`list-ref`

Category: List procedures

Documentation

;;; (list-ref lst pos) -> any?
;;;   lst : list?
;;;   pos : non-negative-integer? (less than (length lst))
;;; Grabs the element at index `pos` from `lst`. Elements in lists are 
;;; indexed starting at 0, so `(list-ref lst 0)` grabs the first element 
;;; of the list.

Examples

> (list-ref (list "a" "b" "c") 0)
"a"

Sample implementation

(define list-ref
  (lambda (lst pos)
    (cond
      [(null? lst)
       (error "list-ref: Invalid position")]
      [(zero? pos)
       (car lst)]
      [else
       (list-ref (cdr lst) (- pos 1))])))