;;; ********************************************************************
;;; util.ss
;;; Susan Fox
;;; December 27, 2000

;;; Contains basic utility operations


(define UTIL.SSLOADED #t)

;;; *********************************************************************

;;; warn takes a string and prints a warning to the screen
;;;
(define warn
  (lambda (str)
    (printf "Warning: ~s~n" str)))


;;; outliers takes two lists, and returns a list containing all
;;; those elements from the first list that are not in the second
;;; list
;;;
(define outliers
  (lambda (lst1 lst2)
    (cond
      [(null? lst1) '()]
      [(member (car lst1) lst2)
       (outliers (cdr lst1) lst2)]
      [else (cons (car lst1)
	      (outliers (cdr lst1) lst2))])))



;;; list-at-i takes a position, i and a list and returns the
;;; list starting at that position.  position zero returns the
;;; whole list, position 1 returns the cdr of the list, etc.
;;;
(define list-at-i
  (lambda (i lst)
    (if (null? lst)
	(error 'list-at-i "Index value out of range")
	(if (zero? i)
	    lst
	    (list-at-i (sub1 i) (cdr lst))))))



;;; int-average takes some number of numbers and computes the
;;; average of them, returning it as an integer value by
;;; using inexact->exact and truncate
;;;
(define int-average
  (lambda nums
    (if (and (= (length nums) 1)
	     (list? (car nums)))
	(apply int-average (car nums))
	(if (null? nums)
	    (error 'int-average "Must pass arguments to int-average")
	    (round (inexact->exact (/ (apply + nums) (length nums))))))))


;;; average returns a real or rational average of its arguments
;;;
(define average
  (lambda nums
    (if (and (= (length nums) 1)
	     (list? (car nums)))
	(apply average (car nums))
	(if (null? nums)
	    (error 'average "Must pass arguments to int-average")
	    (/ (apply + nums) (length nums))))))