Variables in C

Variables in C are memory locations with help of which we can be assigned values and are given names .
To store data in memory for later use,we use variables.
In C, a variable must have to be declared before it can be used.
You can declare Variables at the start of any block of code, but most are found at the start of each function.
Most local variables are destroyed on return from that function and created when the function is called.
A declaration begins with the type, followed by the name of one or more than one variables.

The following code describe the declaration of variable:

int main()
{
int a;
char b;
return 0;
}

C keeps a small set of keywords for its own use only.These keywords are given below:

auto	break	case	char	const	continue	default	do
double	else	enum	extern	float	for	goto	if
int	long	register	return	short	signed	sizeof	static
struct	switch	typedef	union	unsigned	void	volatile	while

Identifiers in C

Identifiers" or "symbols" are the names you supply for variables, types,labels, and functions in your program. Identifier names must differ in case and spelling from any keywords. You cannot use keywords as the identifiers; they are reserved for special use only. You create an identifier by specifying it in the declaration of a variable,function,or type. In this example which is given below result is an identifier for an integer variable, and printf and main are identifier names for functions.

Identifiers provide names for the given language elements:

Functions
Function parameters
Macros and macro parameters
Type definitions
Objects
Labels
Enumerated types and enumerators
Structure and union names

#include <stdio.h>

int main()
{
int result;

if ( result != 0 )
printf_s( "Bad file handle\n" );
}

Variable declarations

Variables are of three different types which are as follows:

Global Variable
Local Variable
Static Variable

Global Variable:

These variables can be accessed by any function of the program. By associating memory locations with variable names,they are implemented. If the function is recalled,they do not get recreated . Declare it outside of all the functions if you want to declare a global variable. The function will use the variable that was declared within it and ignore the global one,if a variable of the same name is declared both within a function and outside of it.

Local Variable:

Inside the specific function that creates them,these variables only exist. They are unknown to to the main program and to the other functions. In this case,they are normally implemented using a stack. Local variables cease to exist once if the function that created them is completed. Each time a function is executed or called,they are recreated.

Outside a function. These declarations declare global variables which are visible throughout the program (i.e. they have global scope). Use of global variables is always a big mistake.
In the header of a function in the argument list . These variables are the parameters to the function. They are only visible inside the function body and their local scope), exist only from when the function is called to when the function returns (bounded extent---note that this is different from what happens in some garbage-collected languages like Scheme), and get their initial values from the arguments to the function when it is called.

At the start of any block delimited by curly braces only. Such variables are exist only when the containing function is active (bounded extent) and visible only within the block (local scope again). The convention in C is generally to declare all such local variables at the top of a function; this is different from the convention in C++ or Java, which encourage variables to be declared when they are first time used

The following program demonstrate the use of global and local variables.

#include <stdio.h>

int counter = 0; /* global because we are outside all blocks.*/
int func(void);

main()
{
counter++; /* global because it has not been declared within this block */
printf("counter is %2d before the call to func\n", counter);

func(); /* call a function. */
printf("counter is %2d after the call to func\n", counter);
}

int func(void)
{
int counter = 10; /* local variable because it has declared within this block */
*/ printf("counter is %2d within func\n", counter);
}

Using static variables

Another important feature of the variable scoping is the static variable. In a local function scope,a static variable exists only , but it does not lose its value when program execution leaves this scope. Consider the example which is given below:

#include <stdio.h>

main()
{
Test();
}
function Test()
{
int a = 0;
printf("a is %d within func\n", a)
a++;
}

Since every time the function is called it sets a to 0 and prints "0",this function is quite useless . The a++ which increments the variable serves no purpose since as soon as the function exits then a variable disappears. The a variable is declared static to make a useful counting function which will not lose track of the current count:

#include <stdio.h>

main()
{
Test();
}
function Test()
{
static int a = 0;
printf("a is %d within func\n", a)
a++;
}

Basic types

There are 4 basic types of variable in C; they are: char, int, double and float

Type name	Meaning
char	The most basic unit addressable by the machine; typically a single octet. This is an integral type.
int	The most natural size of integer for the machine; typically a whole 16, 32 or 64 bit addressable word.
float	A single-precision floating point value.
double	A double-precision floating point value.