C Tokens

C-Tokens

•  Identifier
•  Variable
•  Constants
•  Keywords
•  Data types

Identifier

•  Simply references to memory locations,which can hold values (data).
• Formed by combining
• Letters (both upper and lowercase)
• Digits (0–9)
• Underscore ( _ )

Identifier Rules

• The first character of an identifier must be a letter,an underscore ( _ ) also counts as a letter
• The blank or white space character(ascii value) is not permitted in an identifier.
• Can be any length. Internal identifier (do not have the external linkage) such as preprocessor macro names at least the first 31 characters are significant, also implementation dependent.
• Reserved words/keywords and characters such as main and # also cannot be used.

Variable

• Identifier that value may change during the program execution.
• Initializing a variable means, give a value to the variable, that is the variable’s initial value and can be changed later on.

Syntax - variable

1.  Type
2.  Name
3.  Value

Syntax :
            datatype name = <value>;
                   Ex: int x=4;

Keywords

 C - keyword List

• Keywords are the words whose meaning has already been explained to the ‘C’ compiler.
• The keywords are also called as reserved words.
• There are 32 keywords available in ‘C’.

Constants

• integer-constant
• character-constant
• floating-constant
• enumeration-constant

Integer-constant

• consisting of a sequence of digits
• number preceded by 0 : octal number
• number preceded by 0x: hexadecimal
• integer constant may be suffixed by letter

     – The suffix U (or u) forces the constant to be unsigned. It is                unsigned long if the value of the number itself is greater than            decimal 2,147,483,647, regardless of which base is used.
     – The suffix L (or l) attached to any constant forces the constant          to be represented as a long.
      – UL or ul : unsigned long


character-constant

• sequence of one or more characters enclosed in single quotes    Ex: ‘a’
• Character constants do not contain the newlines character in order to represent them following escape sequences may be used
• There is an extended set of characters that cannot be represented in the char type. 
• A constant in this extended set is written with a preceding L

                     Ex: L‘a’

Float & Enumeration Constant

Floating constant

• floating constant consists of a decimal part and a fraction part.
• A floating-point constant that is not suffixed is of type double.
• F or f for a float and L and I for long double.

Enumeration constant

• Identifiers declared as enumerators are constants of type int.
• Enumeration constants are identifiers defined in enum type declarations

        Ex: enum group { green, blue, red, white}


Why data type?

Variable

• VALUE (100, 1000 etc…)
• TYPE (char, int etc…)

                    ------- data type

int

• int will normally be the natural size for aparticular machine
• To develop portable programs, two suffixes are used : “short” and “long”
• Short is often 16 bits, long 32 bits

Categories in data types

Type Specifiers:
                     char       double         enum
                      int         struct           typedef
                      float      union           void

Special type specifiers:

• signed short
• unsigned long

 Type qualifiers

• Const
• Volatile

Special Type Specifiers

Signed & unsigned

• shows effect on sign of the variable.
• can be used with int and char.
• cannot be used with float and double.

Long & short

• shows effect on size of the variable.
• can be used with int and char.
• cannot be used with float and double.
• long can be used with double.