; Association lists
; built-in data structure which consists of a list of pairs
; the idea is that the first item in each pair is "associated" with the second
; for example:

(define science-professors-directory
  (list (cons "Bruce Voyles" "3038")
        (cons "Diane Robertson" "3039")
        (cons "Martin Minelli" "3007")
        (cons "Arnold Adelberg" "4201")
        (cons "Mark Schneider" "3018")
        (cons "Janet Gibson" "3168")))

; you can call assoc in the following way:
; (assoc KEY ASSOCIATION-LIST)
; and then Scheme will return the pair in ASSOCIATION-LIST whose first
; element is KEY, and returns #f if none exists

(define my-assoc
  (lambda (key alist)
    (if (null? alist)
        #f
        ; note that (caar alist) is short for (car (car alist)) 
        (if (equal? (caar alist) key)
            (car alist)
            (my-assoc key (cdr alist))))))