CHARACTER SET
The character set that can be used to form words, numbers and expressions are called character set.
Elements of character set :-
1.Letters --> A to Z , a to z
2.Digits --> 0 to 9
3. Special Characters --> +,-,*,/,:,;,(,),[,],{,},<,>,=,&,%,_,^,,,",',.,\,etc.
4.White spaces --> Newline,horizontal tab,carriage return,blank space.

IDENTIFIER

Identifier reverse to the name of variables,constants,functions and arrays.These are the user defined names end consist sequence of letters,digits and under score.

Rules for identifiers :

(i) It can contain letters,digits and underscore but the first character must
always be a letter.
(ii) No special character except underscore is allowed even space are not
allowed.
(iii) Both uppercase & lowercase characters are allowed but lowercase will be
treated as different from uppercase.
(iv) Keyword can not be used as identifier.

KEYWORD (RESERVED WORDS)


All keywords have fixed meaning and these meaning can not be changed. Keywords are predefined. They are 32 keywords in 'C' language.

Example : int,float,char,for,break,if,etc.
Keyword must be always written in lowercase.

VARIABLE

Variable is use to store a data value. The value of variable can be change during execution of program.

CONSTANT

Constant means fixed value that do not change during the execution of program.