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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

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``````

Find this solution interesting? Ask the author a question to learn more.
Solution Author
commented over 5 years ago

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.

What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

• What compromises have been made?
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