programming-for-dummies

Defining the Scope of a Variable 155
Program
File 1 File 2
Global X : integer
Book II
Chapter 2
Figure 2-5:
Every part of
a program
can access
and change
a global
variable.
File 3 File 4
Variables,
Data Types,
and Constants
Use global variables sparingly. If you create a global variable and some part
of your program keeps modifying that variable’s data by mistake, you have
to search through your entire program to find which command is messing up
that variable. If you have a million-line program, guess what? You have to
examine a million lines of code to find the one line that’s changing that variable
by mistake. If that’s your idea of fun, go ahead and use global variables.
In the old days, all programming languages let you create global variables,
and it was up to the programmer to make sure no commands accidentally
modified that variable in unintended ways. When writing small programs,
programmers can do this easily, but when working on massive programs created
by teams of programmers, the odds of abusing global variables
increases dramatically.
Think of a shelf where you can store your books, wallet, and laptop computer.
If you’re the only person who has access to that shelf, you can be sure
anything you put on that shelf is there when you look for it again.
Now imagine putting your shelf of personal belongings (books, wallet, and
laptop computer) on a shelf located in Grand Central Station where thousands
of people can grab anything they want off that shelf or put something