Custom number system in C#

Question

I have a requirement for a custom number system in C# which goes as following:

A - 1
B - 2
...
Z - 26
AA - 27
AB - 28

I've made a function that converts from arbitrary strings to numbers like this:

    private const int Min = 'A';
    private const int Max = 'Z';
    private const int Base = Max - Min + 1;

    private static int GetCharValue(char c)
    {
        if (c < Min || c > Max)
            throw new ArgumentOutOfRangeException(nameof(c), c, $"Character needs to be between '{Min}' and '{Max}', was '{c}'.");

        return c - Min + 1;
    }

    public static int GetStringValue(string s)
    {
        char[] chars = s.ToCharArray();
        int[] values = new int[chars.Length];
        for (var i = 0; i < chars.Length; i++)
        {
            values[i] = GetCharValue(chars[i]);
        }

        int position = 1;
        int value = 0;
        for (var i = values.Length - 1; i >= 0; i--)
        {
            value += position * values[i];
            position *= Base;
        }

        return value;
    }

I've tested it to be working for up to AAA (not rigorously, just skimming over the output of printing them all). However, I can't for the life of me figure out how to write the reverse function. In other words, I need 1 to return A, 26 to return Z and 27 to return AA. The "problem" is that this number system has no 0, so it doesn't easily convert to any base. For instance, if A was 0, then AA would also be 0, but it's not. So how do I solve this?

Answer 1

you can simply generate it like this....

    public static IEnumerable<string> generate()
    {
        long n = -1;
        while (true) yield return toBase26(++n);
    }

    public static string toBase26(long i)
    {
        if (i == 0) return ""; i--;
        return toBase26(i / 26) + (char)('A' + i % 26);
    }



    public static void BuildQuery()
    {
        IEnumerable<string> lstExcelCols = generate();
        try
        {

            string s = lstExcelCols.ElementAtOrDefault(1) ;
        }
        catch (Exception exc)
        {

        }


    }