VBA Excel Formula with Dynamic Range And Variable - vba

Question

I want to do a dynamic sum formula in VBA and it's some how very difficult for me because I don't use well integer variables.
the last row might change in the future and I need that the range will be dynamic.
thanks to those who will help me.
Sub SumColumns()
Sheets("data").Select
Range("A1").End(xlDown).Offset(1, 0).Select
Selection.Value = "sum"
Selection.Interior.ColorIndex = 33
Selection.Font.Bold = True
Dim LastCol As Integer
Dim LastRow As Integer
With Sheets("data")
LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
LastCol = .Cells(1, .Columns.Count).End(xlToLeft).Column
End With
Range("A1").End(xlDown).Offset(0, 1).Select
ActiveCell.FormulaR1C1 = "=SUM(R[- " & LastRow & " + 1]C:R[-1]C)"
Selection.AutoFill Destination:=Range("B" & LastRow, "I" & LastRow), Type:=xlFillDefault
End Sub
that is the line with the error:
ActiveCell.FormulaR1C1 = "=SUM(R[- " & LastRow & " + 1]C:R[-1]C)"

Answer 1

Take the + 1 out of the quotes as that seems to be causing the problem and you need to deduct 1 otherwise you will be on row zero. The code below also removes your selects which are unnecessary and inefficient. And use your LastCol variable to determine across how many columns to copy the formula.
Sub SumColumns()
Dim LastCol As Long 'use Long rather than Integer
Dim LastRow As Long
With Sheets("data")
LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
With .Range("A" & LastRow + 1)
.Value = "sum"
.Interior.ColorIndex = 33
.Font.Bold = True
End With
LastCol = .Cells(1, .Columns.Count).End(xlToLeft).Column
.Range("B" & LastRow + 1).Resize(, LastCol - 1).FormulaR1C1 = "=SUM(R[-" & LastRow - 1 & "]C:R[-1]C)"
End With
End Sub

Answer 2

You can get rid of many select portions and steam line code like below. Test it and see if this is what you are after.
Sub SumColumns()
Dim LastCol As Long
Dim LastRow As Long
With Sheets("data")
LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
LastCol = .Cells(1, .Columns.Count).End(xlToLeft).Column
With .Range("A" & LastRow).Offset(1, 0)
.Value = "SUM"
.Interior.ColorIndex = 33
.Font.Bold = True
End With
LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
.Range("A" & LastRow).Offset(0, 1).FormulaR1C1 = "=SUM(R[-" & LastRow - 1 & "]C:R[-1]C)"
.Range("A" & LastRow).Offset(0, 1).AutoFill Destination:=.Range("B" & LastRow, .Cells(LastRow, LastCol)), Type:=xlFillDefault
.Range("A" & LastRow, .Cells(LastRow, LastCol)).Borders.LineStyle = xlContinuous
.Range("A" & LastRow, .Cells(LastRow, LastCol)).Borders.Weight = xlThin
End With
End Sub