def thousands(n, sep = ',')
  s = n.to_s
  sign = s[0] == '-' ? '-' : ''
  s = s[1..-1] if sign == '-'

  int, frac = s.split('.', 2)

  out = ''
  count = 0

  int.reverse.each_char do |c|
    out << sep if count > 0 && count % 3 == 0
    out << c
    count += 1
  end

  int = out.reverse

  frac ? "#{sign}#{int}.#{frac}" : "#{sign}#{int}"
end