Is there a way to seek through a file without loading the whole thing into an array?

Question

This works:

f = File.new(\"myfile\").readlines
f[0] #=> \"line 1\"
f[21] #=> \"line 22\"

But what if I have a very large file, and on

Answer 1

For the purpose you can use the each_line iterator, combined with with_index to have the line number of the current line (counting from 0):

File.open('myfile') do |file|

  file.each_line.with_index do |line, lineno|
    case lineno
    when 0
      # line 1
    when 21
      # line 22
    end   
  end

end

By using open, passing a block to it, instead of new, you are guaranteed that the file is properly closed at the end of the block execution.

Update The with_index method accepts an optional argument to specify the starting index to use, so che code above could be better written like this:

file.each_line.with_index(1) do |line, lineno|
  case lineno
  when 1
    # line 1
  end
end