Convert a Number into Roman Numerals.

roman.rb

module Roman
  CHAR = "IVXLCDM"
  def self.to_num n
    return "" if n <= 0 || 5000 <= n
    str = sprintf("%04d", n).reverse!
    res = ''
    4.times do|i|
       res = "#{roman_char(str[i], i*2)}#{res}"
    end
    res
  end

  def self.roman_char n, i
    n = n.to_i
    arr = CHAR[i,3]
    case n
    when 1..3
      arr[0] * n
    when 5
      arr[1]
    when 4
      arr[0] + arr[1]
    when 6..8
      arr[1] + (arr[0] * (n-5)).to_s
    when 9
      arr[0] + arr[2].to_s
    else
      ''
    end
  end
end

def test num, expect
  if Roman.to_num(num) === expect
    print "."
  else
    puts "#{num} does't convert to #{expect}" 
  end
end
test 1, 'I'
test 2, 'II'
test 3, 'III'
test 4, 'IV'
test 5, 'V'
test 6, 'VI'
test 7, 'VII'
test 8, 'VIII'
test 9, 'IX'
test 10, 'X'
test 24, 'XXIV'
test 50, 'L'
test 100, 'C'
test 500, 'D'
test 1000, 'M'
test 2751, 'MMDCCLI'
test 1999, 'MCMXCIX'
test -1, ''
test 0, ''
test 5001, ''