From: Mike Steiner <mikejaysteiner@...>
Date: 2007-05-31T03:06:06+09:00
Subject: Re: are MatchData objects mutable?

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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)?

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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