VBA column looping - vba

Question

I have a large Excel file and I need to replace all values in 12 columns completely.
Right now, there is a formula in each one of the cells, and I need to replace that formula with my own.
How do I loop through all those columns, knowing at what row it starts but don't know the end row (file is updated constantly). The hack of "A600000" seems overkill.
I am new to VBA and some guidance would be really appreciated.

Answer 1

ActiveSheet.UsedRange is the range of all the used cells on the current sheet.
You can use ActiveSheet.UsedRange.Rows.Count and .Columns.Count to get the height and widht of this range.
Here's a very crude function that hits every cell in the range:
Sub test()
Dim thisRange As Range
Set thisRange = ActiveSheet.UsedRange
With thisRange
For y = 1 To .Rows.Count
For x = 1 To .Columns.Count
thisRange.Cells(y, x).Value = "Formula here"
Next x
Next
End With
End Sub
But what you want may be different, can you be more specific?

Answer 2

The below will accomplish what you need to do. You just need to supply the startRow, .Sheets("Name"), and i arguments. If the columns are all the same length, then UsedRange will work fine if there are not random cells with values outside and below the columns you are interested in. Otherwise, try this in your code (on a throw away copy of your workbook)
Sub GetLastRowInColumn()
Dim ws as Excel.Worksheet
Set ws = Activeworkbook.Sheets("YOURSHEETNAMEHERE")
Dim startRow as long
startRow = 1
Dim lastRow as long
Dim i as long
For i = 1 to 12 'Column 1 to Column 12 (Adjust Accordingly)
lRow = ws.Cells(ws.Rows.Count, i).End(xlUp).Row
ws.Range(ws.Cells(startRow, i), ws.Cells(lRow, i)).Formula = "=Max(1)" 'Sample Formula
Next
End Sub

Answer 3

EDIT : Fixed typo
The below function will build the range with varying length columns. Use the function to return the desired range and fill all related cells in one shot.
Function GetVariantColumnRange(MySheet As Excel.Worksheet, _
TopRow As Long, StartColumn As Long, LastColumn As Long) As Excel.Range
Dim topAddress As String
Dim bottomAddress As String
Dim addressString As String
Dim i As Long
For i = StartColumn To LastColumn
topAddress = MySheet.Cells(TopRow, i).Address
bottomAddress = MySheet.Cells(MySheet.Rows.Count, i).End(xlUp).Address
addressString = addressString & ", " & topAddress & ":" & bottomAddress
Next
addressString = Right(addressString, Len(addressString) - _
InStr(1, addressString, ", ", vbBinaryCompare))
Set GetVariantColumnRange = MySheet.Range(addressString)
End Function
Usage follows...
Sub Test()
Dim myrange As Range
Set myrange = GetVariantColumnRange(ThisWorkbook.Sheets(1), 1, 1, 12)
myrange.Select 'Just a visual aid. Remove from final code.
myrange.Formula = "=APF($Jxx, "string1", "string2") "
End Sub