From: 7stud -- <bbxx789_05ss@...>
Date: 2009-09-13T04:27:48+09:00
Subject: Re: Backtracking through a file

M. Q. wrote:
> IO.gets steps through a file one line at a time.

As does IO.foreach.  And IO.foreach's block terminates naturally when 
the end of the file is reached:

IO.foreach("data.txt") do |line|
  puts line
end


> I thought possibly setting IO.lineno to the line before the current 
> line that would work, but I think I just misunderstood what lineno
> was for.

$ ri IO.lineno
-------------------------------------------------------------- IO#lineno
     ios.lineno    => integer
------------------------------------------------------------------------
     Returns the current line number in _ios_. The stream must be opened
     for reading. +lineno+ counts the number of times +gets+ is called,
     rather than the number of newlines encountered. The two values will
     differ if +gets+ is called with a separator other than newline. See
     also the +$.+ variable.

        f = File.new("testfile")
        f.lineno   #=> 0
        f.gets     #=> "This is line one\n"
        f.lineno   #=> 1
        f.gets     #=> "This is line two\n"
        f.lineno   #=> 2
-------------------------------------------------

The key line in the description is:

**lineno counts the number of times gets is called**

It's no different than if you wrote:

lineno = 0

IO.foreach("data.txt") do |line|
  lineno += 1
  puts "#{lineno}: #{line}"

  if lineno==1
    lineno = 30
  end
end


data.txt:
---------
hello
world
goodbye

--output:--
1: hello
31: world
32: goodbye


lineno is just a counter--it has no affect on the actual file pointer, 
which marks the location of the current position in the file.

> Anyone know a good way to do this?

prev_lines = []

IO.foreach("data.txt") do |line|
  if line =~ /good/
    prev_lines.reverse_each do |prev_line|
      if prev_line =~ /hel/
        puts prev_line
        prev_lines = []
      end
    end
  end

  prev_lines << line
end

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