方案:我不知道实现给定的功能 [英] Scheme : I have no idea to implement given function

问题描述

这是编程语言语用学，迈克尔·斯科特"的练习.

It's the exercise of "Programming language pragmatics, Michael Scott" .

问.返回包含满足给定谓词的给定列表的所有元素的列表.例如， (filter (lambda (x) (< x 5) '(3 9 5 8 2 4 7)) 应该返回 (3 2 4).

Q. return a list containing all elements of a given list that satisfy a given predicate. For example, (filter (lambda (x) (< x 5) '(3 9 5 8 2 4 7)) should return (3 2 4).

我认为这个问题不仅需要满足上述所有谓词的函数.但我没有任何想法来实现这样的功能.请帮忙.

I think that question require function which satisfy every predicate not only above. But I don't have any idea to implement such function. Please help.

推荐答案

filter 过程已经存在于大多数 Scheme 实现中，它的行为符合预期:

(filter (lambda (x) (< x 5)) '(3 9 5 8 2 4 7))
=> '(3 2 4)

现在，如果问题是如何实现它，那就很简单了——我会给你一些提示，这样你就可以自己编写了.这里的技巧是注意到该过程正在接收另一个过程作为参数，一个谓词，它会返回 #t 或 #f> 当应用于输入列表中的每个元素时.那些评估为 #t 的被保留在输出列表中，那些评估为 #f 的被丢弃.这是解决方案的框架，请填空:

Now, if the question is how to implement it, it's rather simple - I'll give you some hints so you can write it by yourself. The trick here is noticing that the procedure is receiving another procedure as parameter, a predicate in the sense that it'll return #t or #f when applied to each of the elements in the input list. Those that evaluate to #t are kept in the output list, those that evaluate to #f are discarded. Here's the skeleton of the solution, fill-in the blanks:

(define (filter pred? lst)
  (cond (<???>       ; if the list is empty
         <???>)      ; return the empty list
        (<???>       ; apply pred? on the first element, if it's #t
         (cons <???> ; then cons the first element
               (filter pred? <???>))) ; and advance recursion
        (else (filter pred? <???>)))) ; else just advance recursion

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