Pascal Costanza wrote:

There doesn't seem to be a way to return a list of duplicates of a
sequence in ANSI CL -- though there is a remove-duplicates. Is there a reason for this? It would be handy if you could tell remove-duplicates
not to include any duplicated elements so you could do a set-
difference at the end to get a list of duplicates. Feel free to post
code to prove me wrong. Thanks!

(loop
with counts
for element in list
do (incf (getf counts element 0))
finally (return
(loop for (element count) on counts by #'cddr
if (> count 1)
collect element into duplicates
else collect element into uniques
finally (return (values uniques duplicates)))))

Shorter

;; For Gauche Scheme.
(use srfi-1) ;; alist-cons

or

;; For Racket.
(require srfi/1) ;; alist-cons alist-delete
(require srfi/8) ;; receive
(define assoc-ref dict-ref)


(define (alist-inc k a)
(alist-cons k (+ 1 (assoc-ref a k 0)) (alist-delete k a)))

(receive (uniq dup)
(partition
(& = 1 (cdr u))
(fold alist-inc '() '(c xo c f y f g g h z h h)))
(values (map car uniq) (map car dup)))

===>
(z y xo)
(h g f c)


Given:

(define-syntax &-aux
(syntax-rules (u v w & lambda quote)
[(_ () shadow (param ...) original)
(lambda (param ...) original)]
[(_ (u more ...) (x y ...) () original)
(&-aux original original (x) original)]
[(_ (v more ...) (x y ...) (a) original)
(&-aux original original (a x) original)]
[(_ (w more ...) (x y ...) (a b) original)
(&-aux () () (a b x) original)]
[(_ ((lambda x ...) more ...) (y z ...) params original)
(&-aux (more ...) (z ...) params original)]
[(_ ((& x ...) more ...) (y z ...) params original)
(&-aux (more ...) (z ...) params original)]
[(_ ((quote x ...) more ...) (y z ...) params original)
(&-aux (more ...) (z ...) params original)]
[(_ ('x more ...) (y z ...) params original)
(&-aux (more ...) (z ...) params original)]
[(_ ((s ...) more ...) (y z ...) params original)
(&-aux (s ... more ...) (s ... z ...) params original)]
[(_ (x more ...) (y z ...) params original)
(&-aux (more ...) (z ...) params original)]))
;; Lambda with anaphoric parameters u, v, and w.
(define-syntax &
(syntax-rules ()
[(& x ...)
(&-aux (x ...) (x ...) () (x ...))]))

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