From: "Brian Schröder" <ruby.brian@...>
Date: 2005-11-19T00:41:50+09:00
Subject: Re: IO.lineno= not behaving as expected

On 18/11/05, Belorion <belorion@gmail.com> wrote:
> What am I missing here (apologies if this is just my 7am brain fog
> speaking):
>
> The contents of my file (test) are:
> 1
> 2
> 3
> 4
> 5
> 6
>
> f = File.open('test')
> f.lineno = 4
> puts f.readline # <= This returns 1, instead of 4 as expected
>
> I'm trying to do fast lookups in a file based on line number, but this isn't
> working like I expected it to. 1) What am I missing here? 2) If this is not
> what lineno=() is designed for, then what is it supposed to be used for? 3)
> is there another easy way for fast lookups by line#? Thanks.
>
>

Hello Belorion,

it is impossible to do a fast lookup by line without building an index
first (i.e. loading everything into an array or hashing line-number to
byte position) because the program has to count the number of newlines
to the line. So the best would be to simply do

DATA = File.read("file").split(/\n/)

and use

DATA[linenumber-1]

to access the data. Try to restructure your program in such a way that
it need not read the file repeatedly. You could even go overboard and
create a multiton class that reads the file into memory and checks on
each access if the file has changed. Something like this: (Untested)

class DataFile
  @@files = {}

  def self.open(file)
    @@files[file] ||= DataFile.new(file)
  end

  def initialize(filename)
    @filename = filename
    reread()
  end

  def read(line)
    mt = File.mtime(@filename)
    reread() if not @mt or @mt < mt
    @mt = mt
    @data[line]
  end

  private
  def reread
    @data = File.read(@filename).split(/\n/)
  end

  private :new
end

But that was more for the fun of it.

Brian

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