Re: unable to do "down" range?

Now what should string[1..-2] do in your version? And would the end
result with at least 3 different behaviors for ranges make any sense?

Ruby ranges are currently not sequences and not really intervals
either, but a horrible mixture of the two, and they can't be "fixed"
without breaking, like, all existing code.

2012/4/13, Ralph Shnelvar <[e-mail]>:
> Roger and Jan,
>
> Friday, April 13, 2012, 1:23:54 PM, you wrote:
>
> JE> Hi,
>
> JE> No, this doesn't work, because 40..10 is an empty range. A range
> JE> consists of all objects between the left limit and the right limit.
>
> JE> We've already discussed this topic here (I cannot find it right now).=
 My
> JE> personal opinion is that many people are confusing ranges with
> JE> sequences, because the ".." syntax looks like you're enumerating
> JE> numbers:
>
> 4..10 =3D>> 4, 5, 6, ..., 10
>
> JE> But a range isn't a sequence. You're not saying "count from ... to ..=
.".
> JE> The numbers are rather limits of a "static" interval. So there's isn'=
t a
> JE> real direction implied, the elements are simply put out in the most
> JE> obvious way: from low to high.
>
> JE> I think Integer#downto is what you're looking for:
>
> JE> 40.downto 10 do |i|
> JE>   puts i
> JE> end
>
> I have thought about this problem and I think I may make my contribution =
to
> the Ruby language an extension that would allow negative ranges as you
> describe.
>
> There are problems, though.
>
> How many pieces of code would break if
>  range =3D 40..10
>  range.each{|n| puts n} # it doesn't
> actually did what you want?
>
> Probably not many.
>
>
>
> The documentation in http://www.ruby-doc.org/core-1.9.3/Range.html says
>   each {| i | block } =E2=86=92 rng click to toggle source
>   each =E2=86=92 an_enumerator
>
>   Iterates over the elements rng, passing each in turn to the block. You =
can
> only iterate if the start object of the range supports the succ method
> (which means that you can=E2=80=99t iterate over ranges of
>   Float objects).
>
> To do what Roger wants, we would need to define a prev function (similar =
to
> the succ function) on Fixnums.
>
>  3.succ # 4
>  4,prev # Undefined!
>
>
> Fortunately, this is easy to do in Ruby
>
>   class Fixnum
>     def prev
>       self - 1
>     end
>   end
>
>   4.prev # 3
>
>
>
> So, should we clutter Ruby with a collection of prev functions (because w=
e'd
> also need to do it for all the things that succ is defined on)?  Possibly
> break existing code?
>
> I'd vote yes.
>
> There would be at least two advantages to having a well-defined way of
> dealing with negative ranges beyond the fact that we'd have the relativel=
y
> minor advantage of having a negative range.
>
> As I understand the code right now,
>
>   (1..10_000_000).last(5) # [9999996, 9999997, 9999998, 9999999, 10000000=
]
>
> will create a temporary array of ten million elements and then peel off t=
he
> last five array elements.
>
> Now from my perspective, having to allocate that relatively huge temporar=
y
> array is awful; truly Rube Goldberg-esque.  Implementing a prev function
> that would operate on the "end object" (in this instance, 10_000_000) the
> way that succ works on the "start object" would eliminate the need for th=
at
> temporary array.  If we had that in the implementation of Ruby, then we
> could get access to the last five elements more-or-less directly.
>
> Ralph Shnelvar
>
>
>
>


--=20
-- Matma Rex