VBA - Excel Append Every Active Row With File Name - excel

Question

I am new to VBA so bear with me. I would like to append in the last cell of each active row the filename. So for example Row/Column A1, B1, C1, and D1 are populated I would like to add the filename to cell E1. The filename should only be appended to active rows. I have played around with different iterations without much luck. Below is what I have so far and the logic is clearly incorrect. Any help would be appreciated. Thanks!
Sub InsertFilename()
Dim Count1 As Long
Count1 = 1
Dim ColumnE As String
ColumnE = "E1"
While Cells(Count1, 1) <> ""
Range(ColumnE).Select
ActiveCell.FormulaR1C1 = _
"=MID(CELL(""filename""),SEARCH(""["",CELL(""filename""))+1, SEARCH(""]"",CELL(""filename""))-SEARCH(""["",CELL(""filename""))-1)"
ColumnE = Range(ActiveCell, ActiveCell.Offset(1, 0)).Select
Count1 = Count1 + 1
Wend
End Sub

Answer 1

This code iterates from the first row to the last, and it finds the last column in each row by imitating the CTRL+LEFT from the edge of the sheet.
It does not assume that all the rows have the same number of columns
Dim LastRow As Long
Dim LastColumn As Long
Sub InsertFileName()
Application.ScreenUpdating = False
Dim i as Long
LastRow = ActiveSheet.Cells(ActiveSheet.Rows.Count, "A").End(xlUp).Row
For i=1 To LastRow
LastColumn = ActiveSheet.Cells(i, ActiveSheet.Columns.Count).End(xlToLeft).Column
ActiveSheet.Cells(i,LastColumn+1)="=CELL(""filename"")"
Next i
Application.ScreenUpdating = True
End Sub

Answer 2

A simple solution. Add your values for fileName to be appended and the start row stRow of your data.
Sub InsertFilename()
Dim stRow As Long, endRow As Long, endCol As Long, c As Long
Dim fileName As String
fileName = "C:\Data\somefile.xlsx"
stRow = 1
With ActiveSheet
endRow = .Cells(Rows.Count, 1).End(xlUp).Row
For c = stRow To endRow
endCol = .Cells(c, Columns.Count).End(xlToLeft).Column
If endCol > 1 Then
.Cells(c, endCol + 1) = fileName
End If
Next
End With
End Sub

Answer 3

This solution tests for activity using Counta and uses each active row's last column to use the activeworkbook's fullname.
Dim lngLastRow As Long
Dim lngLastCol As Long
Dim lngCounter As Long
lngLastRow = Cells.Find(What:="*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
lngLastCol = Cells.Find(What:="*", SearchOrder:=xlByColumns, SearchDirection:=xlPrevious).Column
For lngCounter = 1 To lngLastRow
If WorksheetFunction.CountA(Range(Cells(lngCounter, 1), Cells(lngCounter, lngLastCol))) > 0 Then
Cells(lngCounter, lngLastCol + 1).End(xlToLeft).Offset(0, 1).Value = ActiveWorkbook.FullName
End If
Next lngCounter