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# Instructions | ||
# Introduction | ||
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Write a function to convert from normal numbers to Roman Numerals. | ||
Your task is to convert a number from Arabic numerals to Roman numerals. | ||
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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 programs. | ||
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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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). | ||
~~~~exercism/note | ||
There are lots of different ways to convert between Arabic and Roman numerals. | ||
We recommend taking a naive approach first to familiarise yourself with the concept of Roman numerals and then search for more efficient methods. | ||
```text | ||
1 => I | ||
10 => X | ||
7 => VII | ||
``` | ||
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The maximum number supported by this notation is 3,999. | ||
(The Romans themselves didn't tend to go any higher) | ||
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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. | ||
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To see this in practice, consider the example of 1990. | ||
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In Roman numerals 1990 is MCMXC: | ||
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1000=M | ||
900=CM | ||
90=XC | ||
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2008 is written as MMVIII: | ||
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2000=MM | ||
8=VIII | ||
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Learn more about [Roman numerals on Wikipedia][roman-numerals]. | ||
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[roman-numerals]: https://wiki.imperivm-romanvm.com/wiki/Roman_Numerals | ||
Make sure to check out our Deep Dive video at the end to explore the different approaches you can take! | ||
~~~~ |
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# Description | ||
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Today, most people in the world use Arabic numerals (0–9). | ||
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 | | ||
| ---- | --- | --- | --- | --- | --- | --- | | ||
| 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. | ||
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**. | ||
That means that we can't express numbers such as 4 with the seemingly natural `IIII`. | ||
Instead, for those numbers, we use a subtraction method between two letters. | ||
So we think of `4` not as `1 + 1 + 1 + 1` but instead as `5 - 1`. | ||
And slightly confusingly to our modern thinking, we write the smaller number first. | ||
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! | ||
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 | ||
105 => CV | ||
---- => -- | ||
100 => C | ||
+ 5 => V | ||
``` | ||
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```text | ||
106 => CVI | ||
---- => -- | ||
100 => C | ||
+ 5 => V | ||
+ 1 => I | ||
``` | ||
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```text | ||
104 => CIV | ||
---- => --- | ||
100 => C | ||
+ 4 => IV | ||
``` | ||
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And a final more complex example: | ||
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```text | ||
1996 => MCMXCVI | ||
----- => ------- | ||
1000 => M | ||
+ 900 => CM | ||
+ 90 => XC | ||
+ 5 => V | ||
+ 1 => I | ||
``` |
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