Read lines by number from a large file

Question

I have a file with 15 million lines (will not fit in memory). I also have a small vector of line numbers - the lines that I want to extract.

How can I read-out the l

Answer 1

Before I was able to get an R solution/answer, I've done it in Ruby:

#!/usr/bin/env ruby

NUM_SEQS = 14024829

linenumbers = (1..10).collect{(rand * NUM_SEQS).to_i}

File.open("./data/uniprot_2011_02.tab") do |f|
  while line = f.gets
    print line if linenumbers.include? f.lineno 
  end
end

runs fast (as fast as my storage can read the file).