From: Brian Candler <b.candler@...>
Date: 2009-04-29T18:40:35+09:00
Subject: Re: Random Access using IO#pos in code blocks

Arun Kumar wrote:

>          unless freq.has_key?(word)
>            freq[word] = [1,f.pos,file_name] # freq is a hash, that
> stores an array containing index, position of word (THE PROBLEM)..
>         else
>           freq[word].to_a[0] += 1
>            freq[word].to_a<< f.pos << file_name
>         end

BTW, you can replace all that by:

    freq[word] ||= [0]
    freq[word][0] += 1
    freq[word] << f.pos << file_name

As for the pos, since you've already slurped in the data you'll need to 
remember where you are within your buffer. Your outer loop could become 
something like this:

  para_pos = 0
  f.readlines("\r\n\r\n").each do |para|
    ...
    para_pos += para.size + 4
  end

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

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.
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