From: Tim Hunter <TimHunter@...>
Date: 2008-11-16T02:27:45+09:00
Subject: Re: IO#lineno= doesn't work the way I expected

Chad Perrin wrote:
> I'm working on something that operates on each line of a file
> individually.  So far, the relevant part looks something like this:
> 
>     in_fh = File.open(infile, 'r')
>     out_fh = File.open(outfile, 'w')
> 
>     in_fh.each_line do |l|
>       out_fh << l
>     end
> 
> Obviously not the actual working code (since it doesn't do anything other
> than copy the file), but it's the simplified version I'm using to test a
> feature, anyway.
> 
> This works exactly as expected.  I get a duplicate of the input file,
> with the name assigned to the outfile variable.
> 
> So . . . I want to be able to skip a number of lines at the beginning of
> the input file.  It seemed like IO#lineno= would be the obvious solution:
> 
>     in_fh = File.open(infile, 'r')
>     out_fh = File.open(outfile, 'w')
> 
>     in_fh.lineno = 1
> 
>     in_fh.each_line do |l|
>       out_fh << l
>     end
> 
> Unfortunately, that doesn't give me what I expected at all.  In fact,
> what I end up with is an empty file.  Am I misunderstanding what
> IO#lineno= does?  Is there something about the way IO#each_line works
> that is interacting badly with an incremented line number?  What do I
> need to do differently to make this work?
> 

The way I read the ri doc for lineno=, it appears that all it does is 
determine the value that lineno returns the next time you call it. That 
is, it doesn't move the read position in the file.

I think what you're looking for is IO#seek, but notice that it seek 
doesn't operate on lines, only on byte offsets.


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