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From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.lang.lisp
Subject: Re: Exercises please
Date: Sat, 21 Jun 2025 21:12:02 -0000 (UTC)
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On 2025-06-21, B. Pym <Nobody447095@here-nor-there.org> wrote:
> Pascal J. Bourguignon wrote:
>
>> and my solutions (still incomplete): 
>> http://www.informatimago.com/develop/lisp/l99/index.html
>
> Where we find:
>
> (---------------------------------------------------------------
> P13 (**) Run-length encoding of a list (direct solution).
>
>    Example:
>     * (encode-direct '(a a a a b c c a a d e e e e))
>     ((4 A) B (2 C) (2 A) D (4 E))
> "

1> (flow '(a a a a b c c a a d e e e e)
     (partition-by identity)
     (map [juxt car len]))
((a 4) (b 1) (c 2) (a 2) (d 1) (e 4))

Oops.

2> (flow '(a a a a b c c a a d e e e e)
     (partition-by identity)
     (map [juxt len car]))
((4 a) (1 b) (2 c) (2 a) (1 d) (4 e))

Oops.

3> (flow '(a a a a b c c a a d e e e e)
     (partition-by identity)
     (mapcar [iff cdr [juxt len car] car]))
((4 a) b (2 c) (2 a) d (4 e))

I get working code before I'm done properly scanning the requirements and can
fix it in a few keystrokes.

Shorter:


> ;; Iterative solution, uses only O(r) space:

TL; DR.