在球拍中定义mylength函数

Question

I'm trying to figure out how to define a length function in racket. Here's my code:

(define mylength
  (lambda (lst)
    (cond
     ((null? lst) 0)
     (else (+ 1 (mylength (cdr lst))))
     )))

If I try (mylength '(1 2 3)) the result is 3. When I try (mylength '(1 (2 3) 4)) the result still is 3. But the result should be 4. How do I do that? in Racket

Answer 1

If you want to count nested elements separately, you need to recurse into (car lst) as well.

(define mylength
  (lambda (lst)
    (cond
     ((null? lst) 0)
     ((atom? lst) 1)
     (else (+ (mylength (car lst))
              (mylength (cdr lst))))
     )))

Note that this means that a nested empty list doesn't add to the length.

(mylength '(1 () 3))

will be 2 , not 3 .