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davearonson's solution

to Roman Numerals in the Ruby Track

Published at Jul 13 2018 · 1 comment
Instructions
Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Write a function to convert from normal numbers to Roman Numerals.

The Romans were a clever bunch. They conquered most of Europe and ruled it for hundreds of years. They invented concrete and straight roads and even bikinis. One thing they never discovered though was the number zero. This made writing and dating extensive histories of their exploits slightly more challenging, but the system of numbers they came up with is still in use today. For example the BBC uses Roman numerals to date their programmes.

The Romans wrote numbers using letters - I, V, X, L, C, D, M. (notice these letters have lots of straight lines and are hence easy to hack into stone tablets).

 1  => I
10  => X
 7  => VII

There is no need to be able to convert numbers larger than about 3000. (The Romans themselves didn't tend to go any higher)

Wikipedia says: Modern Roman numerals ... are written by expressing each digit separately starting with the left most digit and skipping any digit with a value of zero.

To see this in practice, consider the example of 1990.

In Roman numerals 1990 is MCMXC:

1000=M 900=CM 90=XC

2008 is written as MMVIII:

2000=MM 8=VIII

See also: http://www.novaroma.org/via_romana/numbers.html


For installation and learning resources, refer to the exercism help page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby roman_numerals_test.rb

To include color from the command line:

ruby -r minitest/pride roman_numerals_test.rb

Source

The Roman Numeral Kata http://codingdojo.org/cgi-bin/index.pl?KataRomanNumerals

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

roman_numerals_test.rb

require 'minitest/autorun'
require_relative 'roman_numerals'

# Common test data version: 1.0.0 070e8d5
class RomanNumeralsTest < Minitest::Test
  def test_1
    # skip
    assert_equal 'I', 1.to_roman
  end

  def test_2
    skip
    assert_equal 'II', 2.to_roman
  end

  def test_3
    skip
    assert_equal 'III', 3.to_roman
  end

  def test_4
    skip
    assert_equal 'IV', 4.to_roman
  end

  def test_5
    skip
    assert_equal 'V', 5.to_roman
  end

  def test_6
    skip
    assert_equal 'VI', 6.to_roman
  end

  def test_9
    skip
    assert_equal 'IX', 9.to_roman
  end

  def test_27
    skip
    assert_equal 'XXVII', 27.to_roman
  end

  def test_48
    skip
    assert_equal 'XLVIII', 48.to_roman
  end

  def test_59
    skip
    assert_equal 'LIX', 59.to_roman
  end

  def test_93
    skip
    assert_equal 'XCIII', 93.to_roman
  end

  def test_141
    skip
    assert_equal 'CXLI', 141.to_roman
  end

  def test_163
    skip
    assert_equal 'CLXIII', 163.to_roman
  end

  def test_402
    skip
    assert_equal 'CDII', 402.to_roman
  end

  def test_575
    skip
    assert_equal 'DLXXV', 575.to_roman
  end

  def test_911
    skip
    assert_equal 'CMXI', 911.to_roman
  end

  def test_1024
    skip
    assert_equal 'MXXIV', 1024.to_roman
  end

  def test_3000
    skip
    assert_equal 'MMM', 3000.to_roman
  end

  # Problems in exercism evolve over time, as we find better ways to ask
  # questions.
  # The version number refers to the version of the problem you solved,
  # not your solution.
  #
  # Define a constant named VERSION inside of the top level BookKeeping
  # module, which may be placed near the end of your file.
  #
  # In your file, it will look like this:
  #
  # module BookKeeping
  #   VERSION = 1 # Where the version number matches the one in the test.
  # end
  #
  # If you are curious, read more about constants on RubyDoc:
  # http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

  def test_bookkeeping
    skip
    assert_equal 2, BookKeeping::VERSION
  end
end
module RomanNumerals

  def self.to_roman(n)
    if n > limit
      raise(ArgumentError,
            "Sorry, this implementation of Roman Numerals is limited to #{limit}.")
    end
    POWERS_OF_TEN.each_with_index.map { |char, zeroes|
      get_roman_letters(n, char, zeroes)
    }.reverse.join
  end

  private

  # we have been promised we won't need any more than M (limited to
  # 3999), though this algorithm will accomodate more, just by adding
  # them to this list.  one more letter would let us to get up to
  # 8999.  the romans actually used a v with a bar over it, for 500,
  # but that is non-ascii, so i'm not going to.
  POWERS_OF_TEN      = %w(I X C M)
  FIVE_POWERS_OF_TEN = %w(V L D)

  def self.get_roman_letters(number, single, zeroes)
    count = (number / 10 ** zeroes) % 10
    case count
    when 0..3 then single * count
    when 4    then single + FIVE_POWERS_OF_TEN[zeroes]
    when 5..8 then FIVE_POWERS_OF_TEN[zeroes] + single * (count - 5)
    when 9    then single + POWERS_OF_TEN[zeroes + 1]
    end
  end

  def self.limit
    # assuming the tens list is never shorter than the fives,
    # nor more than one entry longer.  else it's really weird....
    factor = POWERS_OF_TEN.length > FIVE_POWERS_OF_TEN.length ? 4 : 9
    @limit ||= 10 ** (POWERS_OF_TEN.length - 1) * factor - 1
  end

end

class Fixnum
  def to_roman
    RomanNumerals.to_roman(self)
  end
end

Community comments

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Avatar of davearonson

Incorporated good OO advice (make a separate module and delegate, don't just stuff it all into Fixnum)... and while I was at it, added an error message, incluiding exactly how high we could count under these conditions.

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