Roman numerals to integers

Question

I have a transfer with products that unfortunately has to get matched by product name. The biggest issue here is I might get duplicate products on account of roman numbers.

Answer 1

A more simple and readable C# implementation that:

• maps I to 1, V to 5, X to 10, L to 50, C to 100, D to 500, M to 1000.
• uses one single foreach loop (foreach used on purpose, with previous value hold).
• adds the mapped number to the total.
• subtracts twice the number added before, if I before V or X, X before L or C, C before D or M (not all chars are allowed here!).
• returns 0 (not used in Roman numerals) on empty string, wrong letter or not allowed char used for subtraction.
• remark: it's still not totally complete, we didn't check all possible conditions for a valid input string!

Code:

private static Dictionary _romanMap = new Dictionary
{
   {'I', 1}, {'V', 5}, {'X', 10}, {'L', 50}, {'C', 100}, {'D', 500}, {'M', 1000}
};

public static int ConvertRomanToNumber(string text)
{
    int totalValue = 0, prevValue = 0;
    foreach (var c in text)
    {
        if (!_romanMap.ContainsKey(c))
            return 0;
        var crtValue = _romanMap[c];
        totalValue += crtValue;
        if (prevValue != 0 && prevValue < crtValue)
        {
            if (prevValue == 1 && (crtValue == 5 || crtValue == 10)
                || prevValue == 10 && (crtValue == 50 || crtValue == 100)
                || prevValue == 100 && (crtValue == 500 || crtValue == 1000))
                totalValue -= 2 * prevValue;
            else
                return 0;
        }
        prevValue = crtValue;
    }
    return totalValue;
}