programming-for-dummies

156
Defining the Scope of a Variable
else on there instead. Would you feel safe leaving your wallet and laptop on
such a shelf? If not, you probably wouldn’t feel safe putting your data in a
global variable either.
Because global variables can be so dangerous, most programming languages
don’t let you create a global variable unless you specifically tell the computer
to create one. To create a global variable, you often have to use a special
keyword, such as global or public, such as
Global X : Integer
The preceding command tells the computer to create a global variable,
called X, that can hold an integer data type. You type a global variable
declaration in any file that contains your main program or a library of
subprograms.
Restricting scope to a module
A module is another term for a separate file. If you divide a large program
into subprograms and store those subprograms in a separate file, each file is
a module. A module variable lets you create a variable that can be accessed
only by code stored in that particular module, as shown in Figure 2-6.
The advantage of module variables is that they give you the convenience of
a global variable but without the danger of letting every part of a program
access that variable. Instead, module scope limits access only to code in
that file.
Although an improvement over global variables, module variables also have
some of the disadvantages of global variables. If a file contains a lot of subprograms,
trying to find which line of code is messing up your module variable
can still be troublesome.
To create a module variable, you generally just declare a variable at the top
of any file except you don’t use the Global or Public keyword. In some
programming languages, you declare a module variable with the Private
keyword, such as
Private X : integer;
The preceding code would declare a module variable called X, which can
hold an integer data type.