roman_numeral.rb

def roman_numeral(n) = case n
  in 1000..3000 then 'M'  + roman_numeral(n - 1000)
  in 900..999   then 'CM' + roman_numeral(n - 900)
  in 500..899   then 'D'  + roman_numeral(n - 500)
  in 400..499   then 'CD' + roman_numeral(n - 400)
  in 100..399   then 'C'  + roman_numeral(n - 100)
  in 90..99     then 'XC' + roman_numeral(n - 90)
  in 50..89     then 'L'  + roman_numeral(n - 50)
  in 40..49     then 'XL' + roman_numeral(n - 40)
  in 10..39     then 'X'  + roman_numeral(n - 10)
  in 9          then 'IX' + roman_numeral(n - 9)
  in 5..9       then 'V'  + roman_numeral(n - 5)
  in 4          then 'IV' + roman_numeral(n - 4)
  in 1..3       then 'I'  + roman_numeral(n - 1)
  else ''
end

# Shorter variant
def roman_numeral(n) = case n
  in 1000..3000 then 'M'  + roman_numeral(n - 1000)
  in 900..      then 'CM' + roman_numeral(n - 900)
  in 500..      then 'D'  + roman_numeral(n - 500)
  in 400..      then 'CD' + roman_numeral(n - 400)
  in 100..      then 'C'  + roman_numeral(n - 100)
  in 90..       then 'XC' + roman_numeral(n - 90)
  in 50..       then 'L'  + roman_numeral(n - 50)
  in 40..       then 'XL' + roman_numeral(n - 40)
  in 10..       then 'X'  + roman_numeral(n - 10)
  in 9          then 'IX' + roman_numeral(n - 9)
  in 5..        then 'V'  + roman_numeral(n - 5)
  in 4          then 'IV' + roman_numeral(n - 4)
  in 1..        then 'I'  + roman_numeral(n - 1)
  else ''
end