From: Jonas Pfenniger <zimbatm@...>
Date: 2008-01-01T22:59:54+09:00
Subject: Re: RCR allow indexing last n items

2008/1/1, David A. Black <dblack@rubypal.com>:
> Hi --
>
> On Tue, 1 Jan 2008, Michal Suchanek wrote:
>
> > On 31/12/2007, David A. Black <dblack@rubypal.com> wrote:
> >> Hi --
> >>
> >> On Mon, 31 Dec 2007, Michal Suchanek wrote:
> >>
> >>> On 31/12/2007, David A. Black <dblack@rubypal.com> wrote:
> >>>> I think it's a matter of defining what it means to start taking a
> >>>> substring at a non-existent point. As it now stands, you can never do
> >>>> that, whether the way you reach that non-existent point is with a
> >>>> positive or negative index:
> >>>>
> >>>>>> str = "abc"
> >>>> => "abc"
> >>>>>> str[4..10]
> >>>> => nil
> >>>>>> str[-4..-1]
> >>>> => nil
> >>>>
> >>>> Starting at a non-existent point is not the same as *stopping* at that
> >>>> point. In other words, this is meaningful:
> >>>>
> >>>>>> str[0..10]
> >>>> => "abc"
> >>>>
> >>>> because you're starting in the string, at a defined point, and going
> >>>> as far as you can. The problem with str[-4..-1] is not how far it
> >>>> goes, but the fact that it doesn't start in the string. At least,
> >>>> that's what makes it feel to me like it's not just the mirror case of
> >>>> the positive one.
> >>>
> >>> It's just a matter of point of view. You advocate the point of the
> >>> string implementation which always counts from the start of the string
> >>> but I am advocating the view from the interface side that pretends to
> >>> count from the end. And that's what the people using ruby see.
> >>
> >> I don't mean the start of the string; I mean the starting index, into
> >> the string, of the first character you want. If it's -10 on a
> >> 3-character string, then it's an index that's out of range.
> >>
> >> I guess you could look at it by saying there's no starting point;
> >> there's just n characters, with leading or trailing non-existent ones
> >> chopped off. I'm just not sure I like the semantics of having str[-10]
> >> be nil, but str[-10..anything] being a string.
> >>
> > no, it's not -10..anything, it's -10..anything as long as it defines a
> > part of the string.
>
> What I mean is: if str[-10] is nil, then I'm not sure about there
> being any x, such that str[-10..x] is a string.
>
> I understand your reasoning, in terms of intervals and intersections.
> It's definitely a less "magic" way of looking at it.

And there is yet another case : "abc"[0,0] #=> ""

if "abc"[0,0] would return nil, then you could coerce the range so
that [-10, -5] would give [0,0], which would return nil.


-- 
Cheers,
  zimbatm