Scope

1. Scope

2. Scope of Names • In a small program written by one person, it is easy to be sure that each variable has a unique name • In a large program, or one written by many people, it is not so easy • In fact, we may want to be able to use the same name in different parts of a large program, without conflicts

3. Scope • Recall our family analogy. Normally in a family each person has a different first (given) name (or if not they are distinguished by nicknames) • People in different families can have the same name; there are lots of Mary’s and William’s • If there is a famous person with a certain name, then most people think of that person when they hear the name (Oprah, for example) • But if a person in your family has that name, you think of them first

4. Scope and Lifetime of Variables • Variables can be declared within a subprocedure or function, or at the level of the module • A variable that is declared at the level of the module (also called a global variable) can be seen by all the subprocedures and functions in the module, and is in existence as long as the program is running. It is like a famous person that everyone knows by name • A variable declared within a subprocedure or function (also called a local variable) can only be seen there, and exists only as long as the subprocedure or function is running

5. An Example (code follows) • To see the difference between local and global variables, try running the ScopeIllustration Excel sheet • In this sheet, the value of global variable varA is displayed in Cell(1,1) (A1) and the value of local variable varB is displayed in Cell(1,2) (B1)

6. Global vs Local: The Code Option Explicit '************************************************************************* ' Illustrate scope '************************************************************************* DimvarA As Double 'create a global variable Sub WorkBook_Open() varA = 0 End Sub Sub ChangeValues() Dim varB As Double ‘varB is local varA = varA + 1 varB = varB + 1 Cells(1, 1).Value = varA Cells(1, 2).Value = varB End Sub This procedure sets varA to 0 when the workbook opens Each time you click the button, varA and varB are incremented

7. What happens… • varA exists as long as the workbook is open. Each time we add 1, its value increases • varB is created anew each time we press the button, when procedure ChangeValues is called. VBA sets a new variable to 0, so each time the value we see, after adding 1, is 1 • This illustrates the difference in lifetime between local and global variables

8. Tricky Case for Scope • Normally a global variable is visible everywhere in the module, but what if a local and a global variable have the same name? • It’s as if you have a sister named Oprah. Everyone else thinks Oprah is Oprah Winfrey, but you think first of your sister • Likewise, inside the procedure with the local variable named varA, the code cannot see or access the global variable; it uses that name for its local variable

9. Advice on Global Variables • Use global variables very sparingly. A program with all variables global easily becomes confusing and error-prone • Use them for quantities that have to be used by different procedures • If a quantity is only needed within a single procedure, use a local variable for it • Give global variables names that are unique; don’t reuse the same name for a local variable! • Global constants are fine; just give them unique names