Sum columns in a new column next to the selection - vba

Question

I'm trying to sum a number of columns together in a new column.
I have been able to get to the point where I take A+B and place the values in C. However, the actual columns I will need to sum vary. Is there a way I can edit my code so that any selected columns can be summed in a new column to the right of the selection?
For example. If I select columns B-D, it would insert a new column in E that houses the sums of columns B,C, and D. Or if I selected E-F, it would insert a new column in G that houses the sums of columns E and F.
Sub SumColumns()
Lastrow = ActiveSheet.Cells(Rows.Count, 1).End(xlUp).row
For i = 1 To Lastrow
Range("C" & i).Value = Range("A" & i).Value + Range("B" & i).Value
Next i
End Sub

Answer 1

Here is my (rather sloppy) solution:
Sub Test()
Dim col1 As String, col2 As String, lastrow As Long
col1 = Split(Selection.Address, "$")(1)
col2 = Split(Selection.Address, "$")(3)
lastrow = Cells(Rows.Count, col2).End(xlUp).Row
Columns(col2 & ":" & col2).Offset(0, 1).Insert Shift:=xlToRight
For i = 1 To lastrow
Range(col2 & i).Offset(0, 1).Value = WorksheetFunction.Sum(Range(col1 & i & ":" & col2 & i))
Next i
End Sub
I say this is sloppy, because if you have the entire column selected, you won't Split out the column values appropriately. So it depends on how you're trying to do this.

Answer 2

This procedure will let you select any range. It will add a column at the end of the range and sum each row to the new column.
Sub test()
Call InsertSumCol(Sheet1.Range("B2:E4"))
Call InsertSumCol(Sheet2.Range("E1:F3"))
End Sub
Private Sub InsertSumCol(ByVal oRange As Range)
'Add Sum Column to end of Range
oRange.Worksheet.Columns(oRange.Column + oRange.Columns.Count).Insert shift:=xlToRight
' Sum Each Row in Range
Dim oRow As Range
For Each oRow In oRange.Rows
oRow.Cells(1, oRow.Columns.Count + 1) = WorksheetFunction.Sum(oRow)
Next
End Sub

Answer 3

I assume that you want to sum vertically all cells in a row starting from column A.
Try this (some tips and comments are in code):
'use this in order to avoid errors
Option Explicit
Sub SumColumns()
'always declare variables!
Dim LastRow As Long, i As Long, ws As Worksheet, lastCol As Long, firstCol As Long
'if you can, avoid using ActiveSheet, Selection, etc. it's prone to errors
Set ws = ActiveSheet
i = 1
Do
firstCol = ws.Cells(i, 1).End(xlToRight).Column
lastCol = ws.Cells(i, ws.Columns.Count).End(xlToLeft).Column
'there isn't any filled cells in a row
If lastCol = 1 Then Exit Do
ws.Cells(i, lastCol + 1).Value = Application.WorksheetFunction.Sum(ws.Range(ws.Cells(i, firstCol), ws.Cells(i, lastCol)))
i = i + 1
Loop While True
End Sub
Before and after: