 Improving The Union Function Introductcion

Excel/VBA's Union method is used to combine two ranges into a single range. For example,

```     Dim RR As Range
Set RR = Applicaiton.Union(Range("A1:A10"),Range("B1:B10"))
```

This combines A1:A10 and B1:B10 into a single range, RR, which now references A1:B10.

The Union method, though, has two shortcomings. First, none of its input parameters may be Nothing. If any parameter is Nothing, the Union operation fails. A better union, described below, will ignore parameters that are Nothing and uses only the parameters that not Nothing to create the new range. The second shortcoming of Union is that it duplicates cells when the regions specified by the input parameters overlap. This can cause logic problems in your code since overlapping cells are included twice in the Union result. Allowing Union To Handle Nothing Parameters

Union does not allow any parameters to be Nothing. For example, the Union method in the code below will raise an error 5, Invalid Procedure Call because R3 is Nothing.

```         Dim R1 As Range
Dim R2 As Range
Dim R3 As Range
Dim RR As Range
Set R1 = Range("A1")
Set R2 = Range("B1")
Set RR = Application.Union(R1, R2, R3)' Error 5 - Invalid Parameter
```

A better UnionR33 parameter since it is Nothing. The following function allows for parameters that are Nothing.

```    Function Union2(ParamArray Ranges() As Variant) As Range
''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Union2
' A Union operation that accepts parameters that are Nothing.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Dim N As Long
Dim RR As Range
For N = LBound(Ranges) To UBound(Ranges)
If IsObject(Ranges(N)) Then
If Not Ranges(N) Is Nothing Then
If TypeOf Ranges(N) Is Excel.Range Then
If Not RR Is Nothing Then
Set RR = Application.Union(RR, Ranges(N))
Else
Set RR = Ranges(N)
End If
End If
End If
End If
Next N
Set Union2 = RR
End Function
```

By using Union2 the code above will work, setting RR to reference A1:B10. Since R3 is Nothing, the code ignores it rather than raising an error.

```        Dim R1 As Range
Dim R2 As Range
Dim R3 As Range
Dim RR As Range
Set R1 = Range("A1")
Set R2 = Range("B1")
Set RR = Union2(R1, R2, R3) ' success
```

Handling Duplicates In A Union

If two or more of the parameters to Union overlap, the overlapping cells are counted twice (or more). For example, consider the following code:

```    Dim RR As Range
Set RR = Application.Union(Range("A1:C3"), Range("B3:D5"))
```

Each of the two ranges contain 9 cells, so Union result returns 18 cells. But the two ranges overlap, and the overlapping cells B3 and C3 are counted twice. A proper count of cells shows that there are 16, not 18, cells in the combined range. A loop through the cells iterates 18, not 16, times.

```    Dim RR As Range
Dim R As Range
Dim N As Long
Set RR = Application.Union(Range("A1:C3"), Range("B3:D5"))
For Each R In RR
N = N + 1
Next R
Debug.Print N ' 18 cells. incorrect
```

A better Union would not duplicate cells. Such a function, named ProperUnion is shown below:

```     Function ProperUnion(ParamArray Ranges() As Variant) As Range
''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' ProperUnion
' This provides Union functionality without duplicating
' cells when ranges overlap. Requires the Union2 function.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Dim ResR As Range
Dim N As Long
Dim R As Range

If Not Ranges(LBound(Ranges)) Is Nothing Then
Set ResR = Ranges(LBound(Ranges))
End If
For N = LBound(Ranges) + 1 To UBound(Ranges)
If Not Ranges(N) Is Nothing Then
For Each R In Ranges(N).Cells
If Application.Intersect(ResR, R) Is Nothing Then
Set ResR = Union2(ResR, R)
End If
Next R
End If
Next N
Set ProperUnion = ResR
End Function
```

Using the code above but calling ProperUnion instead of Union, the correct range and cell count is calculated as the combined range.

```    Dim RR As Range
Dim R As Range
Dim N As Long
Set RR = ProperUnion(Range("A1:C3"), Range("B3:D5"))
For Each R In RR
N = N + 1
Next R
Debug.Print N ' 16 cells. correct
```

The ProperUnion function can accept up to 30 parameters. As written, it uses the Union2 function described above, but you could easily incorporate the functionality of Union2 directly into ProperUnion, eliminating the function call.  This page last updated: 12-January-2011.

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