C LANGUAGE GENERAL TERM

C LANGUAGE GENERAL TERM

Variable

A variable is used to hold data within your program. A variable represents a location in your computer's memory. You can put data into this location and retrieve data out of it. Every variable has two parts, a name and a data type.

Variable Names

• Valid names can consist of letters, numbers and the underscore
• The first character must not start with a number.
• A variable name may not be a C keyword.
• Variable names are case sensitive. So, Age, AGE, aGE and AgE could be names for different variables
• No special symbols other than underscore (such as fourth_Dimension).

Keyword

Keywords are reserved identifier and they can not be used as names for the program variables or other user defined program elements. The meanings of the keywords have already been given to the compiler. The keywords are also called reserved words.

List of C Keywords

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

Data Types

C provides built in data types for character, float and integer data. As an aside, in C you may assign a value to a variable when you declare it.

Integer variables

Integer variables are used to store whole numbers. There are several keywords used to declare integer variables, including int, short, long, unsigned short, unsigned long. The difference deals with the number of bytes used to store the variable in memory, long vs. short, or whether negative and positive numbers may be stored, signed vs. unsigned. These differences will be explained in more advanced tutorials. For now, use int to declare integer variables. On most 32-bit systems, int is synonymous with signed long.
Examples:

int count;
int number_of_students = 30;

Float variables

Float variables are used to store floating point numbers. Floating point numbers may contain both a whole and fractional part, for example, 52.7 or 3.33333333. There are several keywords used to declare floating point numbers in C including float, double and long double. The difference here is the number of bytes used to store the variable in memory. Double allows larger values than float. Long double allows even larger values. These differences will be explained in more advanced tutorials. For now, use float to declare floating point variables.
Examples:

float owned = 0.0;
float owed = 1234567.89;

Character variables

Character variables are used to store character values. The use of characters and strings will be covered in a latter tutorial. Character variables are declared with the keyword char.
Examples:

char firstInitial = 'J';
char secondInitial = 'K';

Datatypes in C

Data Type	Category	Range	Bytes	Format
Character	signed char	-128 to +127	1	%c
	unsigned char	0 to 255	1	%c
Integer	short signed int	-32768 to +32767	2	%d
	short unsigned int	0 to 65535	2	%u
	signed int	-32768 to +32767	2	%d
	unsigned int	0 to 65535	2	%u
	long signed int	-2147483648 to +2147483647	4	%ld
	long unsigned int	0 to 4294967295	4	%lu
Float	float	-3.4e38 to +3.4e38	4	%f
Double	double	-1.7e308 to +1.7e308	8	%lf
	long double	-1.7e4932 to +1.7e4932	10	%Lf

Constants

A constant is similar to a variable in the sense that it represents a memory location. It differs, as I sure you can guess, in that it cannot be reassigned a new value after initialization. In general, constants are a useful feature that can prevent program bugs and logic errors. Unintended modifications are prevented from occurring. The compiler will catch attempts to reassign new values to constants.

#include
void main()
{
       int pi=3.1415; /*constant value of pi*/
       int r=5;    /*constant value given to r varable */
       printf("Area of circle=%d",pi*r*r);
}

Using const variables

The second technique is to use the keyword const when defining a variable. When used the compiler will catch attempts to modify variables that have been declared const.

const float pi = 3.1415;
const int id_no = 12345;

There are two main advantages over the first technique. First, the type of the constant is defined. "pi" is float. "id_no" is int. This allows some type checking by the compiler. Second, these constants are variables with a definite scope. The scope of a variable relates to parts of your program in which it is defined. Some variables may exist only in certain functions or in certain blocks of code. You may want to use "id_no" in one function and a completely unrelated "id_no" in your main program. Sorry if this is confusing, the scope of variables will be covered in a latter