From: David Alan Black <dblack@...>
Date: 2002-04-30T07:07:42+09:00
Subject: Re: [RFC] Ranges - enable (10..-10)

Hello --

On Tue, 30 Apr 2002, Michal Rokos wrote:

> Hello,
> 	I've looked that negative ranges are not allowed in Ruby.
>
> 	I'd like to discuss, if it is possible permit them...
>
> Example:
> 	(10..-10).each |i| do puts i end
> 		does nothing in current Ruby
> 	(10..-10).step(2)
> 		does nothing in current Ruby
>
> This patch make the example works (and fixes some funny step's value
> like 0 (causes never ending loop) and < 0 (causes overroll).
>
> 	I'm waiting for your comments :-)
>
> 		Michal

In [ruby-talk:8993], matz said:

  The truth are:

    * A range enumeration cannot go back, it's upward only.

    * But A range with end point less than start point is valid.

    * Thus 1..-1 is a completely valid range object.

    * Array and String internally convert negative end point into
      size+n, where n is a negative number.

    * So range n..-m is interpreted as n..(size-m), thus works fine (if
      n <= (size-m)).

I can't claim I've totally grasped why a range is upward by
definition... but apparently it is, so that's *my* problem :-)  You'll
find a few other posts if you search for "backwards range" or "reverse
range" at ruby-talk.org.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav