How do I get an item from a list at a given index in racket language?

Question

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?

Solution 1

Example:

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

See:

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

Solution 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))))))

Comments

[Zelphir Kaltstahl]

@leo-the-manic: Easy: To understand how it works internally and to be able to use the concepts within in other contexts. You wouldn't do it in production setting, but for learning purposes it is justified.