Constants And Variables

The alphabets, numbers and special symbols when properly combined form constants, variables and keywords. A constant is an entity that does not change.

Variables:
A variable is an entity that may change it value. In any program we typically do lots of calculations. The results of these calculations are stored in computer memory locations. To make the retrieval and usage of these values we give names to the memory locations. These names are called variables.

Keywords:
A keyword is a word that is part of C Language itself. These words have predefined meanings and these words cannot be used as variable names.

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

Types of Variables
There are two main types of variables in C: numeric variables that hold only numbers or values, and string variables that hold text, from one to several characters long.

Basic fundamental data types in C Language

Name	Description	Size	Range
char	Character or small integer.	1byte	signed: -128 to 127 unsigned: 0 to 255
short int	Short Integer.	2bytes	signed: -32768 to 32767 unsigned: 0 to 65535
long int (long)	Long integer.	4bytes	signed: -2147483648 to 2147483647 unsigned: 0 to 4294967295
bool	Boolean value. It can take one of two values: true or false.	1byte	true or false
float	Floating point number.	4bytes	+/- 3.4e +/- 38 (~7 digits)
double	Double precision floating point number.	8bytes	+/- 1.7e +/- 308 (~15 digits)
long double	Long double precision floating point number.	8bytes	+/- 1.7e +/- 308 (~15 digits)