Understanding Subroutines

When you write macros in Excel, you use a programming language called Visual Basic for Applications (VBA). This is based on the BASIC programming language, with extensions specific to Excel. One of the features of the language is the capability to use subroutines in your programs. For instance, consider the following VBA macro:

Sub Macro1()
    TestSub
End Sub

Sub TestSub()
    MsgBox "In the subroutine"
End Sub

This simple macro (Macro1) does nothing but call a subroutine (TestSub), which in turn displays a message box to inform you that it is in the subroutine. When you click on OK to dismiss the message box, the subroutine ends and returns control to the main program. You can have as many subroutines in a VBA program as you desire. The purpose of each should be to perform common tasks so you don't have to rewrite the same code all the time.

You can also pass parameters to your subroutines. These parameters can then be acted upon by your subroutine. For instance, consider the following macro:

Sub Macro1()
    A = 1
    PrintIt A
End Sub

Sub PrintIt(x)
    MsgBox "Value: " & x
End Sub

This is a simple macro that sets a variable, and then passes it in a subroutine call to PrintIt. This subroutine displays the value of the variable in a message box, and then (after you press OK) returns to the calling program.

Notice that the subroutine does not use the same variable name as it was passed. This is because VBA reassigns the value of x (what the subroutine expects to receive) so that it matches the value of A (what the program is passing to the subroutine). The important thing to remember in passing parameters to subroutines is that your program must pass the same number of parameters as the subroutine expects, and that the parameters must be of matching types and in the proper order.