A structure that holds data within an executable program. Mostly created by and uniquely named by the programmer, variables may store predefined data at the start of the program or be empty containers until a value is placed in them. The values may remain constant or be updated in RAM as the program runs. Variables are used as counters to sum totals and do math as well as to keep track of processes that are repeated within the program. There may be dozens or hundreds of variables defined in a single application.
Using C/C++ as the example language, the statement
int counter; defines an integer variable for whole numbers named COUNTER. The statement
counter=1; stores a 1 in COUNTER, and
counter++; adds 1 to COUNTER. A "char" variable holds character data. A single character variable requires single quotes, but a "string" of characters uses double quotes; for example,
mode='A'; and
product="abc"; places A in the variable MODE and ABC in PRODUCT. See
string,
integer and
RAM.
Variables Are Control Values
Variables are widely used to repeat a process. In the following
for statement, the function DO-SOMETHING is performed five times. At first,
x is set to 0. All statements between the left and right curly braces ({ }) are performed. Then,
x is incremented by 1 until
x is no longer less than 5.
for (x=0; x<5; x++)
{
do-something();
}
Local and Global Variables
A local variable is one that is referenced only within the subprogram, function or procedure it was defined in. A global variable can be used by the entire program. See
environment variable,
undefined variable and
local variable.
Actual Variable Examples
The following integer variables and character strings were extracted from a program written in C that converts an XML encyclopedia feed to an HTML feed. The
=NO is the same as inserting a 0. Variables are often used to keep track of internal modes that are turned on and off while the program is running. YES and NO are the same as 1 and 0 to the compiler.
int ForceOne=NO, ForceAll=NO, NoMoreFiles=NO;
int XMLfileNewer=NO, EndFile=NO;
int TermsWith1Link=0, TermsWith2Links=0;
int TermsWith3Links=0, TermsWithNoLinks=0;
int AFTermCount=0;
int AF_results=0;
int AF_EOF=0;
int hFileAF;
int ARTICLES=0;
char AFbuff[2000];
char *AFptr;
char HREF1buff[500];
char HREF2buff[500];
char HREF3buff[500];