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

_{I landed here searching for a small implementation of a Roman Numerals parser but wasn't satisfied by the provided answers in terms of size and elegance. I leave my final, recursive implementation here, to help others searching a small implementation.}

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Convert Roman Numerals by Recursion

• The algorithm is able to non-adjacent numerals as well (f.e. XIIX).
• This implementation may only work with well-formed (strings matching /[mdclxvi]*/i) roman numerals.
• The implementation is not optimized for speed.

// returns the value for a roman literal
private static int romanValue(int index)
{
    int basefactor = ((index % 2) * 4 + 1); // either 1 or 5...
    // ...multiplied with the exponentation of 10, if the literal is `x` or higher
    return index > 1 ? (int) (basefactor * System.Math.Pow(10.0, index / 2)) : basefactor;
}

public static int FromRoman(string roman)
{
    roman = roman.ToLower();
    string literals = "mdclxvi";
    int value = 0, index = 0;
    foreach (char literal in literals)
    {
        value = romanValue(literals.Length - literals.IndexOf(literal) - 1);
        index = roman.IndexOf(literal);
        if (index > -1)
            return FromRoman(roman.Substring(index + 1)) + (index > 0 ? value - FromRoman(roman.Substring(0, index)) : value);
    }
    return 0;
}

Try it using this .Netfiddle: https://dotnetfiddle.net/veaNk3

How does it work?

This algorithm calculates the value of a Roman Numeral by taking the highest value from the Roman Numeral and adding/subtracting recursively the value of the remaining left/right parts of the literal.

ii X iiv # Pick the greatest value in the literal `iixiiv` (symbolized by uppercase)

Then recursively reevaluate and subtract the lefthand-side and add the righthand-side:

(iiv) + x - (ii) # Subtract the lefthand-side, add the righthand-side
(V - (ii)) + x - ((I) + i) # Pick the greatest values, again
(v - ((I) + i)) + x - ((i) + i) # Pick the greatest value of the last numeral compound

Finally the numerals are substituted by their integer values:

(5 - ((1) + 1)) + 10 - ((1) + 1)
(5 - (2)) + 10 - (2)
3 + 10 - 2
= 11