Making VLOOKUP Trigger a Macro

Mike uses VLOOKUP regularly in his worksheets, but wonders if there is a way to make the function run a macro if it fails to return a value.

There are a couple of ways you could approach this problem. First, you could use a conditional formula to determine whether VLOOKUP will return a value or an error. If it will return an error, then you can have the formula run a user-defined function (MyUDF), as shown here:

=IF(ISERROR(VLOOKUP(B2,CODES,1,FALSE)),MyUDF(),
VLOOKUP(B2,CODES,1,FALSE))

All you need to do is make sure that you put your actual VLOOKUP code in the formula (twice) and replace MyUDF with the name of the user-defined function you want to trigger.

Another approach is to set up an event handler for the Calculate event. This can be rather simple, as in the following:

Private Sub Worksheet_Calculate()
   If IsError(Range("A1")) Then Call Macro1
End Sub

This example assumes that the VLOOKUP formula is in cell A1 and that you want to run a macro called Macro1 if the VLOOKUP returns an error. Your macro could then do whatever you need it to do. Remember, as well, that the Calculate event handler should be placed in the ThisWorksheet object.

You could also make the Calculate event handler a bit more robust, as shown here:

Private Sub Worksheet_Calculate()
    On Error GoTo myMac
    Worksheets(1).Select
    If Range("A1").Value Then
        Exit Sub
    End If
myMac:
    Macro1   'macro to run if VLOOKUP fails
End Sub