From: Brian Candler <b.candler@...>
Date: 2011-10-06T00:09:10+09:00
Subject: Re: Best way to compare lines in 2 files

Guillaume Bloss wrote in post #1025125:
> I have 2 list in 2 separate files
> I call them 1st file = old file, 2nd file = new file
> They are similar, except there is some lines added and some lines
> removed in both files, and I only want to keep : lines common in both
> files and line only existing in my 2nd file.

You want two separate output files, one containing lines common to both, 
and one containing lines only in the 2nd file?

As long as preserving the order isn't important, look at the manpages 
for 'sort' and 'join'.

Otherwise, since the files aren't too big, you can read all of file1 
into a Hash of {linedata=>true}. Then read through file2 and print a 
line only if the corresponding Hash entry is true.

seen = {}
File.open("file1") do |f|
  f.each_line do |line|
    seen[line] = true
  end
end
File.open("file2") do |f|
  f.each_line do |line|
    if seen[line]
      print "In both,#{line}"
    else
      print "Only in file 2,#{line}"
    end
  end
end

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