Trying to fill blanks with alpha characters based on number above - excel

Question

I have this.
3
3.1
3.2
3.3
3.4
4
4.1
NULL
NULL
NULL
NULL
5
I would like this.
3
3.1
3.2
3.3
3.4
4
4.1
4.A
4.B
4.C
4.D
5
I have about 22k rows to fill in. How can I do it? I put together the code below, bu tit doesn't do what I want. I need to start over with prior number + 'A' whenever a new blank cell is found.
This is what I have so far, but it doesn't work.
Sub AlphaFill()
Dim Cell, CellChars
Dim Default, Prompt, Title
Dim rangeSelected As Range
Dim UpperCase As Boolean
On Error Resume Next
Set rangeSelected = Range("F1:F21400")
For Each Cell In rangeSelected
If Cell.Value <> "" Then
i = 1
End If
If Cell.Value = "" Then
CellChars = Chr(64 + i)
If Not UpperCase Then CellChars = UCase(CellChars)
Cell.Value = Cell.Value & CellChars
i = i + 1
End If
Debug.Print Cell.Value
Next
End Sub
The problem is, I can't seem to preserve the prior cell, for instance, the 4.A, 4.B, 4.C, and 4.D

Answer 1

If I understand correctly then you need to store the last whole number you see then use that on the next blank cell, resetting the counter between gaps:
Sub AlphaFill()
Dim Cell As Range
Dim UpperCase As Boolean
Dim LastWholeNumber As String
Dim LastLetter As Long
Dim CurrentNumber As String
UpperCase = True
For Each Cell In Range("F1:F21400")
CurrentNumber = Cell.Value
If CurrentNumber = "" Then
LastLetter = LastLetter + 1
Cell.Value = LastWholeNumber & "." & ChrW$(LastLetter)
ElseIf InStr(CurrentNumber, ".") = 0 Then
'// whole number - store it & reset to A/a
LastLetter = IIf(UpperCase, 64, 97)
LastWholeNumber = CurrentNumber
End If
Next
End Sub

Answer 2

Oh, I got it. This works!!
Sub AlphaFill()
Dim Cell, CellChars
Dim Default, Prompt, Title
Dim rangeSelected As Range
Dim UpperCase As Boolean
On Error Resume Next
Set rangeSelected = Range("F1:F21400")
For Each Cell In rangeSelected
If Cell.Value <> "" Then
KeepValue = Cell.Value
i = 1
End If
If Cell.Value = "" Then
CellChars = Chr(64 + i)
If Not UpperCase Then CellChars = UCase(CellChars)
Cell.Value = KeepValue & CellChars
i = i + 1
End If
Next
End Sub

Answer 3

Assuming your data is as
Try this
Sub AlphaFill()
Dim Cell, CellChars
Dim Default, Prompt, Title
Dim rangeSelected As Range
Dim UpperCase As Boolean
Dim charIndex As Long
On Error Resume Next
Set rangeSelected = Range("F1:F21400")
UpperCase = True
charIndex = 65
For Each Cell In rangeSelected
If Cell.Value = "" Then
If InStr(Cell.Offset(-1, 0), ".") > 0 Then 'check if value contains "."
Cell.Value = Left(Cell.Offset(-1, 0), WorksheetFunction.Find(".", Cell.Offset(-1, 0))) & Chr(charIndex) 'extract the string till "."
Else
Cell.Value = Cell.Offset(-1, 0) & "." & Chr(charIndex) 'get the number and add "."
End If
charIndex = charIndex + 1
Else
If charIndex <> 65 Then charIndex = 65 'if cell is not blank set charIndex to 65
End If
Next
End Sub
This will give result as