Keywords and Identifiers

Keywords and Identifiers

C TOKENS:

• C tokens are the basic buildings blocks in C language which are constructed together to write a C program.
• Each and every smallest individual units in a C program are known as C tokens.

• C tokens are of six types. They are,

1. Keywords               (eg: int, while),
2. Identifiers               (eg: main, total),
3. Constants              (eg: 10, 20),
4. Strings                    (eg: “total”, “hello”),
5. Special symbols  (eg: (), {}),
6. Operators              (eg: +, /,-,*)

Keywords are the reserved words in programming and are part of the syntax. 

Character set

Character set is a set of alphabets, letters and some special characters that are valid in C language.

Alphabets

Uppercase: A B C ................................... X Y Z

Lowercase: a b c ...................................... x y z

Digits

0 1 2 3 4 5 6 7 8 9

Special Characters

,	<	>	.	_
(	)	;	$	:
%	[	]	#	?
'	&	{	}	"
^	!	*	/	|
-	\	~	+

White space Characters

blank space, new line, horizontal tab, carriage return and form feed.

Carriage return:

Carriage return means to return to the beginning of the current line without advancing downward. The name comes from a printer's carriage, as monitors were rare when the name was coined. This is commonly escaped as "\r", abbreviated CR, and has ASCII value 13 or 0x0D.

Line feed:

Linefeed means to advance downward to the next line; however, it has been repurposed and renamed. Used as "newline", it terminates lines (commonly confused with separating lines). This is commonly escaped as "\n", abbreviated LF or NL, and has ASCII value 10 or 0x0A. CRLF (but not CRNL) is used for the pair "\r\n".

Form feed:

Form feed means advance downward to the next "page". It was commonly used as page separators, but now is also used as section separators. (It's uncommonly used in source code to divide logically independent functions or groups of functions.) Text editors can use this character when you "insert a page break". This is commonly escaped as "\f", abbreviated FF, and has ASCII value 12 or 0x0C.

Keywords

Keywords are predefined, reserved words used in programming that have special meaning. 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.

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

Standard Identifies in c (Predefined)

Identifiers

Identifiers are the names that are given to various program elements such as variables, symbolic constants and functions.Variable or function identifier that is called a symbolic constant name.

Identifier names must be unique. They are created to give unique name to a C entity to identify it during the execution of a 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

1. A valid identifier can have letters (both uppercase and lowercase letters), digits and underscore only.
2. 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. It is because identifier that starts with an underscore can conflict with system names.
3. In such cases, compiler will complain about it. Some system names that start with underscore are _fileno, _iob, _wfopen etc.
4. There is no rule on the length of an identifier. However, the first 31 characters of identifiers are discriminated by the compiler. So, the first 31 letters of two identifiers in a program should be different.
5. There should be no blank space while declaring an identifier
6. No special characters, such as semicolon,period,blank space, slash or comma are permitted.

Good Programming Practice:

You can choose any name for an identifier. However, if the programmer choose meaningful name for an identifier, it will be easy to understand and work on.