Duplicates in list...

[cormullion]

How do you get all the duplicates in a list? Ie the opposite of unique...

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(set 'l '(5 7 1 3 5 2 9 12 6 4 8 5 10 5 5))
(???  l)
;-> (5 5 5 5 5)

I though I could persuade difference/intersect to do it, but no luck yet..

[itistoday]

Finding duplicates requires keeping a memory of what you've seen, while making a note as to whether you've already seen it before.

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(new Tree 'dups.mem)
(define (dups l (convert int))
	(delete 'dups.mem nil)
	(new Tree 'dups.mem)
	(dolist (el l)
		(dups.mem (string el) (if $it 2 1))
	)
	(intersect l (find-all '(? 2) (dups.mem) (convert (first $it))) true)
)
(set 'l '(5 7 1 3 5 2 9 12 6 4 8 5 10 5 5))
(println (dups l))
;=> (5 5 5 5 5)

[cormullion]

Cool - thanks! I'll use that as a working solution for now. However, I have the vague feeling that there should be a cleaner (or shorter) solution than this... ! :)

[itistoday]

Cool - thanks! I'll use that as a working solution for now. However, I have the vague feeling that there should be a cleaner (or shorter) solution than this... ! :)

If you find out let me know. ;-p

BTW, I updated the code, I made a small mistake. I changed this:

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(delete 'dups.mem true)

To this:

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(delete 'dups.mem nil) ; fast delete on 10.1.9 and later

[Kazimir Majorinc]

I think count has exactly the information one needs, just in different form.

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> (set 'l '(5 7 1 3 5 2 9 12 6 4 8 5 10 5 5 6 6)) ; I appended few sixes
(5 7 1 3 5 2 9 12 6 4 8 5 10 5 5 6 6)
> (flat (map dup l (replace 1 (count l l) 0)))
(5 5 5 5 5 6 6 6)
> (flat (map (lambda(x y)(if (!= y 1)(list x)(list))) l (count l l)))
(5 5 6 5 5 5 6 6)

[itistoday]

Nicely done Kazimir! I'm not very familiar with the 'count' function, but it looks like I should be!

That looks like the solution cormullion is looking for, and it's faster than mine too! :-)

In my tests, this method is the fastest so far (and it's the shortest too):

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(flat (map dup l (replace 1 (count l l) 0)))

[cormullion]

Clever, Kazimir - I wouldn't have thought of using count on the same list, but it's the right solution. The second one keep the order of the original too.

[Kazimir Majorinc]

Perhaps it could be nice additional function, say redundant as a pair to unique.
Although I am not sure whether it would be better that

(set 'l '(5 7 1 3 5 2 9 12 6 4 8 5 10 5 5 6 6))
(redundant l) => (5 5 6 5 5 5 6 6)

or

(redundant l) => (5 5 5 5 6 6), without first 5 and first 6

(flat (map (lambda(x y)(if (zero? y) (list x) '())) l (count l l)))

[kks]

A "imperative" solution:

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(let (R '()  L l  E nil)
	(while L
		(setq E (pop L))
		(when (or (find E R) (find E L))
			(push E R -1) ))
	R )

or in one line ;-)

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(let (R '()  L l  E nil) (while L (setq E (pop L)) (when (or (find E R) (find E L)) (push E R -1))) R)

But I love Kazimir's solution. Thanks.

Happy New Year 2010 to all of you newLISPer!