roman numerals

はじめに

今回は以下の記事にある，アラビア数字を受け取って，ローマ数字を返す関数to_romanをrubyで実装する．
- roman numerals

roman numerals

境界値が沢山あってifだらけになってしまうような場合は，配列に境界値を入れてループで回すという実装テクニックが使える場合がある．今回はこの方法が有効なので使う．

roman_numerals.rb

def to_roman(n)
  raise RangeError if n <= 0 or 4000 <= n

  arabic_num = [1000,900,500,400,100,90,50,40,10,9,5,4,1]
  roman_num = ["M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"]
  ret = ""
  arabic_num.zip(roman_num) do |arabic, roman|
    while n >= arabic do
      n -= arabic
      ret << roman
    end
  end
  return ret
end

発展課題

この関数をIntegerクラスのメソッドとして追加してみる．

roman_numerals.rb

class Integer
  def to_roman
    n = self
    raise RangeError if n <= 0 or 4000 <= n

    arabic_num = [1000,900,500,400,100,90,50,40,10,9,5,4,1]
    roman_num = ["M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"]
    ret = ""
    arabic_num.zip(roman_num) do |arabic, roman|
      while n >= arabic do
    n -= arabic
    ret << roman
      end
    end
    return ret
  end
end

すると以下のように使える．

roman_numerals.rb

3999.to_roman # => "MMMCMXCIX"

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• source ~/grad_members_20f/members/ryuta-kikuchi/qiita_articles/roman_numerals.org