identifiers

Identifiers are names for entities in a C program, such as variables, arrays, functions, structures, unions and labels. An identifier can be composed only of uppercase, lowercase letters, underscore and digits, but should start only with an alphabet or an underscore. If the identifier is not used in an external link process, then it is called asinternal. Example: Local variable. If the identifier is used in an external link process, then it is called as external. Example: Global variable

An identifier is a string of alphanumeric characters that begins with an alphabetic character or an underscore character that are used to represent various programming elements such as variables, functions, arrays, structures, unions and so on. Actually, an identifier is a user-defined word. There are 53 characters, to represent identifiers. They are 52 alphabetic characters (i.e., both uppercase and lowercase alphabets) and the underscore character. The underscore character is considered as a letter in identifiers. The underscore character is usually used in the middle of an identifier. There are 63 alphanumeric characters, i.e., 53 alphabetic characters and 10 digits (i.e., 0-9).

Rules for constructing identifiers

1.     The first character in an identifier must be an alphabet or an underscore and can be followed only by any number alphabets, or digits or underscores.
2.     They must not begin with a digit.
3.     Uppercase and lowercase letters are distinct. That is, identifiers are case sensitive.
4.     Commas or blank spaces are not allowed within an identifier.
5.     Keywords cannot be used as an identifier.
6.     Identifiers should not be of length more than 31 characters.
7.     Identifiers must be meaningful, short, quickly and easily typed and easily read.

Valid identifiers:      total    sum     average          _x        y_        mark_1           x1

Invalid identifiers    
                                                     1x       -           begins with a digit
                                                    char    -           reserved word
                                                    x+y      -           special character

Note: Underscore character is usually used as a link between two words in long identifiers.

Identifier must be unique. They are created to give unique name to a entity to identify it during the execution of the program. For example:

int money;
double accountBalance;

Here, money and accountBalance are identifiers.

Also remember, identifier names must be different from keywords. You cannot use int as an identifier because int is a keyword.

kinds of identifiers
        
C defines two kinds of identifiers:

• Internal
• External

Internal identifier    

                                If the identifier is used in an external link process, then it is called as external. These identifiers are also known as external names; include function names and global variable names that are shared between source files. It has at least 63 significant characters.

External identifier
                                    If the identifier is not used in an external link process, then it is called as internal. These identifiers are also known as internal names; includes the names of local variables. It has at least 31 significant characters.

Differentiate between Keywords words and identifiers

                            Keyword                                                                                                                     Identifier

Predefined-word                                                                                                        User-defined word
Must be written in lowercase only                                                                          Can written in lowercase and uppercase
Has fixed meaning                                                                                                    Must be meaningful in the program
Whose meaning has already been explained to the C compiler                     Whose meaning not explained to the C compiler
Combination of alphabetic characters                                                                  Combination of alphanumeric characters
Used only for it intended purpose                                                                          Used for required purpose
Underscore character is not considered as a letter                                           Underscore character is considered as a letter