Another Version

roman.rb

def digit_to_roman digit, one, five, ten
  case d = digit % 10
  when 0..3 then one * d
  when 4    then one + five
  when 5..8 then five + one * (d - 5)
  when 9    then one + ten
  end
end

def to_roman n
  digit_to_roman(n / 1000, 'M', '', '') +
    digit_to_roman(n / 100, 'C', 'D', 'M') +
    digit_to_roman(n / 10, 'X', 'L', 'C') +
    digit_to_roman(n, 'I', 'V', 'X')
end

def from_roman n
  @roman_to_arabic ||= Hash[(1..3999).map{|i| [to_roman(i), i]}]
  @roman_to_arabic[n.chomp]
end

def convert n
  n =~ /\d/ ? to_roman(n.to_i) : from_roman(n)
end

ARGF.each_line {|l| puts convert(l)}  if $0 == __FILE__