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To become an efficient programmer, you need to stop writing one long subroutine and start writing lots of small ones instead - and to do this, you're going to need to know everything there is to know about passing arguments.
- Passing Arguments
- Passing Arguments by Reference and Value
- Optional Arguments and ParamArrays (this blog)
You can find more articles on this subject in our Excel Visual Basic tutorial blog - or book onto one of our Excel or VBA training courses.
Posted by Andy Brown on 09 February 2012
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Optional Arguments and ParamArrays
Optional Arguments
In our BuildSnowman routine at the start of this blog, sometimes we wanted buttons and a bow tie, and sometimes we didn't. The way to get round this is to make arguments optional.
Suppose we want to be able to construct an address (perhaps for use in a mail merge, although this isn't shown here), and we don't know how many lines it will have. Here are 2 possible addresses we might have:
|
|
| One address line only | A full postal address |
A routine which would display an address whether it has one line, two lines or two lines and a postcode is shown below:
Sub CreateAddress( _
PersonName As String, _
AddressLine1 As String, _
Optional AddressLine2 As String = "", _
Optional PostCode As String = "")
'create the full address for mail merge
Dim FullAddress As String
'start with first line of address
FullAddress = PersonName & vbCrLf & AddressLine1
'if there is a second line, add it on with line break
If Len(AddressLine2) > 0 Then _
FullAddress = FullAddress & vbCrLf & AddressLine2
'if there is a postcode, add this on with line break
If Len(PostCode) > 0 Then _
FullAddress = FullAddress & vbCrLf & PostCode
'display result
MsgBox FullAddress
End Sub
By making the last two arguments optional you can miss them out (in which case they'll take the default value shown). The syntax of an optional argument is thus:
Optional ArgumentName as ArgumentType = DefaultValue
Here's how you could call the CreateAddress above with different argument combinations:
Sub TestAddresses()
'3 different combinations of arguments (the message box for the first
'and the last are shown above)
CreateAddress "Bob the Builder", "Sunflower Valley"
CreateAddress "Tinky-Winky", "1 The Avenue", "Teletubbieland"
CreateAddress "William Pitt", "10 Downing Street", "London", "SW1"
End Sub
Optional Argument Syntax
The main rule of optional arguments is that they should come at the end of the list.:
This code shows an error because the optional arguments aren't at the end of the list
If you think hard about it, you'll see why: otherwise, how would VBA know which value to assign to which argument?
Using ParamArray
In our address example above, we could have addresses with anything from 0 to 6 lines. Rather than having to list 6 optional arguments, we could create an optional array, called a ParamArray. The easiest way to explain this is by example:
Sub CreateAddress( _
PersonName As String, _
ParamArray AddressLines() As Variant)
'create the full address for mail merge
Dim FullAddress As String
'start with first line of address
FullAddress = PersonName
'add on any other lines
Dim AddressLine As Variant
For Each AddressLine In AddressLines
FullAddress = FullAddress & vbCrLf & AddressLine
Next AddressLine
'display result
MsgBox FullAddress
End Sub
You could then call this CreateAddress routine with any number of arguments you liked (including none at all):
Sub TestAddresses()
'3 different combinations of arguments
CreateAddress "Wise Owl"
CreateAddress "Bob the Builder", "Sunflower Valley"
CreateAddress "Tinky-Winky", "1 The Avenue", "Teletubbieland"
CreateAddress "William Pitt", "10 Downing Street", "London", "SW1"
End Sub
- Passing Arguments
- Passing Arguments by Reference and Value
- Optional Arguments and ParamArrays (this blog)