How do I get an item from a list at a given index

问题:

I'm trying to get an item from a list at a given index for a loop statement.

(define decision-tree-learning
  (lambda (examples attribs default)
    (cond
      [(empty? examples) default]
      [(same-classification? examples) (caar examples)] ; returns the classification
      [else (lambda () 
              (let ((best (choose-attribute attributes examples))
                    (tree (make-tree best))
                    (m (majority-value examples))
                    (i 0)
                    (countdown (length best)) ; starts at lengths and will decrease by 1
                  (let loop()
                    (let example-sub ; here, totally stuck now
                      ; more stuff
                      (set! countdown (- countdown 1))
                      ; more stuff
                      )))))])))

In this case, best is the list and I need to get its value at the countdown index. Could you help me on that?

回答1:

Example:

> (list-ref '(a b c d e f) 2)
'c

See:

http://docs.racket-lang.org/reference/pairs.html

回答2:

Or build this yourself:

(define my-list-ref
    (lambda (lst place)
      (if (= place 0)
          (car lst)
          (my-list-ref (cdr lst) (- place 1)))))

but if you want to check if the list is done and don't worry by error yo can do this as well:

(define my-list-ref
    (lambda (lst place)
      (if (null? lst)
          '()
          (if (= place 0)
          (car lst)
          (my-list-ref (cdr lst) (- place 1))))))