RomanConvert.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "roman_converter.h"

char *hundreds[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
char *tens[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
char *ones[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};

/*
 * Function: ConvertToRoman
 * ----------------------------
 *   Convert Arabic number to Roman number.
 *   number: Arabic number.
 *   returns: Roman number.
 */
char* ConvertToRoman(int number) {

    if (number <= 0 || number >= 4999)
        return NULL;
    else {
        char roman[12] = "";
        char* thousands[] = {"", "M", "MM", "MMM", "MMMM"};
        int thousand, hundred, ten, one;
        one = number % 10;
        ten = (number / 10) % 10;
        hundred = (number / 100) % 10;
        thousand = (number / 1000) % 10;

        strcat(roman, thousands[thousand]);

        strcat(roman, hundreds[hundred]);
        strcat(roman, tens[ten]);
        strcat(roman, ones[one]);
//        return roman;
        char* Return = malloc(sizeof (char)*12);
        strcpy(Return, strcat);
        return Return;
    }

}

int main() {
    printf("========== Arabic-Roman numerals converter ==========\n");
    printf("This program is about to convert a number from Arabic to Roman.\n");
    printf("(number must be an integer, larger than 0 and smaller than 4000\nEnter number: ");
    int number;
    scanf("%d", &number);
    char* roman = ConvertToRoman(number);
    if (roman == NULL)
        printf("Invalid input!\n");
    else
        printf("Value of Roman number is: %s", roman);
}

RomanConvert.h
#ifndef ROMAN_CONVERTER_H
#define	ROMAN_CONVERTER_H

char* ConvertToRoman(int number);

#endif	/* ROMAN_CONVERTER_H */