十进制到罗马数字转换器-输出不正确的输出没有明显的原因

程序在接收到"1984"输入时，应该输出"MCMLXXXIV"。它正在输出"MDCCLXXIV"。"

代码似乎是正确的思想。我已经想过很多很多次了，但我不知道我做错了什么。也不知道如何让输入失败捕获工作…它被注释掉了…任何帮助都太好了!

    #include <iostream>
    #include <iomanip>
    #include <string>
    using namespace std;

    string convert(int, string, string, string);

    int main()
    {
        int userNum = 0;
        const int MIN_NUM = 0;
        const int MAX_NUM = 3999;
        const int ARRAY_SIZE = 4;
        string answers[ARRAY_SIZE] = { "", "", "", "", };
        cout << "Enter a negative number to end the program.n";
        cout << "Enter an arabic number between 1 and 3999: ";
        cin >> userNum;
        while (userNum <= MIN_NUM || userNum >= MAX_NUM)
        {
            cout << "nInvalid Value. Number must be between 1 and 3999: ";
            cin >> userNum;
        }
        if (userNum < 0)
        {
            cout << "Program ending due to user request.n";
            EXIT_SUCCESS;
        }


       /*{
            cout << "enter an integer: ";
            if (cin >> num)
                      cout << "num: " << num << endl; }
            else 
            {
                   cin.clear(); 
                   cin.ignore(1000, 'n'); 
                   cout << "Bad data detected..." << endl;
                   system("pause");
                   system("cls");
                 }
            }
                     return 0;
            } */
        int thous = userNum / 1000;   // Extract digits and store as digit 
        cout << thous;                // also printing them out 
        int hund = userNum % 1000 / 100;
        cout << "hundreds:" << hund;
        int tens = userNum % 100 / 10;
        cout << "tens:" << tens;
        int ones = userNum % 10 / 1;
        cout << "Ones: " << ones << endl;
    /* Attempting to make four calls to a function named convert
       with returns a string and accepts arguments of the digit 
       from above extraction and the low, mid, and high, 
       numeral for that place (i.e. ones, tens, hundreds,
       attempting to store as a string. */
        answers[0] = convert(thous, "M", "M", "M");
        answers[1] = convert(hund, "C", "D", "M");
        answers[2] = convert(tens, "X", "L", "C");
        answers[3] = convert(ones, "I", "V", "X");
        cout << answers[0] << endl << answers[1] << endl << answers[2];
        cout << endl <<  answers[3] << endl;
        system("PAUSE");
        return 0;
    }
    string convert(int digit, string low, string mid, string high)
    {
        cout << digit << endl;
        if (digit == 1)
       {
            return low;
        }
        if (digit == 2)
        {
            return low + low;
        }
        if (digit == 3)
        {
            return low + low + low; 
        }
        if (digit == 4) 
         {
            return low + mid;
        }
        if (digit == 5)
        {
            return mid;
        }
        if (digit == 6)
        {
            return mid + low;
        }
        if (digit == 7);
        {
            return mid + low + low;
        }
        if (digit == 8);
        {
            return mid + low + low + low;
        }
        if (digit == 9)
        {
            return low + high;
        }
        if (digit == 0)
        {
            return "";
        } 
      }

当用户输入一个负数时，整个程序也应该结束。我在想，最好把整个事情在一个do - while循环，做int main，而userNum> 0??这能行吗?

任何帮助都非常感谢!谢谢各位。