How to get a particular line from a file

These solutions work if you want only one line from a file, or if you want multiple lines from a file small enough to be read repeatedly. Large files (for example, 10 million lines) take much longer to search for a specific line so it's better to get the necessary lines sequentially in a single read so the large file doesn't get read multiple times.

Create a large file:

File.open('foo', 'a') { |f| f.write((0..10_000_000).to_a.join("\n")) }

Pick which lines will be read from it and make sure they're sorted:

lines = [9_999_999, 3_333_333, 6_666_666].sort

Print out those lines:

File.open('foo') do |f|
  lines.each_with_index do |line, index|
    (line - (index.zero? ? 0 : lines[index - 1]) - 1).times { f.gets }
    puts f.gets
  end
end

This solution works for any number of lines, does not load the entire file into memory, reads as few lines as possible, and only reads the file one time.