一个假定两个列表且列表长度相同的实现)是:

(define (zip l1 l2) (map list l1 l2))

Answer 2

你可以用以下方式表达它foldl:

(define (reduce f xs)
  (and (not (empty? xs)) (foldl f (first xs) (rest xs))))