Re: Negative nth index

[Thierry Volpiatto <thievol@posteo.net>]

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