Stop show with sound  Next slide in show -- Keyboard shortcut: 'n'  1 minute, 39 secondsLecture 4 - slide 5 : 34
Program 1
;; Returns a function, which selects element number n in a list.
;; The second parameter, which is optional, is used for error message purposes.
;; In general, this parameter should be a string corresponding to the name of the selector function.
;; If the second parameter is given, we check whether the list is long enough for selection.
;; If not, we give a decent error message. We recommend use of the second parameter in order to
;; avoid meaningless error messages.
;; The first element is number 1.
;; (make-selector-function 1) corresponds to car, (make-selector-function 2) corresponds to cadr, etc.
;; .form (make-selector-function n [selector-name])
(define (make-selector-function n . optional-parameter-list)
 (let ((selector-name (optional-parameter 1 optional-parameter-list #f)))
   (if selector-name
       (lambda (lst)   
         (let ((lgt (length lst)))
            (if (> n lgt)
                (display-error (string-append "The selector function " (as-string selector-name) ": " 
                               "The list "  (as-string lst) " is is too short for selection. "
                               "It must have at least " (as-string n) " elements."
                (list-ref lst (- n 1)))))
       (lambda (lst) (list-ref lst (- n 1))))))