Increment an index that uses numbers and characters (aka Base36 numbers)

I have a string based code that can be either two or three characters in length and I am looking for some help in creating a function that will increment it.

Each 'digi开发者_如何学Ct' of the code has a value of 0 to 9 and A to Z.

some examples:

the first code in the sequence is 000

009 - next code is - 00A

00D - next code is - 00E

AAZ - next code is - AB0

the last code is ZZZ.

Hope this makes some sense.

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Thanks for the advice guys.

This is what I independently came up with.

    private static String Increment(String s)
    {
        String chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

        char lastChar = s[s.Length - 1];
        string fragment = s.Substring(0, s.Length - 1);

        if (chars.IndexOf(lastChar) < 35)
        {
            lastChar = chars[chars.IndexOf(lastChar) + 1];

            return fragment + lastChar;
        }

        return Increment(fragment) + '0';
    }

I don't know if it is better/worse but seems to work. If anyone can suggest improvements then that is great.

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Maintain the counter as an int and increment this. Convert the int to your character representation by modding and dividing by 36 iterativly. Map the modded range (0-35) to 0-Z.

Example

Updated with functions to go in either direction:

internal class Program
{
    const int Base = 36;

    public static void Main()
    {
        Console.WriteLine(ToInt("0AA"));
        Console.WriteLine(ToString(370));
    }

    private static string ToString(int counter)
    {
        List<char> chars = new List<char>();

        do
        {
            int c = (counter % Base);

            char ascii = (char)(c + (c < 10 ? 48 : 55));

            chars.Add(ascii);
        }
        while ((counter /= Base) != 0);

        chars.Reverse();

        string charCounter = new string(chars.ToArray()).PadLeft(3, '0');

        return charCounter;
    }

    private static int ToInt(string charCounter)
    {
        var chars = charCounter.ToCharArray();

        int counter = 0;

        for (int i = (chars.Length - 1), j = 0; i >= 0; i--, j++)
        {
            int chr = chars[i];

            int value = (chr - (chr > 57 ? 55 : 48)) * (int)Math.Pow(Base, j);

            counter += value;
        }

        return counter;
    }

For more variants of conversion code see Quickest way to convert a base 10 number to any base in .NET?.

---

Does this do what you need?

public class LetterCounter
{
    private static readonly string[] _charactersByIndex = new string[] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };

    public string GetStr(int i)
    {
        if (i < _charactersByIndex.Length)
            return _charactersByIndex[i];

        int x = i / (_charactersByIndex.Length - 1) - 1;
        string a = _charactersByIndex[x];
        string b = GetStr(i - (_charactersByIndex.Length - 1));
        return a + b;
    }
}

}

---

Based on @Martin answer, I found some error when two ZZ comes, this makes exception in code

private static String Increment(String s,bool IsFromRecursion=false)
{
    String chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    //Added this condition
    if (IsFromRecursion && string.IsNullOrEmpty(number))
    {
         return "1";
    }
    //Added this condition

    char lastChar = s[s.Length - 1];
    string fragment = s.Substring(0, s.Length - 1);

    if (chars.IndexOf(lastChar) < 35)
    {
        lastChar = chars[chars.IndexOf(lastChar) + 1];

        return fragment + lastChar;
    }

    return Increment(fragment,true) + '0';
}

When we call this method we pass first parameter only.