ANSI Common Lisp (exercises)

gistfile1.lisp

;; Exercise 3.2, p. 56
(defun union-in-order (&rest lists)
  "Returns a union of lsts, preserving the order"
  (remove-duplicates (reduce #'append lists)
                     :from-end t
                     :test #'equal))

;; Exercise 3.3, p. 56
(defun sort-by-frequency (list)
  "Returns sorted by frequency alist of occurrences,
e.g. '(a a b a c c) -> '((A .3) (C . 2) (B . 1))"
  (sort (mapcar #'(lambda (x) `(,x . ,(count x list :test #'equal)))
		(remove-duplicates list :test #'equal))
	#'>
	:key #'cdr))