From: Robert Klemme <shortcutter@...>
Date: 2009-04-29T20:02:40+09:00
Subject: Re: Random Access using IO#pos in code blocks

2009/4/29 Brian Candler <b.candler@pobox.com>:
> Unfortunately, I don't think string#scan will give you offsets into the
> strings found.
>
> In ruby 1.8 you can write this:
>
>  pos = 0
>  while md = /\b\w+\b/.match(para[pos..-1])
>    word = md[0]
>    puts "Match #{word} at #{para_pos+pos+md.begin(0)}"
>    pos += md.end(0)
>    ...
>  end
>
> In ruby 1.9 (but not 1.8.6/1.8.7), Regexp.match takes a start pos, so
> you could optimise it to this:
>
>  pos = 0
>  while md = /\b\w+\b/.match(para, pos)
>    word = md[0]
>    puts "Match #{word} at #{para_pos+md.begin(0)}"
>    pos = md.end(0)
>    ...
>  end

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"

> However in ruby 1.9 the offsets used will be in terms of number of
> characters, not number of bytes. It would be up to you to convert this
> back into byte offsets into the file, if that's what you're after.

This is an important point to remember!

Kind regards

robert

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