programming-for-dummies

154
Defining the Scope of a Variable
Now if the sales tax changes from 0.075 to 0.080, you just need to change this
value in one constant declaration, such as
Const sales_tax as Single = 0.080
This one change effectively plugs in the value of 0.080 everywhere your program
uses the sales_tax constant. So constants offer two advantages:
✦ They let you replace fixed values with descriptive constant names.
✦ They let you change the value of a constant once and have those
changes occur automatically in the rest of your program.
So use constants to replace fixed values and use variables to store different
types of data retrieved from the outside world. Every program needs
to use variables, but not every program needs to use constants.
After you understand how to store data temporarily in variables, your program
can start manipulating that data to do something useful.
Defining the Scope of a Variable
The scope of a variable defines which part of your program can store and
retrieve data in a variable. Because variables store data that your program
needs to work correctly, your program must make sure that no other part of
the program accidentally modifies that data.
If your program stores a person’s credit card number in a variable, you don’t
want another part of your program to accidentally retrieve that data and
change the numbers around or send a hundred copies of each credit card
number to customers outside the company.
So when creating variables, limit the variables’ scope. The scope simply
defines which parts of your program can access a variable. When you
declare a variable, you also define one of three possible scope levels for that
variable:
✦ Global
✦ Module
✦ Subprogram
Handling global variables with care
In a global variable, any part of your program can access that variable,
including storing new data in that variable (and wiping out any existing data
already stored in that variable), changing the data in a variable, or wiping
out the data in a variable altogether, as shown in Figure 2-5.