Converting integers to roman numerals

Question

I\'m trying to write a function that converts numbers to roman numerals. This is my code so far; however, it only works with numbers that are less than 400. Is there a quick

Answer 1

I've created this class that does decimal <=> roman

public static class Roman
{
    public static readonly Dictionary RomanNumberDictionary;
    public static readonly Dictionary NumberRomanDictionary;

    static Roman()
    {
        RomanNumberDictionary = new Dictionary
        {
            { 'I', 1 },
            { 'V', 5 },
            { 'X', 10 },
            { 'L', 50 },
            { 'C', 100 },
            { 'D', 500 },
            { 'M', 1000 },
        };

        NumberRomanDictionary = new Dictionary
        {
            { 1000, "M" },
            { 900, "CM" },
            { 500, "D" },
            { 400, "CD" },
            { 100, "C" },
            { 90, "XC" },
            { 50, "L" },
            { 40, "XL" },
            { 10, "X" },
            { 9, "IX" },
            { 5, "V" },
            { 4, "IV" },
            { 1, "I" },
        };
    }

    public static string To(int number)
    {
        var roman = new StringBuilder();

        foreach (var item in NumberRomanDictionary)
        {
            while (number >= item.Key)
            {
                roman.Append(item.Value);
                number -= item.Key;
            }
        }

        return roman.ToString();
    }

    public static int From(string roman)
    {
        int total = 0;

        int current, previous = 0;
        char currentRoman, previousRoman = '\0';

        for (int i = 0; i < roman.Length; i++)
        {
            currentRoman = roman[i];

            previous = previousRoman != '\0' ? RomanNumberDictionary[previousRoman] : '\0';
            current = RomanNumberDictionary[currentRoman];

            if (previous != 0 && current > previous)
            {
                total = total - (2 * previous) + current;
            }
            else
            {
                total += current;
            }

            previousRoman = currentRoman;
        }

        return total;
    }
}

Some Unit Tests for To method:

[TestClass]
public class DecimalToRomanTest
{
    [TestMethod]
    public void Roman_1_I()
    {
        Assert.AreEqual("I", Roman.To(1));
    }

    [TestMethod]
    public void Roman_2_II()
    {
        Assert.AreEqual("II", Roman.To(2));
    }

    [TestMethod]
    public void Roman_3_III()
    {
        Assert.AreEqual("III", Roman.To(3));
    }

    [TestMethod]
    public void Roman_4_IV()
    {
        Assert.AreEqual("IV", Roman.To(4));
    }

    [TestMethod]
    public void Roman_5_V()
    {
        Assert.AreEqual("V", Roman.To(5));
    }

    [TestMethod]
    public void Roman_9_IX()
    {
        Assert.AreEqual("IX", Roman.To(9));
    }

    [TestMethod]
    public void Roman_10_X()
    {
        Assert.AreEqual("X", Roman.To(10));
    }

    [TestMethod]
    public void Roman_49_XLIX()
    {
        Assert.AreEqual("XLIX", Roman.To(49));
    }

    [TestMethod]
    public void Roman_50_L()
    {
        Assert.AreEqual("L", Roman.To(50));
    }

    [TestMethod]
    public void Roman_100_C()
    {
        Assert.AreEqual("C", Roman.To(100));
    }

    [TestMethod]
    public void Roman_400_CD()
    {
        Assert.AreEqual("CD", Roman.To(400));
    }

    [TestMethod]
    public void Roman_500_D()
    {
        Assert.AreEqual("D", Roman.To(500));
    }

    [TestMethod]
    public void Roman_900_CM()
    {
        Assert.AreEqual("CM", Roman.To(900));
    }

    [TestMethod]
    public void Roman_1000_M()
    {
        Assert.AreEqual("M", Roman.To(1000));
    }

    [TestMethod]
    public void Roman_11984_MMMMMMMMMMMCMLXXXIV()
    {
        Assert.AreEqual("MMMMMMMMMMMCMLXXXIV", Roman.To(11984));
    }
}

Some Unit Tests for From method:

[TestClass]
public class RomanToDecimalTest
{
    [TestMethod]
    public void Roman_I_1()
    {
        Assert.AreEqual(1, Roman.From("I"));
    }

    [TestMethod]
    public void Roman_II_2()
    {
        Assert.AreEqual(2, Roman.From("II"));
    }

    [TestMethod]
    public void Roman_III_3()
    {
        Assert.AreEqual(3, Roman.From("III"));
    }

    [TestMethod]
    public void Roman_IV_4()
    {
        Assert.AreEqual(4, Roman.From("IV"));
    }

    [TestMethod]
    public void Roman_V_5()
    {
        Assert.AreEqual(5, Roman.From("V"));
    }

    [TestMethod]
    public void Roman_IX_9()
    {
        Assert.AreEqual(9, Roman.From("IX"));
    }

    [TestMethod]
    public void Roman_X_10()
    {
        Assert.AreEqual(10, Roman.From("X"));
    }

    [TestMethod]
    public void Roman_XLIX_49()
    {
        Assert.AreEqual(49, Roman.From("XLIX"));
    }

    [TestMethod]
    public void Roman_L_50()
    {
        Assert.AreEqual(50, Roman.From("L"));
    }

    [TestMethod]
    public void Roman_C_100()
    {
        Assert.AreEqual(100, Roman.From("C"));
    }

    [TestMethod]
    public void Roman_CD_400()
    {
        Assert.AreEqual(400, Roman.From("CD"));
    }

    [TestMethod]
    public void Roman_D_500()
    {
        Assert.AreEqual(500, Roman.From("D"));
    }

    [TestMethod]
    public void Roman_CM_900()
    {
        Assert.AreEqual(900, Roman.From("CM"));
    }

    [TestMethod]
    public void Roman_M_1000()
    {
        Assert.AreEqual(1000, Roman.From("M"));
    }

    [TestMethod]
    public void Roman_MMMMMMMMMMMCMLXXXIV_11984()
    {
        Assert.AreEqual(11984, Roman.From("MMMMMMMMMMMCMLXXXIV"));
    }
}