How to end infinite "change" loop in VBA - vba

Question

I have a problem with visual basic. I want to make a macro/function that will multiply a number I enter by 3 and give a result in the same cell. I tried something like this:
Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$Q$21" Then
Target.Value = Target.Value * 3
End If
End Sub
but it doesn't work - I'm getting results like "xE25" because it keeps multiplying.
I'd like it to stop after first iteration or work only when I press "enter" not with every change in the cell.
It's quite easy to put a result in different cell, but it's not my point.-----Edit:
I edited "If" line to :
If (Target.Column = 5 Or Target.Column = 11 Or Target.Column = 17 Or Target.Column = 23) And (Target.Row >= 19 And Target.Row <= 24) And Target.Value <> "" Then so it would work on all cells that I need. After that, the best solution is the way given by #Chrismas007, because it doesn't prompt an error when trying to delete data in few cells at once.

Answer 1

With error handling to ensure .EnableEvents goes back to True:
Sub Worksheet_Change(ByVal Target As Range)
On Error GoTo CleanExit
If Target.Address = "$Q$21" Then
Application.EnableEvents = False
Target.Value = Target.Value * 3
End If
CleanExit:
Application.EnableEvents = True
On Error GoTo 0
End Sub

Answer 2

When a Worksheet_Change event macro changes a value, you need to set Application.EnableEvents to false or risk triggering another event and having the macro run on top of itself.
Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$Q$21" Then
Application.EnableEvents = False
Target.Value = Target.Value * 3
Application.EnableEvents = True
End If
End Sub
While I would usually involve some error control in order that the .EnableEvents was always reset to true, the above should get you started.

Answer 3

You need to disable events to prevent recursion:
Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$Q$21" Then
application.enableevents = false
Target.Value = Target.Value * 3
application.enableevents = true
End If
End Sub
Just as an alternative, you can also use your own variable rather than tinkering with EnableEvents, though it will mean your code responds twice even though it doesn't actually do anything the second time round:
Dim bSkipEvents as Boolean
Sub Worksheet_Change(ByVal Target As Range)
If bSkipEvents then exit sub
If Target.Address = "$Q$21" Then
bSkipEvents = True
Target.Value = Target.Value * 3
bSkipEvents = False
End If
End Sub
This is also the sort of approach you need with userforms and most events for activex controls on worksheets.