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Tip of the Day
Language: VB4,VB5,VB6
Expertise: Intermediate
Jun 5, 1999

Items of ParamArray can be Missing

When using the ParamArray keyword within a procedure, always remember that when the procedure is invoked from elsewhere in the program one of the argument might be omitted, and you should keep this into account. Here is an example of a routine that uses ParamArray:


Function Max(ParamArray args() As Variant) As Variant
  Dim result As Variant, index As Integer
  result = args(LBound(args))
  For index = LBound(args) + 1 To UBound(args)
    If result < args(index) Then result = args(index)
  Next
  Max = result
End Function
The above routine works fine, but only if the programmer doesn't omit any argument in the list. If you recall the function as in:


Print Max(1, , 3)    
the function will raise a "Type Mismatch" error. Here's a better version, that also accounts for missing arguments:

Function Max(ParamArray args() As Variant) As Variant
  Dim result As Variant, index As Integer
  For index = LBound(args) To UBound(args)
    If IsMissing(args(index)) Then
      ' ignore this argument
    ElseIf IsEmpty(result) Then
      ' the first time we set a possible return value
      result = args(index)
    ElseIf result < args(index) Then 
      result = args(index)
    End If
  Next
  Max = result
End Function
Also, remember that to a Variant argument you can pass almost anything, including objects and arrays. If you really want to create a general-purpose routine you should account for these cases too.
Francesco Balena
 
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