在Racket中，如何使用struct而不是仅使用高阶函数或递归来查找列表的长度

Question

I'm pretty bad at understanding data structures in Racket right now so I'm currently working on a question where all I have to do is find the length of the list.

I understand there are many ways to find the length of a list using recursion and foldr and map but I'm trying to find the length of a list a specific way right now.

Here are the details of the question;

For this question, use the following data definition:

(define-struct ls (first rest))

;; a Ls is either

;; '(), or

;; (make-ls first rest) where first is an Int and rest is a Ls.

Keep in mind that since the structure is named ls, and its fields are named first and rest,you will access these fields using ls-first and ls-rest.

Here is the question

Length. Write a function (ls-length L) that consumes a Ls and returns the number of values in it.

For example, (check-expect (ls-length (make-ls 5 (make-ls 7 (make-ls 11 '())))) 3)

Here's a common way to find the length

(define (length lst)
  (cond
    [(empty? lst)  0]
    [(cons? lst)   (+ 1 (length (rest lst)))]))

I however want the question solved using

(define-struct ls (first rest))

This is what I have so far but I know that this code seems very wrong, I feel my base case should be correct though.

(define (ls-length L)
  (cond
    [(empty? L) 0]
    [(cons? (ls-first (first L))) (+ 1 (length (rest (ls-rest))))]))

Answer 1

You have to remember that you can't use cons? , rest and first on the new data structure, only empty? , ls-first and ls-rest are allowed. The conditions are quite simple: the list is either empty or not empty, we don't really need cons? . Also, be careful when calling the recursion, the procedure is called ls-length , not length :

(define (ls-length lst)
  (cond
    [(empty? lst) 0]
    [else (+ 1 (ls-length (ls-rest lst)))]))

Compare the above with the well-known length implementation:

(define (length lst)
  (cond
    [(empty? lst) 0]
    [else (+ 1 (length (rest lst)))]))

You see what happened? we only had to replace length with ls-length , and rest with ls-rest . So simple!