On Apr 13, 2012 5:04 PM, "Ralph Shnelvar&qu= ot; <ralphs@dos32.com> wrote:=

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.<= br>
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 e= numerating
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 t= here's isn't a
JE> real direction implied, the elements are simply put out in the most<= br> 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> =C2=A0 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 des= cribe.

There are problems, though.

How many pieces of code would break if
=C2=A0range =3D 40..10
=C2=A0range.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 say= s
=C2=A0each {| i | block } =E2=86=92 rng click to toggle source
=C2=A0each =E2=86=92 an_enumerator

=C2=A0Iterates over the elements rng, passing each in turn to the block. Y= ou can only iterate if the start object of the range supports the succ meth= od (which means that you can=E2=80=99t iterate over ranges of
=C2=A0Float objects).

To do what Roger wants, we would need to define a prev function (similar to= the succ function) on Fixnums.

=C2=A03.succ # 4
=C2=A04,prev # Undefined!

Fortunately, this is easy to do in Ruby

=C2=A0class Fixnum
=C2=A0 =C2=A0def prev
=C2=A0 =C2=A0 =C2=A0self - 1
=C2=A0 =C2=A0end
=C2=A0end

=C2=A04.prev # 3

So, should we clutter Ruby with a collection of prev functions (because we&= #39;d also need to do it for all the things that succ is defined on)? =C2= =A0Possibly break existing code?

I'd vote yes.

There would be at least two advantages to having a well-defined way of deal= ing with negative ranges beyond the fact that we'd have the relatively = minor advantage of having a negative range.

As I understand the code right now,

=C2=A0(1..10_000_000).last(5) # [9999996, 9999997, 9999998, 9999999, 10000= 000]

will create a temporary array of ten million elements and then peel off the= last five array elements.

Now from my perspective, having to allocate that relatively huge temporary = array is awful; truly Rube Goldberg-esque. =C2=A0Implementing a prev functi= on that would operate on the "end object" (in this instance, 10_0= 00_000) the way that succ works on the "start object" would elimi= nate the need for that temporary array. =C2=A0If we had that in the impleme= ntation of Ruby, then we could get access to the last five elements more-or= -less directly.

Ralph Shnelvar