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From: Philip Kaludercic <philipk@posteo.net>
To: Thierry Volpiatto <thievol@posteo.net>
Cc: Stefan Monnier <monnier@iro.umontreal.ca>,
	 Stefan Kangas <stefankangas@gmail.com>,
	 Tassilo Horn <tsdh@gnu.org>,  Eli Zaretskii <eliz@gnu.org>,
	 Anand Tamariya <atamariya@gmail.com>,
	emacs-devel@gnu.org
Subject: Re: Negative nth index
Date: Tue, 24 Dec 2024 13:09:22 +0000	[thread overview]
Message-ID: <877c7pb531.fsf@posteo.net> (raw)
In-Reply-To: <87v7v9d1ae.fsf@posteo.net> (Thierry Volpiatto's message of "Tue,  24 Dec 2024 06:48:25 +0000")

Thierry Volpiatto <thievol@posteo.net> writes:

> Stefan Monnier <monnier@iro.umontreal.ca> writes:
>
>>>>>> Should negative index for nth be valid? e.g.
>>>>>> (nth -1 '(1 2)) returns 1
>>
>> [ I'd make it return 2 otherwise you can't use it to return the
>>   last element.  ]
>>
>>>>> What does this do in other Lisps?
>>>> In CL, it errors (ditto for nthcdr with negative index).  Same for (nth
>>>> '(1 2 3) -1) in Clojure or (list-ref '(1 2 3) -1) in Scheme.
>>> Stefan Monnier, any comments here?
>>
>> I don't see a strong argument in favor of making it index from the end
>> rather than signal an error.  What would the implementation look like?
>> Would it be significantly more efficient than doing it "by hand" e.g.:
>>
>>     (let ((l (length X)))
>>       (nth (if (< i 0) (- l i) i) X))
>
> I guess you meant (+ l i) no?

And do we need a check to see if (< (- i) (length l)),  otherwise

  (nth -10 '(1 2 3))

could still return a non-nil value, while

  (nth 10 '(1 2 3))

just returns nil.

Using (mod i l) would also be too DWIM-y, right?

>>
>> - Stefan
>>
>>
>>



  reply	other threads:[~2024-12-24 13:09 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-12-22  5:43 Negative nth index Anand Tamariya
2024-12-22  7:39 ` Eli Zaretskii
2024-12-22  9:01   ` Tassilo Horn
2024-12-23  5:40     ` Stefan Kangas
2024-12-24  5:11       ` Stefan Monnier
2024-12-24  6:42         ` Teemu Likonen
2024-12-24  6:48         ` Thierry Volpiatto
2024-12-24 13:09           ` Philip Kaludercic [this message]
2024-12-24 15:18         ` Stefan Monnier
2024-12-24 15:24         ` Sebastián Monía
2024-12-24 21:33         ` Pip Cet via Emacs development discussions.
2024-12-25 10:30           ` Mattias Engdegård
2024-12-25 10:56             ` Pip Cet via Emacs development discussions.
2024-12-25 12:14               ` Mattias Engdegård
2024-12-25 13:04                 ` Pip Cet via Emacs development discussions.
2024-12-25 14:23                   ` Stefan Kangas
2024-12-25 14:18         ` Stefan Kangas
2024-12-22 10:16 ` Andreas Schwab

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