12.c

char *getOne(char c)
{
    switch (c)
    {
    case '9':
        return "IX";

    case '8':
        return "VIII";

    case '7':
        return "VII";

    case '6':
        return "VI";

    case '5':
        return "V";

    case '4':
        return "IV";

    case '3':
        return "III";

    case '2':
        return "II";

    case '1':
        return "I";

    case '0':
        return "";

    default:
        return NULL;
    }
}

char *getTen(char c)
{
    switch (c)
    {
    case '9':
        return "XC";

    case '8':
        return "LXXX";

    case '7':
        return "LXX";

    case '6':
        return "LX";

    case '5':
        return "L";

    case '4':
        return "XL";

    case '3':
        return "XXX";

    case '2':
        return "XX";

    case '1':
        return "X";

    case '0':
        return "";

    default:
        return NULL;
    }
}

char *getHundred(char c)
{
    switch (c)
    {
    case '9':
        return "CM";

    case '8':
        return "DCCC";

    case '7':
        return "DCC";

    case '6':
        return "DC";

    case '5':
        return "D";

    case '4':
        return "CD";

    case '3':
        return "CCC";

    case '2':
        return "CC";

    case '1':
        return "C";

    case '0':
        return "";

    default:
        return NULL;
    }
}

char *getThousand(char c)
{
    switch (c)
    {
    case '3':
        return "MMM";

    case '2':
        return "MM";

    case '1':
        return "M";

    default:
        return NULL;
    }
}

char *intToRoman(int num)
{
    int length;
    char number[5];
    char *s = malloc(16 * sizeof(char));

    sprintf(number, "%i", num);

    length = strlen(number);

    switch (length)
    {
    case 4:
        sprintf(s, "%s%s%s%s", getThousand(number[0]), getHundred(number[1]),
                getTen(number[2]), getOne(number[3]));
        break;

    case 3:
        sprintf(s, "%s%s%s", getHundred(number[0]), getTen(number[1]),
                getOne(number[2]));

        break;

    case 2:
        sprintf(s, "%s%s", getTen(number[0]), getOne(number[1]));

        break;

    case 1:
        s = getOne(number[0]);
        break;

    default:
        break;
    }
    return s;
}