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 number`.

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	 IX	  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 number
student@ubuntu:~/[[ROOT]]/test$
```