Keywords and Identifiers in C Programming

Back to: C Programming for Beginners

In this lesson, we will talk about allowed characters, numbers, special symbols, keywords and identifiers in C Programming.

Character Set in C Programming

C supports a wide range of Characters (256) which can be used for various purposes. The entire character set is divided into 2 parts i.e. the ASCII characters set and the extended ASCII characters set. An extensive list of all of these is given below:

Alphabets

C supports the English alphabet in both lower and upper cases. We should also keep in mind that C is a case sensitive language. This means that "a" and "A" are not the same and will be treated differently by the C compiler.

Supported Alphabets in C Language
a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z
A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

Numbers

Supported Numbers in C Language
0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Special Symbols

The below list shows the special symbols used extensively in the C language:

Symbol	Name	Use
+	Plus Sign	Used for Arithmetic Operations, Increment Operator
-	Minus Sign	Used for Arithmetic Operations, Decrement Operator
*	Multiplication Sign	Used for Arithmetic Operations, Pointers
/	Division Sign	Used for Arithmetic Operations
%	Percentage / Modulus	Used for Arithmetic Operations
()	Parenthesis	Used in Function Declarations
{}	Curly Braces	Used in Function/Structure Body
[]	Square Brackets	Used in Arrays
=	Equal To Sign	Used for Arithmetic Operations, Assignment Operator
,	Comma	Used during Multiple Declarations
;	Semicolon	Used to Terminate Statements
:	Colon	Used in Bit-Fields
' '	Single Quotes	Used in Character Variable Declaration
" "	Double Quoted	Used in String Declaration
?	Question Mark	Used in Ternary Operator
.	Period / Dot	Used in Member Selection via Object Name
#	Hash Sign	Used in Preprocessor Directive Declaration
^	Caret Sign	Used for Arithmetic and Logical Operations
~	Tilde Sign	Used for Logical Operations
!	Exclamation Mark	Used for Logical Operations
&	Ampersand Sign	Used for Logical Operations
|	Pipe Sign	Used for Logical Operations

Escape Sequences in C Programming

Escape Sequences are characters that do not fall directly into the 256 Character set, supported by C. They do not represent themselves when used inside a string and are difficult or impossible to be represented directly._[1] What this means is these special characters can’t be printed directly using the keyboard. An Escape Sequence always starts with the backslash character \ followed by a particular escape character. The below table gives a detailed list of all Escape Sequences supported in the C language:

Escape Sequence	Name	Use
\n	Newline	Used to move the cursor to the beginning of a new line
\t	Tab	Used to move the cursor to the next Tab stop
\b	Backspace	Used to move the cursor back by one space on the current line
\r	Carriage Return	Used to move the cursor to the beginning of the current line
\a	Bell(alert)	Used to produce a beep sound from the system
\\	Backslash	Used to print the Backslash (\) character
\0	Null	Denotes a null character
\'	Single Quote	Used to print the Single Quote (') character
\"	Double Quote	Used to print the Double Quote (") character

Newline (\n), Tab (\t), Backspace (\b) and Carriage Return(\r) are also known as Whitespace characters.

Keywords and Identifiers in C Programming

Keywords

Keywords are reserved words that have special meaning to the C compiler. They are used in the C language for denoting something specific and can not be declared as Identifiers. In C, Keywords are written in lowercase. Originally C had only 32 Keywords but they have been increased with newer C standards._[2]

auto	extern	short	while
break	float	signed	_Alignas2, a
case	for	sizeof	_Alignof2, a
char	goto	static	_Atomic2, b
const	if	struct	_Bool1, a
continue	inline	switch	_Complex1, b
default	int	typedef	_Generic2, a
do	long	union	_Imaginary1, b
double	register	unsigned	_Noreturn2, a
else	restrict	void	_Static_assert2, a
enum	return	volatile	_Thread_local2, b

Identifiers

Identifiers are user-given names to various C entities such as variables, functions, array, structure, symbolic constant etc. Identifiers are used extensively by the Programmer to access various entities of the C language. There are strict rules in C for naming these entities. These rules have been mentioned below:_[3]

• Identifiers can only have alphanumeric characters (a-z, A-Z, 0-9) and underscore (_) only in their name.
• Identifier names must be unique. No two Identifiers in the same program can have the same name.
• The first character of an Identifier must be an alphabet or the underscore (_). Numbers are not allowed in the first character.
• The Identifier name can’t be the same as that of a Keyword.
• Identifiers are case-sensitive.
• There must not be white spaces inside the Identifier name.

Valid Examples of Identifier Names

• Identifiers with only alphabets: var, Var, VAR.
• Identifiers with alphabets and numbers: var1, VAR123, V123ar.
• Identifiers with underscore: var_, Var_123, _VAR,var_VAR .

Invalid Examples of Identifier Names

• Identifiers starting with Numbers: 1Var, 123VAR.
• Use of space between alphabets and numbers: var 123.
• Use of special symbols: var#, Var@123.

References

1. Escape Sequences in C (GeekForGeeks)
2. C standard keywords (Microsoft Docs)
3. C Identifiers (W3Schools)