From: Robert Klemme <shortcutter@...>
Date: 2009-04-30T17:26:00+09:00
Subject: Re: Random Access using IO#pos in code blocks

2009/4/30 Brian Candler <b.candler@pobox.com>:
> Robert Klemme wrote:
>> String#scan is likely faster than manually matching portions with
>> #match.  In both versions of Ruby you can do this to get the
>> /character/ offset:
>>
>> irb(main):001:0> s=%{foo bar baz}
>> => "foo bar baz"
>> irb(main):002:0> s.scan(/\w+/) { p $`.length }
>> 0
>> 4
>> 8
>> => "foo bar baz"
>
> Well, my guess is that would be *less* efficient for large paragraphs,
> since $` forces allocation of a new string containing all the text from
> the start to the current point.

Last time I checked the actual string buffer was shared so the
overhead is just a single instance.  I do have to admit though that I
do not know when the object is allocated (i.e. at time of match or
when referencing $`).

> But that reminds me, there is a global
> variable containing a MatchData object: $~
>
> So you can write:
>
> irb(main):001:0> s=%{foo bar baz}
> => "foo bar baz"
> irb(main):002:0> s.scan(/\w+/) { p $~.begin(0) }
> 0
> 4
> 8
> => "foo bar baz"

Also a good variant! (Btw, MatchData might be even more heavyweight
than a sub string.)

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/