list - 如何从 Racket 语言中给定索引处的列表中获取项目？

Question

我正在尝试从循环语句的给定索引处的列表中获取项目。

(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
                      )))))])))

在这种情况下， best是列表，我需要在 countdown 处获取它的值指数。你能帮我吗？

Answer 1

例子:

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

看:

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