Fastest function to generate Excel column letters in C#

Question

What is the fastest c# function that takes and int and returns a string containing a letter or letters for use in an Excel function? For example, 1 returns \"A\", 26 return

Answer 1

Try this function.

// Returns name of column for specified 0-based index.
public static string GetColumnName(int index)
{
    var name = new char[3]; // Assumes 3-letter column name max.
    int rem = index;
    int div = 17576; // 26 ^ 3

    for (int i = 2; i >= 0; i++)
    {
        name[i] = alphabet[rem / div];
        rem %= div;
        div /= 26;
    }

    if (index >= 676)
        return new string(name, 3);
    else if (index >= 26)
        return new string(name, 2);
    else
        return new string(name, 1);
}

Now it shouldn't take up that much memory to pre-generate each column name for every index and store them in a single huge array, so you shouldn't need to look up the name for any column twice.

If I can think of any further optimisations, I'll add them later, but I believe this function should be pretty quick, and I doubt you even need this sort of speed if you do the pre-generation.