Sams Teach Yourself C in 21 Days

Understanding Variable Scope 291

12

Creating Local Variables ....................................................................................

Alocal variableis one that is defined within a function. The scope of a local

variable is limited to the function in which it is defined. Day 5 describes local

variables within functions, how to define them, and what their advantages are. Local

variables aren’t automatically initialized to 0 by the compiler. If you don’t initialize a

local variable when it’s defined, it has an undefined or garbagevalue. You must

explicitly assign a value to local variables before they’re used for the first time.

A variable can be local to the main()function as well. This is the case for xin Listing

12.2. It is defined within main(), and as compiling and executing that program illus-

trates, it’s also only visible within main().

DOuse local variables for items such as

loop counters.

DOuse local variables to isolate the val-

ues the variables contain from the rest of

the program.

DON’Tuse external variables if they

aren’t needed by a majority of the pro-

gram’s functions.

DO DON’T

Static Versus Automatic Variables ................................................................

Local variables are automaticby default. This means that local variables are created

anew each time the function is called, and they are destroyed when execution leaves the

function. What this means, in practical terms, is that an automatic variable doesn’t retain

its value between calls to the function in which it is defined.

Suppose your program has a function that uses a local variable x. Also suppose that the

first time it is called, the function assigns the value 100 tox. Execution returns to the

calling program, and the function is called again later. Does the variable xstill hold the

value 100? No, it does not. The first instance of variable xwas destroyed when execution

left the function after the first call. When the function was called again, a new instance of

xwas created. The old xis gone.

What if the function needs to retain the value of a local variable between calls? For

example, a printing function might need to remember the number of lines already sent to

the printer to determine when it is necessary to start a new page. In order for a local vari-

able to retain its value between calls, it must be defined as staticwith the statickey-

word. For example:

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