Wendy has a worksheet that has quite a bit of data in it, with the main data in the range C3:P312. She would like to have a macro that, if she clicks a cell in this data range, would put an "x" into the cell.

There is no event that Excel can recognize as a "click" on a cell. Perhaps the closest event is the SelectionChange event, which is triggered every time the cell selection changes. The event handler could then check to make sure that the cell selection is within the C3:P312 range, and then place an "x" in the cell if it is. The following event handler will do that:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Dim rInt As Range
    Dim rCell As Range

    Set rInt = Intersect(Target, Range("C3:P312"))
    If Not rInt Is Nothing Then
        For Each rCell In rInt
            rCell.Value = "x"
        Next
    End If
    Set rInt = Nothing
    Set rCell = Nothing
End Sub

There is a problem with this approach, however: Not only will the SelectionChange event trigger when you click on a different cell, it also triggers if you use the keyboard to move from one cell to another in the worksheet. This means that if you use the keyboard to move about the worksheet you will leave a tail of "x" characters in each cell you transit.

One way around this is to change the event that triggers the check and change of the cells. While Excel has no "click" event, there is a "double click" event. This means that you can change the cell on which you double click, as shown here:

Private Sub Worksheet_BeforeDoubleClick( _
            ByVal Target As Range, Cancel As Boolean)
    Dim rInt As Range
    Dim rCell As Range

    Set rInt = Intersect(Target, Range("C3:P312"))
    If Not rInt Is Nothing Then
        For Each rCell In rInt
            rCell.Value = "x"
        Next
    End If
    Set rInt = Nothing
    Set rCell = Nothing
    Cancel = True
End Sub