Character set

The set of characters that are used to form words, numbers, and expressions in C is called a character set of C. The combination of these characters forms words, numbers, and expressions. Characters and tokens of C are vital elements of C.

The C character set is grouped into the following four categories:
1) Letters or alphabets
2) Digits
3) Special Characters
4) White Spaces

Letters

1. Uppercase: A……Z
2. Lowercase: a……z

Digits

All decimal digits: 0……9

Special Characters

 

1) , comma	2) . period	3) ; semicolon	4) : colon
5) ? question mark	6) ‘ apostrophe	7) “ quotation mark	8) ! exclamation mark
9) | vertical bar	10) / slash	11) \ backslash	12) ~ tilde
13) _ underscore	14) $ dollar sign	15) % percent sign	16) & ampersand
17) ^ caret	18) *asterisk	19) -minus sign	20) + plussign
21) < opening angle bracket (or less than sign)	22) >closing angle bracket (or greater than sign)	23) ( left parenthesis	24) ) right parenthesis
25) [ left bracket	26) ] right bracket	27) { left-brace	28) } right brace
29) # number sign

White Spaces

1. Blank space
2. Horizontal tab
3. Carriage return
4. Newline
5. Form feed

C tokens

1. In a passage of text, individual words and punctuation marks are called tokens. Similarly, in a C program, the smallest individual units are known as tokens of C.

2. The basic elements recognized by the C compiler are known as “C tokens”.

3. E.g. of C tokens are: keywords (e.g. float, while), identifiers (e.g. num, sum), constants (e.g. 15.5, 100), string literals (e.g. “ABC”, “year”), operators (e.g. +, ,*,/) and special symbols (e.g. [], {}, (), ,).

Every C word is classified as either a keyword or an identifier.

Keywords are predefined words in C programming language.
• All keywords have fixed meaning and these meanings cannot be changed.
• Keywords serve as basic building blocks for program statements.
• Keywords are also called reserved words because they are used for pre-defined purposes and cannot be used as identifiers.

There are generally 32 keywords:

1) auto	2) break	3) case	4) char
5) const	6) continue	7) default	8) do
9) double	10) else	11) enum	12) extern
13) float	14) for	15) goto	16) if
17) int	18) long	19) register	20) return
21) short	22) signed	23) sizeof	24) static
25) struct	26) switch	27) typedef	28) union
29) unsigned	30) void	31) volatile	32) while

note: keywords are written in lower case

Identifiers

• Every word used in the C program to refer to the names of variables, functions, arrays, pointers and symbolic constants are called identifiers.
• These are user-defined names and consist of a sequence of letters and digits, with a letter as the first character.
• Both uppercase and lowercase letters can be used, although lowercase letters are commonly preferred.
• The underscore character can also be used to link between two words in long identifiers.

Rules for identifiers

1) The first character must be an alphabet (or underscore).
2) Must consist of only letters, digits, or underscore.
3) It must not contain white space. The only underscore is permitted.
4) Keywords cannot be used.
5) Only the first 31 characters are significant.
6) It is case sensitive, i.e. uppercase and lowercase letters are not interchangeable.