comp.lang.ruby

Brian Candler

5/30/2007 8:17:00 PM

On Thu, May 31, 2007 at 03:06:06AM +0900, Mike Steiner wrote:
> Sorry, but now I'm completely confused. Arrays don't have a .sub! method,
> and m[1] is a string. And I can reference m[1], m[2], etc. in any order and
> get the same strings each time.
>
> Why are the object id's of m[1] different each time you reference it (in
> your example)?

Because m isn't an Array, it's a MatchData object.

$ irb1.8
irb(main):001:0> m = /(b+)/.match("abbbc")
=> #<MatchData:0xb7d40020>

MatchData has a [] method, but that doesn't mean it has to behave like an
Array. If m[1] were to return the same string each time, it would have to
cache its own answers, or build an array containing all the captures (which
would be wasteful in those cases where they're not used).

Regards,

Brian.

>
> Mike Steiner
>
>
> On 5/26/07, dblack@wobblini.net <dblack@wobblini.net> wrote:
> >
> >Hi --
> >
> >On Sun, 27 May 2007, Mike Steiner wrote:
> >
> >> A few times in my programs I've tried to modify elements of the array
> >> returned by .match, and it always causes strange errors later in my
> >code.
> >>
> >> For example, if I do:
> >>
> >> s = "aaa bbb ccc"
> >> m = /(.*) (.*) (.*)/.match(s)
> >> m[1].sub! ( "aa" , "" )
> >>
> >> I get strange results, but if I instead do:
> >>
> >> s = "aaa bbb ccc"
> >> m = /(.*) (.*) (.*)/.match(s)
> >> x = m[1].dup
> >> x.sub! ( "aa" , "" )
> >>
> >> it works fine. What's going on?
> >
> >Looking at the source (fairly quickly, but I think this is what's
> >happening), it looks like m[1] calls match_aref, which calls
> >rb_reg_nth_match, which hands back a different string object each
> >time. So doing a sub! on it doesn't have any impact on the string you
> >get the next time you do the [1] operation.
> >
> >You can also see this like this, where each [1] string is a different
> >object:
> >
> >irb(main):006:0> m = /(abc)/.match("abc")
> >=> #<MatchData:0x34bf98>
> >irb(main):007:0> m[1].object_id
> >=> 1713410
> >irb(main):008:0> m[1].object_id
> >=> 1698110
> >irb(main):009:0> m[1].object_id
> >=> 1684420
> >
> >etc.
> >
> >
> >David
> >
> >--
> >Q. What is THE Ruby book for Rails developers?
> >A. RUBY FOR RAILS by David A. Black (http://www.manning...)
> > (See what readers are saying! http://www.r.../r...)
> >Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
> >A. Ruby Power and Light, LLC (http://www.r...)
> >
> >