103 lines
2.6 KiB
Markdown
103 lines
2.6 KiB
Markdown
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# Roman Numerals
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Welcome to Roman Numerals on Exercism's Elixir Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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## Introduction
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Today, most people in the world use Arabic numerals (0–9).
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But if you travelled back two thousand years, you'd find that most Europeans were using Roman numerals instead.
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To write a Roman numeral we use the following Latin letters, each of which has a value:
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| M | D | C | L | X | V | I |
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| ---- | --- | --- | --- | --- | --- | --- |
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| 1000 | 500 | 100 | 50 | 10 | 5 | 1 |
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A Roman numeral is a sequence of these letters, and its value is the sum of the letters' values.
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For example, `XVIII` has the value 18 (`10 + 5 + 1 + 1 + 1 = 18`).
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There's one rule that makes things trickier though, and that's that **the same letter cannot be used more than three times in succession**.
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That means that we can't express numbers such as 4 with the seemingly natural `IIII`.
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Instead, for those numbers, we use a subtraction method between two letters.
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So we think of `4` not as `1 + 1 + 1 + 1` but instead as `5 - 1`.
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And slightly confusingly to our modern thinking, we write the smaller number first.
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This applies only in the following cases: 4 (`IV`), 9 (`IX`), 40 (`XL`), 90 (`XC`), 400 (`CD`) and 900 (`CM`).
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Order matters in Roman numerals!
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Letters (and the special compounds above) must be ordered by decreasing value from left to right.
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Here are some examples:
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```text
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105 => CV
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---- => --
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100 => C
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+ 5 => V
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```
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```text
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106 => CVI
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---- => --
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100 => C
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+ 5 => V
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+ 1 => I
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```
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```text
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104 => CIV
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---- => ---
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100 => C
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+ 4 => IV
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```
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And a final more complex example:
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```text
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1996 => MCMXCVI
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----- => -------
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1000 => M
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+ 900 => CM
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+ 90 => XC
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+ 5 => V
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+ 1 => I
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```
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## Instructions
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Your task is to convert a number from Arabic numerals to Roman numerals.
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For this exercise, we are only concerned about traditional Roman numerals, in which the largest number is MMMCMXCIX (or 3,999).
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~~~~exercism/note
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There are lots of different ways to convert between Arabic and Roman numerals.
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We recommend taking a naive approach first to familiarise yourself with the concept of Roman numerals and then search for more efficient methods.
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Make sure to check out our Deep Dive video at the end to explore the different approaches you can take!
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~~~~
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## Source
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### Created by
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- @rubysolo
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### Contributed to by
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- @andrewsardone
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- @angelikatyborska
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- @cetinajero
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- @Cohen-Carlisle
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- @dalexj
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- @devonestes
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- @jinyeow
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- @lpil
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- @neenjaw
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- @parkerl
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- @sotojuan
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- @Teapane
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- @waiting-for-dev
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### Based on
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The Roman Numeral Kata - https://codingdojo.org/kata/RomanNumerals/
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