Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Kaz Kylheku <643-408-1753@kylheku.com>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Another code review perhaps?
Date: Thu, 12 Sep 2024 12:22:24 -0000 (UTC)
Organization: A noiseless patient Spider
Lines: 64
Message-ID: <20240912051355.168@kylheku.com>
References: <vbuba3$68dk$1@dont-email.me>
Injection-Date: Thu, 12 Sep 2024 14:22:24 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="fc240c90c706e031471363b92b2225a2";
	logging-data="270113"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/A+ZGG7qbH18lEm7w3yeNr/hXvn9BRdEU="
User-Agent: slrn/pre1.0.4-9 (Linux)
Cancel-Lock: sha1:hfbmZj0Yy+LRXc9myX03csW3d/E=
Bytes: 2279

On 2024-09-12, B. Pym <Nobody447095@here-nor-there.org> wrote:
> Arthur Lemmens wrote:
>
>> > Define iterative and recursive versions of a function that takes an
>> > object x and a vector v, and returns a list of all the objects that
>> > immediately precede x in v.
>> 
>> Graham doesn't like LOOP, but I do. So here's a LOOP-version:
>> 
>> (defun precedes (object vector)
>>   (loop for x across vector
>>         and i from 0
>>         when (and (equal x object) (> i 0))
>>         collect (elt vector (1-i))))
>
>
> Look at that:
>
> (1-i)
>
> Don't you think that that should be:
>
> (1- i)
>
> or
>
> (- i 1)
>
> ?
>
> It's shorter when you use a Lispy language instead of CL.
>
> Gauche Scheme
>
> (use srfi-42) ; list-ec
>
> (define (precedes obj vec)
>   (list-ec (: x (index i) vec)
>     (if (and (> i 0) (equal? x obj)))
>     (ref vec (- i 1))))
>
> (precedes 5 #(5 0 4 5 8 9 5))
>  ===>
> (4 9)
>
> Another way:
>
> (define (precedes o v)
>   (let ((l (vector->list vec)))
>     (filter-map
>       (^(a b) (and (equal? o a) b))
>       (cdr l)
>       l)))

1> (match @(scan-all (@x 5 . @nil)) '(5 0 4 5 8 9 5) x)
(4 9)
2> (window-mappend 1 nil (do if (and @1 (= @2 5)) (list @1)) #(5 0 4 5 8 9 5))
#(4 9)


-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca