C Keywords, Variables, Constants

Getting Ready to Learn C

Like any natural language, C computer language too has its own character set, grammar, and rules. Before start writing programs, you should first learn these basics of C language.

C Character Set:

C computer language allows following valid alphabets, numbers and special symbols-

A, B, ……., Z

a, b, …….., z

0, 1, …….., 9

~  `  !  @  #  %  ^  & *  (  )  _  +  -  =  |  \  {  }  [  ]  :  ;   “  ‘  <  > , .  ?  /  $ 

C Keywords:

Keywords are the reserved words or terms which have a special meaning. This meaning has been described to compiler. These keywords can only be used for the purpose they are designed to. They cannot be used for anything else. There are 32 keywords provided in the C computer language as 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

 

Meaning of these keywords will be explained later wherever and whenever they are used.

Variables and Constants:

As the name suggests variable refers to the entity that may change, and constant refers to the entity that doesn’t change. During execution, the program resides in computer’s memory (RAM). A program can have many data values and it does lots of calculations during program execution. The results too are stored in this memory. Computer memory consists of millions of cells. The data values are stored in these memory cells. To retrieve or store these data values easily, these memory cells or locations are given some meaningful names. These names are called variable names. Since, one can change the values of these memory locations; hence the name given to these locations is variable-name or variable simply. The data values itself are called constants. For example, you refer to a memory location by the name ‘x’, and give the value to it 5. During program execution, you give another value 7 to ‘x’. In this example, ‘x’ is a variable, because its value is changing and you can give different values to it at different times. 5 and 7 are constants, because these are fixed values. Variable can be called ‘identifiers’ and constants can be called ‘literals’.

Types of Variables and Constants:

Variables and constants can be organised in two broad categories-

• Primary
• Secondary

In primary type, there are three main categories-

• Integer type
• Real or floating point type
• Character type

In secondary type, there come complex data types and user-defined data types. Some of these are given below-

• Array
• Structure
• Union
• Enum

A specific type of variable can contain the same type of constant. For example, an integer type of variable can hold only integer type of constant.

Variable Naming Conventions:

• A variable name can be a combination of alphabets, digits, and an underscore ( _ ) and its size can be of one character to 31 characters.
• A variable name must start with an alphabet or underscore ( _ ).
• No blanks are permitted in the variable name.
• No special character except underscore ( _ ) can be used.
• Variable names should be meaningful.

Rules for Creating Constants:

Rules for creating primary constants are given below-

Integer Constant:

• There should be at least one digit in the integer constant.
• There should be no decimal point.
• An integer constant can be positive or negative.
• Default sign of integer constant is positive. If you don’t specify the sign, it is assumed to be positive.
• There should be no commas or blanks in the constant.
• The range of integer constant is from -32768 to +32767.
• For some compiler, range is from -2147483648 to +2147483647.
• Ex: 3, 56, -657, etc.

Real or Floating Point Constant:

• A real constant must have a decimal point.
• There should be at least one digit before or after decimal point.
• No commas or blanks are permitted in the constant.
• By default, floating point constants are positive, but it can be either positive or negative.
• Ex: 0.5, 238.48, -65754.433, etc.
• Floating point constants can be written in exponential form too. If the number is either too small or too large, exponential form is suitable to use.
• In exponential form, the real constant is written in two parts. The first part is called mantissa, whereas second part is called exponent. These two parts are separated by the letter ‘e’ or ‘E’.
• The first part mantissa can be positive or negative. Default sign is positive.
• Thus, the number 0.000003827 can be written in exponential form as 3.827e-6. In normal mathematics it can be written as 3.827x10^-6.
• The range of real constant is -3.4e38 to 3.4e38.

Character Constants:

A character constant is any single alphabet, or a digit, or any special symbol enclosed within single quotation marks or apostrophes.

Ex: ‘a’, ‘6’, ‘&’, etc.