mouse wheel prob

How can i stop the tileindex from increasing/decreasing constantly?

I thought I could use the code

mousewheel=MouseZ ()

If mousewheel>0 Then tileindex=tileindex+1 : mousewheel=0
If mousewheel<0 Then tileindex=tileindex-1 : mousewheel=0

so that 'tileindex' would go up or down once then set 'mousewheel' to 0. What happens is once I move the mousewheel up or down it cycles through my tiles and doesnt stop.

Any ideas why?
thanks

I think this should do it:

wheel_dir = MouseZSpeed()

If wheel_dir > 0
  tileindex = tileindex + (1 And (tileindex < lastindex))
ElseIf wheel_dir < 0
  tileindex = tileindex - (1 And (tileindex > firstindex))
EndIf

or alternatively:

mousewheel=MouseZ()

If mousewheel>MlastPos Then tileindex=tileindex+1 : MlastPos = mousewheel
If mousewheel<MlastPos Then tileindex=tileindex-1 : MlastPos = mousewheel