From: Mike Steiner <mikejaysteiner@...>
Date: 2007-05-31T06:25:09+09:00
Subject: Re: are MatchData objects mutable?

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Now I get it. Thanks!

Mike Steiner

On 5/30/07, Brian Candler <B.Candler@pobox.com> wrote:
>
> 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.com/black)
> > >    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
> > >Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
> > >A. Ruby Power and Light, LLC (http://www.rubypal.com)
> > >
> > >
>
>

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