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to Roman Numerals in the Elixir Track

Published at Jul 13 2018 · 0 comments
Test suite


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

Running tests

Execute the tests with:

$ elixir roman_numerals_test.exs

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

For more detailed information about the Elixir track, please see the help page.


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.


if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("roman.exs", __DIR__)

ExUnit.configure(exclude: :pending, trace: true)

defmodule RomanTest do
  use ExUnit.Case

  # @tag :pending
  test "1" do
    assert Roman.numerals(1) == "I"

  @tag :pending
  test "2" do
    assert Roman.numerals(2) == "II"

  @tag :pending
  test "3" do
    assert Roman.numerals(3) == "III"

  @tag :pending
  test "4" do
    assert Roman.numerals(4) == "IV"

  @tag :pending
  test "5" do
    assert Roman.numerals(5) == "V"

  @tag :pending
  test "6" do
    assert Roman.numerals(6) == "VI"

  @tag :pending
  test "9" do
    assert Roman.numerals(9) == "IX"

  @tag :pending
  test "27" do
    assert Roman.numerals(27) == "XXVII"

  @tag :pending
  test "48" do
    assert Roman.numerals(48) == "XLVIII"

  @tag :pending
  test "59" do
    assert Roman.numerals(59) == "LIX"

  @tag :pending
  test "93" do
    assert Roman.numerals(93) == "XCIII"

  @tag :pending
  test "141" do
    assert Roman.numerals(141) == "CXLI"

  @tag :pending
  test "163" do
    assert Roman.numerals(163) == "CLXIII"

  @tag :pending
  test "402" do
    assert Roman.numerals(402) == "CDII"

  @tag :pending
  test "575" do
    assert Roman.numerals(575) == "DLXXV"

  @tag :pending
  test "911" do
    assert Roman.numerals(911) == "CMXI"

  @tag :pending
  test "1024" do
    assert Roman.numerals(1024) == "MXXIV"

  @tag :pending
  test "3000" do
    assert Roman.numerals(3000) == "MMM"
defmodule Roman do
  @romans %{
   1000 =>  "M", 
    900 => "CM", 
    500 =>  "D", 
    400 => "CD", 
    100 =>  "C", 
     90 => "XC", 
     50 =>  "L", 
     40 => "XL", 
     10 =>  "X", 
      9 => "IX", 
      5 =>  "V", 
      4 => "IV", 
      1 =>  "I", 

  @doc """
  Convert the number to a roman number.
  @spec numerals(pos_integer) :: String.t
  def numerals(number) do
    # need to reverse 'cuz Elixir defaults to sorted, ascending
    do_numerals(number, Map.keys(@romans) |> Enum.reverse, "")

  defp do_numerals(0, _, so_far), do: so_far

  defp do_numerals(number, [value | rest], so_far) do
    cond do
      value <= number ->
        do_numerals(number - value,
                    [value | rest],  # in case of 1-char repeats
      true ->
        do_numerals(number, rest, so_far)


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