Keywords and Identifiers in C

C Keywords
Keywords are predefined, reserved words used in programming that have special meanings to the compiler. Keywords are part of the syntax and they cannot be used as an identifier. 

 

For example:
int money; 

Here, int is a keyword that indicates 'money' is a variable of type integer. As C is a case sensitive language, all keywords must be written in lowercase. Here is a list of all keywords allowed in ANSI C.There are 32 keywords.

Keywords In C

Along with these keywords, C supports other numerous keywords depending upon the compiler.

C Identifiers

Identifier refers to name given to entities such as variables, functions, structures etc. 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. 

Rules for writing an identifier

• A valid identifier can have letters (both uppercase and lowercase letters), digits and underscores.
• The first letter of an identifier should be either a letter or an underscore. However, it is discouraged to start an identifier name with an underscore.
• There is no rule on length of an identifier. However, the first 31 characters of identifiers are discriminated by the compiler. 
• Identifiers are case sensitive ie; X is different from x.
• Keywords should not be used as identifiers.

The following are valid identifiers

x    y12     sum_1     _temperature     names     area     tax_rate  TABLE

The following names are not valid identifiers for the reasons stated.

2csbmec        The first character must be a letter.

mec#cs2b      Illegal character  #.

order-no         Illegal character (-).

mec cs2b        Illegal character (blank space)

An identifier can be arbitrarily long. Some implementations of C recognize only the first eight characters, though most implementations recognize more (typically, 31 characters).The identifiers file-manager and file-management are both grammatically valid. Some compilers may be unable to distinguish between them, however, because the first eight letters are the same for each identifier. Therefore, only one of these identifiers should be used in a single C program.

Good Programming Practice

You can choose any name for an identifier (excluding keywords). However, if you give meaningful name to an identifier, it will be easy to understand and work on for you and your fellow programmers.