Negative nth index

Negative nth index

[Anand Tamariya]

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Should negative index for nth be valid? e.g.
(nth -1 '(1 2)) returns 1

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Re: Negative nth index

[Eli Zaretskii]

> From: Anand Tamariya <atamariya@gmail.com>
> Date: Sun, 22 Dec 2024 11:13:18 +0530
> 
> Should negative index for nth be valid? e.g. 
> (nth -1 '(1 2)) returns 1

What does this do in other Lisps?

Re: Negative nth index

[Tassilo Horn]

Eli Zaretskii <eliz@gnu.org> writes:

>> Should negative index for nth be valid? e.g. 
>> (nth -1 '(1 2)) returns 1
>
> 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.

Bye,
Tassilo

Re: Negative nth index

[Andreas Schwab]

On Dez 22 2024, Anand Tamariya wrote:

> (nth -1 '(1 2)) returns 1

What would (nth -1 '#1=(1 2 . #1#)) return?

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

Re: Negative nth index

[Stefan Kangas]

Tassilo Horn <tsdh@gnu.org> writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> Should negative index for nth be valid? e.g.
>>> (nth -1 '(1 2)) returns 1
>>
>> 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?

Re: Negative nth index

[Stefan Monnier]

>>>> 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))


- Stefan

Re: Negative nth index

[Teemu Likonen]

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* 2024-12-24 00:11:26-0500, Stefan Monnier wrote:

> 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))

First test if index (i) is negative and only then run the needed
"length" etc.

-- 
/// Teemu Likonen - .-.. https://www.iki.fi/tlikonen/
// OpenPGP: 6965F03973F0D4CA22B9410F0F2CAE0E07608462

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Re: Negative nth index

[Thierry Volpiatto]

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?

>
> - Stefan
>
>
>

-- 
Thierry

Re: Negative nth index

[Philip Kaludercic]

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
>>
>>
>>

Re: Negative nth index

[Stefan Monnier]

> 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))

AFAICT so far people were only able to fix my code but not answer my
question, which suggests the answer is no, which would be a good
argument in favor of staying with the current behavior: the change
wouldn't save us from going down the list twice.


        Stefan

Re: Negative nth index

[Sebastián Monía]

Hello,

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.

And for this reason I _would not_ expect it to work with negative
numbers in Elisp.

> 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?

This is "modern"? behaviour that I would expect from seq-elt, because it
is the newest, bell and whistles library.

I have a feeling that keeping `nth' more limited for reasons of
tradition is silly, but...it feels right.

Regards,
Seb

-- 
Sebastián Monía
https://site.sebasmonia.com/

Re: Negative nth index

[Pip Cet via Emacs development discussions.]

"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.  ]

I think the OP meant that (nth -1 '(1 2)) is 1 on current Emacs, which
is definitely unexpected.

> 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?

I agree, but the current implementation does not signal an error.  It
should, and I think that's what the suggestion was.

To me, (nth -1 list) means "I set list to be the cdr of x, now tell me
what x was", so the right answer is "I can't do that, so here's an error
and go fix your code".  It's perfectly analogous to (/ 0 0), which means
"I multiplied a number x by 0 and got 0.  Tell me what x was."

For circular lists, we could give a possible answer for x (just as we
could give a possible answer for x in the 0/0 example), by moving back
in the cycle, but it's not the only possible answer, so we shouldn't.

(For lists both proper and circular, we could also create a new cons
cell and return it, I guess).

Indexing from the end is even worse than defining 0/0 to be 17.

Pip