Given any number output the roman numeral representation.

roman_numerals.erl

-module(roman_numerals).
-compile(export_all).

% dec_to_roman/1 : integer -> string
%   format the integer num using roman numerals
decimal_to_roman(Num) ->
    decimal_to_roman(Num, "",
        [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1],
        ["M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"]).

% dec_to_roman/4 : integer string [t number) (list string) -> string
%   append the numbers num formatted as roman numerals to the string s,
%   using the roman numerals in roman-numbers with corresponding
%   decimal values in decimal-values
decimal_to_roman(0, Accum, _Decimals, _Romans) ->
    Accum;
decimal_to_roman(Num, Accum, Decimals, Romans) ->
    F_el = hd(Decimals),
    if
        F_el =< Num ->
            decimal_to_roman(Num - F_el, Accum ++ hd(Romans), Decimals, Romans);
        F_el > Num ->
            decimal_to_roman(Num, Accum, tl(Decimals), tl(Romans))
    end.

% roman_to_decimal : string -> integer
%  convert a string with (uppercase) roman numerals to an integer
roman_to_decimal("") -> 0;
roman_to_decimal(Str) ->
    case length(Str) of
        1 ->
            case list_to_atom(Str) of
                'I' ->    1;
                'V' ->    5;
                'X' ->   10;
                'L' ->   50;
                'C' ->  100;
                'D' ->  500;
                'M' -> 1000;
                _   -> 0
            end;
        _ ->
            roman_to_decimal(string:substr(Str, 3))
                + case list_to_atom(string:substr(Str, 1, 2)) of
                      'IV' ->   4;
                      'IX' ->   9;
                      'XL' ->  40;
                      'XC' ->  90;
                      'CD' -> 400;
                      'CM' -> 900;
                      _    ->
                          roman_to_decimal(string:substr(Str, 1, 1))
                              + roman_to_decimal(string:substr(Str, 2, 1))
                  end
    end.