gistfile1.rb

# read_write.rb  

file_A = File.new("parent.txt", "r+")
file_B = File.new("child.txt", "r+")
file_C = File.new("out.txt", "r+")

while (line_A = file_A.gets)
  line_B = file_B.gets
  if (line_A == line_B)
    file_C.puts "#{line_A}"
  else
    file_C.puts "poop"
  end
end

file_A.close
file_B.close
file_C.close