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The identifiers must conform to the following rules.

1. First character must be an alphabet (or underscore)
2. Identifier names must consists of only letters, digits and underscore.
3. A identifier name should have less than 31 characters.
4. Any standard C language keyword cannot be used as a variable name.
5. A identifier should not contain a space.

Constants

A constant value is the one which does not change during the execution of a program. C supports several types of constants.

1. Integer Constants
2. Real Constants
3. Single Character Constants
4. String Constants

Integer 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 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
-31
0
562321
+ 78

Some examples for invalid integer constants are

15 750
20,000
Rs. 1000

Octal Integers constant consists of any combination of digits from 0 through 7 with a O at the beginning. Some examples of octal integers are

O26
O
O347
O676

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
-0.97
435.29
+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.