Learning VBA trying to understand passing paramenters and variables

Hi group, I'm working my way through an Excel Programming book teaching VBA and I've run across an example of passing parameters to a sub routine that calls another sub routine. What is confusing me is that the parameters that are being passed (Val1 and Val2) are different than the variables used in the sub routine which calls the previous sub routine. See below:

Sub AddValues (Val1 As Integer, Val2 As Integer)
Dim Total As Integer
Total = Val1 + Val2
MsgBox (Total)
End Sub

Sub TotalValues
Dim Value1 as Integer
Dim Value2 as Integer
Value1 = 5
Value2 = 7
Call AddValues (Value1, Value2)
End Sub

Shouldn't the first Sub AddValues actually be a Function? does the variable Val1 and Val2 become Value1 and Value2? or should the variables used be the same, ie. Val1 in the first routine stay as Val1 in the second sub routine and Val2 the same?


Sebastien.

Originally Posted by sgrondines

does the variable Val1 and Val2 become Value1 and Value2? or should the variables used be the same, ie. Val1 in the first routine stay as Val1 in the second sub routine and Val2 the same?

I think we should talk about this first. Each subroutine processes within its own scope. There's no need to name them the same or differently, they will not share information or collide. You do want to take care not to use the same name as globally scoped variables, constants, or other functions.

Originally Posted by sgrondines

Shouldn't the first Sub AddValues actually be a Function?

It is a little easier to understand having it be a function, and I myself would write it as a function. That said, it doesn't necessarily have to be a function. Use a function when you want to get a value returned. Use a sub when you just want it to do a thing and don't care about having it return a value.

Here's a function implementation.

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Note: It's best practice to declare the return type of your function as I did in my example, otherwise it'll be declared as a variant. then it will be hard to predict what kind of data you'll get back.

It's not really helpful to think of it like that because it's not really true.

When the AddValues function (or subroutine) is called, it doesn't actually pass the variable. It either passes the value stored in the variable (byVal) or it passes a pointer reference to a memory address (byRef). In this example, they are being passed byVal (because they are integer type).

You can think of it as the VBA interpreter reading the values inside of Value1 and Value2 and swapping them in the call to make it go from AddValues(Value1, Value2) to AddValues(5, 7).
AddValues, then takes the passed data and assigns it to its own locally scoped variables, Val1 and Val2. They are assigned positionally unless you use the name them: AddValues(Val2:=7, Val1:=5)

Edit:

Another way of putting it: The subroutine TotalValues, and its variables don't close and go out of scope until AddValues completes its run and you hit the End Sub line at the end. The whole time AddValues is running, TotalValue's variable Value1 and AddValue's variable Val1 both exist in memory, albeit in isolated scopes so they don't see each other. The initial value for Val1 is set by Value1 but that's where it ends. Further changes to Val1 don't do anything to Value1 therefore they are not the same thing.