How to implement column self-naming from its index

Question

I wish to be able to instantiate my Cell class while naming the cell instance with such name as "A", "B", "C", etc. just like in an Excel spreadsheet.

I have my Cell class like so:

public class Cell {
    public Cell(Range nativeCell) {
        NativeCell = nativeCell;
    }

    public Range NativeCell { get; private set; }
}

And my Sheet class:

public class Sheet {
    private IDictionary<string, Cell> _cells;

    public Sheet(Worksheet nativeSheet) {
        NativeSheet = nativeSheet;

        _cells = new Dictionary<string, Cell>();

        for (int rowIndex = 1; rowIndex <= NativeSheet.Rows.Count; ++rowIndex)
            for (int colIndex = 1; colIndex <= NativeSheet.Columns.Count; ++colIndex) {
                ICell newCell = new Cell(NativeSheet.Cells(rowIndex, colIndex));
                newCell.Name = ?? // This name should look like "A1", "B1", "AA3", "CB20", etc.
                Cells.Add(newCell.Name, newCell);
            }
    }

    public IDictionary<string, Cell> Cells {
        get {
            return _cells;
        }
    }

    public Worksheet NativeSheet { get; private set; }
}

I would need to generate a name based on the alphabetic letters and double and triple them once I encounter the last alphabet letter 'Z'. The algorithm would have to generate the letters that I would concatenate with the rowIndex value that would result to this naming strategy such as Excel.

The letters would be:

A, B, C, D...Z, AA, AB, AC...AZ, BA, BB, BC...BZ, CA...XAA, XAB, XAC...

While we clearly know that colIndex value 1 will definitely designate column "A", value 2 = "B", value 3 = "C", etc.

My problem is particularly when we double the letters.

Do you have any idea on how I could achieve this in the simplest possible form?

Thanks! =)

Answer 1

This function will do it for you. It is in VB.NET but I trust you'll be able to port it to C# if need be.
I have updated the answer with the C# version of the function.

VB.NET

''' <summary>Returns the Excel-style name of the column from the column index.</summary>
''' <param name="colIndex">The column index.</param>
Function GetColumnName(ByVal colIndex As Integer) As String
    If colIndex < 1 Then Throw New ArgumentException("Column number must be greater or equal to 1.")

    Dim result As New List(Of String)

    'letter codes start at Chr(65)'
    Do While colIndex > 0
        'reduce the column number by 1 else the 26th column (Z) will become 0 (@) '
        'add 65 to the result and find the Chr() value.                           '
        'insert the character at position 0 of the character list                 '
        'integer divide by 26 to remove the column from the stack and repeat till '
        'there are no columns in the stack.                                       '
        result.Insert(0, Chr(65 + CInt((colIndex - 1) Mod 26)))
        colIndex = (colIndex - 1) \ 26
    Loop

    Return String.Join("", result.ToArray)
End Function

C#

/// <summary>Returns the Excel-style name of the column from the column index.</summary>
/// <param name="colIndex">The column index.</param>
static string GetColumnName(int colIndex)
{
    if (colIndex < 1)
        throw new ArgumentException("Column number must be greater or equal to 1.");

    var result = new List<char>();

    //letter codes start at Chr(65)'
    while (colIndex > 0)
    {
        //reduce the column number by 1 else the 26th column (Z) will become 0 (@) 
        //add 65 to the result and find the Chr() value.
        //insert the character at position 0 of the char list
        //integer divide the column index by 26 to remove the last calculated column 
        //from the stack and repeat till  there are no columns in the stack.                                       
        result.Insert(0, Microsoft.VisualBasic.Strings.Chr(65 + Convert.ToInt32((colIndex - 1) % 26)));
        colIndex = (int)((colIndex-1)/ 26);
    }

    return new string(result.ToArray());
}

I tested this up to column index 1000 and it worked without fail. I hope you find it useful.

Answer 2

Here is this. Translate a column index into an Excel Column Name

Shouldn't be to hard to make it recursive and give you exactly what you need. I hope this helps.