VBA Function vs VBA Sub

The VBA Function vs VBA Sub

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Today back to basics – the explaining the VBA Function and the VBA Sub procedures. We use Subs and Functions often not thinking about their true potential and how much we can squeeze out of a regular VBA Function. Let’s start with a reminder how both differ from each other and then dive into the details.

VBA Function vs. VBA Sub

We often tend to mix up procedures, Subs and Functions in VBA. So let’s get it right this time.

A VBA procedure is a block of VBA statements enclosed by a declaration statement (Function, Sub, Operator, Get, Set) and a matching End declaration. All executable statements in Visual Basic must be within some procedure.

A VBA Sub procedure is a series of VBA statements enclosed by the Sub and End Sub statements. The Sub procedure performs a task and then returns control to the calling code, but it does not return a value to the calling code. Only VBA Subs can be called directly.

Sub NameOfSub()
 Debug.Print "Hello I am a Sub"
End Sub

A VBA Function procedure is a series of VBA statements enclosed by the Function and End Function statements. The Function procedure performs a task and then returns control to the calling code. When it returns control, it also returns a value to the calling code.

Function NameOfFunction() as String
 Debug.Print "Hello I am a Function"
 NameOfFunction = "Hello I am a Function"
End Sub
In short VBA Functions return values, VBA Sub can’t return any values. Additionally only VBA Subs can be executed directly (VBA Functions can be executed only within other code blocks)

Syntax

VBA Sub procedure syntax

VBA Sub procedureThe code block of a VBA Sub procedure is marked by the Sub and End Sub statements.

VBA Function procedure syntax

VBA FunctionThe code block of a VBA Function is marked by the Function and End Function statements.

Passing arguments ByVal and ByRef

The common knowledge is that only VBA Functions can return values. That indeed is mostly true. However, instead of a value (the result of the Function), we can assign values to variables that can be passed to the procedure by reference. What does that mean?

ByVal vs ByRef
Passing a variable by its Value or its Reference
Variables can either by passed to a procedure (Function, Sub etc.) by their value or their reference.
Passing by value translates to making a copy of the variable. Thus any changes to the copy will not be reflected in the original variable.
Passing by reference is passing the address of the variable to the procedure. This means that any changes to a argument will be reflected in the original variable.

By default all arguments are passed to procedures ByVal, so that no changes can be made to the original variables

ByVal and ByRef examples

Let’s look at some examples now:

Sub SetValueByVal(ByVal someLong As Long)
    someLong = 10
End Sub
Sub SetValueByRef(ByRef someLong As Long)
    someLong = 10
End Sub

Sub Test()
    Dim someLong As Long
    someLong = 1
    
    SetValueByVal someLong
    Debug.Print someLong 'Result: 1 (no change)
    
    SetValueByRef someLong
    Debug.Print someLong 'Result: 10
End Sub

When passing by value the variable someLong is not modified. However, when we pass it by reference its value is changed within the Sub procedure.

Objects and arrays are always passed by reference, because objects are in fact references (e.g. a Collection). Beware in such cases not to modify referenced objects by mistake!

Passing arrays to Subs and Functions

You can also easily pass arrays to Subs or Functions, even redefining their length. See example below:

Sub ChangeLength(arr() As Long)
    ReDim arr(5) As Long
End Sub

Sub Test()
    Dim arr() As Long
    ReDim arr(2) As Long
    Debug.Print UBound(arr) 'Result: 2
    ChangeLength arr
    Debug.Print UBound(arr) 'Result: 5
End Sub

Optional parameters

VBA Functions and Subs permit optional parameters, ones that need not be provided when executing the Function or Sub. It is a good practice to specify the default value for such parameters. See example below:

Sub SayHi(name As String, Optional surname As String = vbNullString)
    Debug.Print "Hi there, " & name & IIf(surname = vbNullString, "", " " & surname)
End Sub

Sub Test()
    Call SayHi("John") 'Result: "Hi there, John"
    
    Call SayHi("John", "Smith") 'Result: "Hi there, John Smith"
End Sub

You can verify if a parameter has not been passed to an Sub or Function by using the IsMissing function. The IsMissing function works however only for Variant type parameters. See the same SayHi procedure as above, this time with the IsMissing function.

Sub SayHi(name As String, Optional surname As Variant)
    Debug.Print "Hi there, " & name & IIf(IsMissing(surname), "", " " & CStr(surname))
End Sub

Sub Test()
    Call SayHi("John") 'Result: "Hi there, John"
    
    Call SayHi("John", "Smith") 'Result: "Hi there, John Smith"
End Sub

Dynamic parameter list

Say you want to create a VBA function like the Excel SUM or AVERAGE formulas – that can be provided with a dynamic list of parameters. VBA extends a neat solution for such a scenario called the ParamArray.

Public Function MySUM(ParamArray args())
    For Each arg In args
        MySUM = MySUM + arg
    Next arg
End Function

Sub Test()
    Debug.Print MySUM(1, 2, 3, 4) 'Result: 10
End Sub

The ParamArray transforms a list of Variant variables passed as parameters into a neat Variant array.

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