Re: RCR allow indexing last n items

Hi --

On Tue, 1 Jan 2008, Michal Suchanek wrote:

> On 31/12/2007, David A. Black <[e-mail]> wrote:
>> Hi --
>>
>> On Mon, 31 Dec 2007, Michal Suchanek wrote:
>>
>>> On 31/12/2007, David A. Black <[e-mail]> 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.


David

-- 
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