theme

Write a method that takes arabic numerals and returns roman numerals. (Issues in https://qiita.com/daddygongon/items/2d0a73a51ddab2d9da1b)

The code I wrote this time

For the time being, I will paste the code I wrote this time before the explanation.


def to_roman (arabic_numerals)
  pair_arabic_roman = [[1,"I"],[4,"IV"],[5,"V"],[9,"IX"],[10,"X"],[40,"XL"],[50,"L"],
		       [90,"XC"],[100,"C"],[400,"CD"],[500,"D"],[900,"CM"],[1000,"M"]]
  roman_numerals = ""

  pair_arabic_roman.reverse_each do |p_a, p_r|
    while arabic_numerals >= p_a do
      arabic_numerals	= arabic_numerals - p_a
      roman_numerals	= roman_numerals  + p_r
    end
  end

  return roman_numerals
end

if $PROGRAM_NAME == __FILE__
  arabic_numerals = ARGV[0].to_i
  print "#{arabic_numerals}\t : #{to_roman(arabic_numerals)}\n"
end

(The version incorporated in Integer class is pasted at the end of the page)

Commentary

First, organize the outline of the method to be created

-** Function name : Anything is fine (here, to \ _roman) - Input : Arabic numerals - Output **: Roman numerals

Let's make it now

Output I for 1

If you write it as you like, it will be like this.


def to_roman (arabic_numerals)
  if arabic_numerals==1
    print "I\n"
  else
    print "hoge\n"
  end
end

if $PROGRAM_NAME == __FILE__
  arabic_numerals = ARGV[0].to_i
  print "#{arabic_numerals}\n"
  to_roman(arabic_numerals)
end

Of course it runs. By the way, it is somehow outputting " hoge " when it is other than 1.

Output II for 2 and IV for 4

When I wrote it obediently, it became like this.


def to_roman (arabic_numerals)
  if arabic_numerals==1
    print "I\n"
  elsif arabic_numerals==2
    print "II\n"
  elsif arabic_numerals==4
    print "IV\n"
  else
    print "hoge\n"
  end
end

if $PROGRAM_NAME == __FILE__
  arabic_numerals = ARGV[0].to_i
  print "#{arabic_numerals}\n"
  to_roman(arabic_numerals)
end

Of course it runs.

Returns V for 5

elsif I got tired of writing, so I wrote it in a loop. Also, instead of outputting in the function, the return value is returned.


def to_roman (arabic_numerals)
  pair_arabic_roman = [[1,"I"],
		       [2,"II"],
		       [4,"IV"],
		       [5,"V"]]

  pair_arabic_roman.each do |arabic, roman|
    if arabic_numerals==arabic
      return roman
    end
  end

  return "hoge"
end

if $PROGRAM_NAME == __FILE__
  arabic_numerals = ARGV[0].to_i
  print "#{arabic_numerals}\n"
  print "#{to_roman(arabic_numerals)}\n"
end

Of course it runs.

Returns Roman numerals for 1 to 10

It is too tedious to write all the correspondence between Arabic numerals and Roman numerals in the correspondence table pair_arabic_roman. So, like this.


def to_roman (arabic_numerals)
  pair_arabic_roman = [[1,"I"],[2,"II"],[4,"IV"],[5,"V"],[6,"VI"],[9,"IX"],[10,"X"]]
  roman_numerals = ""

  pair_arabic_roman.reverse_each do |arabic, roman| #Reverse each
    if arabic_numerals >= arabic
      arabic_numerals	= arabic_numerals - arabic
      roman_numerals	= roman_numerals  + roman
    end
  end

  return roman_numerals
end

if $PROGRAM_NAME == __FILE__
  arabic_numerals = ARGV[0].to_i
  print "#{arabic_numerals}\n"
  print "#{to_roman(arabic_numerals)}\n"
end

Strategy to concatenate character strings steadily by setting the initial value of roman_numerals to"". I referred to the string concatenation operation by seeing it written on this site (https://eigo.rumisunheart.com/2018/03/30/integer-to-roman-number-converter/).

-** For 3 : 3 = 2 + 1, so " " becomes"II"and then"III". - For 8 **: 8 = 6 + 2, so " " becomes"VI"and then"VIII".

For 1 to 20

Improved if to while.


def to_roman (arabic_numerals)
  pair_arabic_roman = [[1,"I"],[2,"II"],[4,"IV"],[5,"V"],[6,"VI"],[9,"IX"],[10,"X"]]
  roman_numerals = ""

  pair_arabic_roman.reverse_each do |p_a, p_r|
    while arabic_numerals >= p_a do
      arabic_numerals	= arabic_numerals - p_a
      roman_numerals	= roman_numerals  + p_r
    end
  end

  return roman_numerals
end

if $PROGRAM_NAME == __FILE__
  arabic_numerals = Range.new(1,20)
  arabic_numerals.each do |i|
    print "#{i}\t : #{to_roman(i)}\n"
  end
end

Almost completed when you come here.

By the way, Range is often seen in python.

For 1 ~ 2000

After organizing the correspondence table pair_arabic_roman to some extent, it became like this.


def to_roman (arabic_numerals)
  pair_arabic_roman = [[1,"I"],[4,"IV"],[5,"V"],[9,"IX"],[10,"X"],[40,"XL"],[50,"L"],
		       [90,"XC"],[100,"C"],[400,"CD"],[500,"D"],[900,"CM"],[1000,"M"]]
  roman_numerals = ""

  pair_arabic_roman.reverse_each do |p_a, p_r|
    while arabic_numerals >= p_a do
      arabic_numerals	= arabic_numerals - p_a
      roman_numerals	= roman_numerals  + p_r
    end
  end

  return roman_numerals
end

if $PROGRAM_NAME == __FILE__
  arabic_numerals = [51,97,99,439,483,499,500,732,961,999,1000,1999,2000]
  arabic_numerals.each do |i|
    print "#{i}\t : #{to_roman(i)}\n"
  end
end

Of course I did it. I did it. With this feeling, the code listed at the beginning is completed.

Development issues

Integer Integer class is extended to

999.to_roman #=> CMXCIX

To return.

I slammed the method I just created into an Integer class.


class Integer
  def to_roman
    arabic_numerals = self
    pair_arabic_roman = [[1,"I"],[4,"IV"],[5,"V"],[9,"IX"],[10,"X"],[40,"XL"],[50,"L"],
			 [90,"XC"],[100,"C"],[400,"CD"],[500,"D"],[900,"CM"],[1000,"M"]]
    roman_numerals = ""

    pair_arabic_roman.reverse_each do |p_a, p_r|
      while arabic_numerals >= p_a do
	arabic_numerals	= arabic_numerals - p_a
	roman_numerals	= roman_numerals  + p_r
      end
    end

    return roman_numerals
  end
end

if $PROGRAM_NAME == __FILE__
  a = ARGV[0].to_i
  print "#{a}\t : #{a.to_roman}\n"
end

Of course, the output is as expected.

Reference material

roman_numerals -C ++ Programming: Convert Arabic Numerals to Roman Numerals

source ~/Lecture/multiscale_simulation/grad_members_20f/members/gagagagazelle/docs/roman_numerals.org