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From: Joerg Mertens <joerg-mertens@t-online.de>
Newsgroups: comp.lang.lisp
Subject: Re: Faster remove-duplicates with sorted list.
Date: Fri, 21 Jun 2024 12:22:31 +0200
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B. Pym <No_spamming@nowhere_7073.org> wrote:
> "Pierre R. Mai" wrote:
> 
>> (defun uniquify-sorted-list (list &key (key #'identity) (test #'eql))
>>   (loop for element in list
>>         for element-key = (funcall key element)
>>         for last-element-key = (load-time-value (gensym))
>>         then element-key
>>         unless (funcall test element-key last-element-key)
>>         collect element))
> 
> Testing under SBCL:
> 
> (defun uniquify-sorted-list (list &key (key #'identity) (test #'eql))
>   (loop for element in list
>         for element-key = (funcall key element)
>         for last-element-key = (load-time-value (gensym))
>         then element-key
>         unless (funcall test element-key last-element-key)
>         collect element))
> 
> (uniquify-sorted-list '(a b b c d d e f f f g))
>   ===>
> (A)
> 
> (uniquify-sorted-list '(2 2 3 4 4 4 5 5 6 7))
>   ===>
> (2)
> 
> (uniquify-sorted-list '(2 2 3 4 4 4 5 5 6 7) :test #'=)
>   ===>
> debugger invoked on a SIMPLE-TYPE-ERROR in thread
> #<THREAD "main thread" RUNNING {23EAC0D1}>:
>   Argument Y is not a NUMBER: #:G5

(defun uniquify-sorted-list (list &key (key #'identity) (test #'eql))
  (loop for element in list
        for last-element-key = nil then element-key
        for element-key = (funcall key element)
        for result = nil then (funcall test element-key last-element-key)
        unless result collect element))

Tested with GNU Clisp:

[3]> (uniquify-sorted-list '(a b b c d d e f f f g))
(A B C D E F G)
[4]> (uniquify-sorted-list '(2 2 3 4 4 4 5 5 6 7))
(2 3 4 5 6 7)
[5]> (uniquify-sorted-list '(2 2 3 4 4 4 5 5 6 7) :test #'=)
(2 3 4 5 6 7)