Question

I want to access a n-dimensional vector but somehow (empty? ‘()) keeps returning false.

;; access n dimensional vectors
;; (access-nd vector a-list-of-numbers) -> element
;; (access-nd (vector (vector ‘x ‘y) ‘a ‘b)) 0 1 ) -> x

(define (access-nd avector . alist)
  (cond
    ((and (not(empty? alist)) (vector? avector))
     (vector-ref (access-nd avector (rest alist)) (first alist)))
    (else avector)))

Please Help.

Edit: CORRECTED CODE

(define (access-nd avector . alist)
  (cond
    ((and (not(empty? alist)) (vector? avector))
     (apply access-nd (vector-ref avector  (first alist)) (rest alist)))
    (else avector)))

Answer 1

Most likely, that one line should read:

     (vector-ref (apply access-nd avector (rest alist)) (first alist)))

Without the “apply”, alist will never be empty. Here’s why:

In the definition of access-nd the alist parameter is an optional parameters list; it’s separated with a dot from normal positional parameters. This means access-nd can be called with 1-n parameters. Any parameters after the first one are collected to a list and bound to alist. For example, a call like

(access-nd v 1 2 3) 

will cause alist to be bound to the list (1 2 3). Similarly, this call in your original code:

(access-nd avector (rest alist))

will cause alist to be bound to a list with one element. That’s why alist will never be empty.

Scheme’s apply, on the other hand, takes a list of arguments as the last parameter and calls the function as if they were passed to it in the normal way.