Being a 10-days-old Rubyist, I have to confest that I spent a lot of time
solving this problem and trying just for fun to make it more and more
concise at the price of clarity and no error handling. I've learned a lot
during the process. A very good problem to solve for beginners.

Yannick

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ARABIC = {"I" => 1, "IV" => 4, "V" => 5, "IX" => 9, "X" => 10, "XL" => 40,
"L" => 50, "XC" => 90, "C" => 100, "CD" => 400, "D" => 500, "CM" => 900,
"M" => 1000}
ROMAN = ARABIC.invert

def toInt(roman)
    roman.reverse.split(//).inject(0) do |acc, letter|
        acc += ARABIC[letter] * (acc > ARABIC[letter] * 3 ? -1 : 1)
    end
end

def toRoman(number)
    ARABIC.values.sort.reverse.inject("") do |acc, value|
        mult, number = number.divmod(value)
        acc << ROMAN[value] * mult
    end  
end

STDIN.readlines.each{|line| line.chomp!; print line, " = ", line.to_i > 0 ?
toRoman(line.to_i) : toInt(line), "\n"}
-------------------

Or Object Oriented:
-------------------
ARABIC = {"I" => 1, "IV" => 4, "V" => 5, "IX" => 9, "X" => 10, "XL" => 40,
"L" => 50, "XC" => 90, "C" => 100, "CD" => 400, "D" => 500, "CM" => 900,
"M" => 1000}
ROMAN = ARABIC.invert

class String
    def roman_to_i()
        self.reverse.split(//).inject(0) do |acc, letter|
            acc += ARABIC[letter] * (acc > ARABIC[letter] * 3 ? -1 : 1)
        end
    end
end

class Integer
    def to_roman()
        remainder = self
        ARABIC.values.sort.reverse.inject("") do |acc, value|
            mult, remainder = remainder.divmod(value)
            acc << ROMAN[value] * mult
        end
    end
end