In a C source program, the basic element recognized by the compiler is the "token." A token is source-program text that the compiler does not break down into component elements.
- TOKENS:
- Keyword
- Identifier
- Constant\Literal
- Variable
- Operator
- Punctuator
- 1. KEYWORD :
- "Keywords" are words that have special meaning to the C compiler. An identifier cannot have the same spelling and case as a C keyword. C language uses the following keywords:You cannot redefine keywords. However, you can specify text to be substituted for keywords before compilation by using C preprocessor directives.
auto double int struct break else long switch case enum register typedef char extern return union const float short unsigned continue for signed void default goto sizeof volatile do if static while
2. IDENTIFIERS :
"Identifiers" or "symbols" are the names you supply for variables, types,
functions, and labels in your program. Identifier names must differ in
spelling andcase from any keywords. You cannot use keywords (either C
or Microsoft) as identifiers; they are reserved for special use. You create
an identifier by specifying it in the declaration of a variable, type, or
function.
Once declared, you can use the identifier in later program statements to
refer to the associated value.
A special kind of identifier, called a statement label, can be used in goto
statements. (Declarations are described in Declarations and Types
Statement labels are described in The goto and Labeled Statements.)
Syntax
- identifier:
- nondigit identifier nondigit identifier digit
- nondigit: one of
- _ 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
- digit: one of
- 0 1 2 3 4 5 6 7 8 9
The first character of an identifier name must be a nondigit (that is,
the first character must be an underscore or an uppercase or lowercase
letter). ANSI allows six significant characters in an external identifier's
name and 31 for names of internal (within a function) identifiers. External
identifiers (ones declared at global scope or declared with storage class
extern) may be subject to additional naming restrictions because these
identifiers have to be processed by other software such as linkers.
the first character must be an underscore or an uppercase or lowercase
letter). ANSI allows six significant characters in an external identifier's
name and 31 for names of internal (within a function) identifiers. External
identifiers (ones declared at global scope or declared with storage class
extern) may be subject to additional naming restrictions because these
identifiers have to be processed by other software such as linkers.
Characteristic of identifier:
- The first character must be an alphabet or an underscore.
- It must consist of only letters, digits or underscore.
- Only first 31 characters are significant, i.e. if the user enters an identifier of more than 31 characters, the compiler considers only first 31 characters & deletes the others.
- A keyword cannot be used as an identifier name.
- It cannot contain any spaces
3. CONSTANTS :
A constant value is the one which does not change during the execution
of a program. C supports several types of constants.
- Integer Constants
- Real Constants
- Single Character Constants
- String Constants
An integer constant is a sequence of digits. There are 3 types of integers
namely decimal integer, octal integers and hexadecimal integer.
Decimal Integers :
Decimal integer constant consists of a set of digits 0 to 9 preceded by an
optional + or - sign. Spaces, commas and non digit characters are not
permitted between digits. Example for valid decimal integer constants
are
123
-310562321
+78
Some examples for invalid integer constants are
15 750
20, 000
Rs. 1000
Octal Integers :
Octal integer constant consists of any combination of digits from 0
through 7 with a O at the beginning. Some examples of octal integers
are
O26O
O347
O676
Hexadecimal integer:
Hexadecimal integer constant is preceded by OX or Ox, they may
contain alphabets from A to F or a to f. The alphabets A to F refers
to 10 to 15 in decimal digits. Example of valid hexadecimal integers
are
OX2
OX8C
OXbcd
Ox
Real Constants
Real Constants consists of a fractional part in their representation.
Integer constants are inadequate to represent quantities that vary
continuously. These quantities are represented by numbers containing
fractional parts like 26.082. Example of real constants are 0.0026
-0.974 35.29 +487.0. Real Numbers can also be represented by
exponential notation. The general form for exponential notation is
mantissa exponent. The mantissa is either a real number expressed in
decimal notation or an integer. The exponent is an integer number with
an optional plus or minus sign
Single Character constants
A Single Character constant represent a single character which is
enclosed in a pair of quotation symbols.Example for character
constants are
'5'
'x'
';'
' '
All character constants have an equivalent integer value which are
called ASCII Values.
String Constants
A string is a sequence of characters enclosed in double quotes. The
sequence of characters may contain letters, numbers, special characters
& blank spaces.
For eg:-
“HELLO!”
“1987”
“Well Done”
“?.....!”
“x”
A subtype of strings are Escape Sequences/Backslash character
constants. They are string s that do not get printed on screen but
have their own importance when executed.
\a - audible alert(bell)
\b - back space
\f - form feed
\n - new line
\r - carriage return
\t - horizontal tab
\v - vertical tab
\' - single quote
\” - double quote
\? - question mark
\\ - backslash
\0 - null
4. VARIABLE :
For details : click here
5. OPERATOR :
For details : click here
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