public/subjects/romannumbers.en.md

## romannumbers

### Instructions

Write a program called `rn`. The objective is to convert a number, given as an argument, into a roman number and print it with its roman number calculation.

The program should have a limit of `4000`. In case of an invalid number, for example `"hello"` or `0` the program should print `ERROR: can not convert to roman digit`.

Roman Numerals reminder:

| I |   1  |
|:-:|:----:|
| V |   5  |
| X |  10  |
| L |  50  |
| C |  100 |
| D |  500 |
| M | 1000 |

For example, the number 1732 would be denoted MDCCXXXII in Roman numerals. However, Roman numerals are not a purely additive number system. In particular, instead of using four symbols to represent a 4, 40, 9, 90, etc. (i.e., IIII, XXXX, VIIII, LXXXX, etc.), such numbers are instead denoted by preceding the symbol for 5, 50, 10, 100, etc., with a symbol indicating subtraction. For example, 4 is denoted IV, 9 as IX, 40 as XL, etc.

The following table gives the Roman numerals for the first few positive integers. 

|  1 |   I  | 11 |   XI  | 21 |   XXI  |
|:--:|:----:|:--:|:-----:|:--:|:------:|
|  2 |  II  | 12 |  XII  | 22 |  XXII  |
|  3 |  III | 13 |  XIII | 23 |  XXIII |
|  4 |  IV  | 14 |  XIV  | 24 |  XXIV  |
|  5 |   V  | 15 |   XV  | 25 |   XXV  |
|  6 |  VI  | 16 |  XVI  | 26 |  XXVI  |
|  7 |  VII | 17 |  XVII | 27 |  XXVII |
|  8 | VIII | 18 | XVIII | 28 | XXVIII |
|  9 |  XIX | 19 |  XIX  | 29 |  XXIX  |
| 10 |   X  | 20 |   XX  | 30 |   XXX  |

## Usage

```console
student@ubuntu:~/[[ROOT]]/test$ go build
student@ubuntu:~/[[ROOT]]/test$ ./rn hello
ERROR: can not convert to roman digit
student@ubuntu:~/[[ROOT]]/test$ ./rn 123
C+X+X+I+I+I
CXXIII
student@ubuntu:~/[[ROOT]]/test$ ./rn 999
(M-C)+(C-X)+(X-I)
CMXCIX
student@ubuntu:~/[[ROOT]]/test$ ./rn 3999
M+M+M+(M-C)+(C-X)+(X-I)
MMMCMXCIX
student@ubuntu:~/[[ROOT]]/test$ ./rn 4000
ERROR: can not convert to roman digit
student@ubuntu:~/[[ROOT]]/test$
```