Variables, Literals and Constants

Variables : In programming, a variable is a container (storage area) to hold data.To indicate the storage area, each variable should be given a unique name (identifier). 

Variable names are just the symbolic representation of a memory location.

For example:   int player=40;

Rules for naming a variable

1. A variable name can have only letters (both uppercase and lowercase letters), digits and underscore.
2. The first letter of a variable should be either a letter or an underscore.
3. There is no rule on how long a variable name (identifier) can be. However, you may run into problems in some compilers if the variable name is longer than 31 characters.

Literals : Literals are data used for representing fixed values. They can be used directly in the code.

For example: 1, 2.5, 'c' etc.

1. Integers : An integer is a numeric literal (associated with numbers) without any fractional or exponential part. 

There are three types of integer literals in C programming:

1.  decimal (base 10)
2.  octal (base 8) 
3.  hexadecimal (base 16)

For example:

Decimal : 0, -9, 23 etc.

Octal : 021, 077, 045 etc.

Hexadecimal : 0x7f, 0x3a, 0x542 etc.

In C programming, octal starts with a 0, and hexadecimal starts with a 0x.

2. Floating-point Literals : A floating-point literal is a numeric literal that has either a fractional form or an exponent form.

 For example: -2.0, 0.00034,-0.33E-6 etc.

3. Characters : A character literal is created by enclosing a single character inside single quotation marks.

 For example: ’a’, ’c’, ‘s’, ‘]’ etc.

4. Escape Sequences : Sometimes, it is necessary to use characters that cannot be typed or has special meaning in C programming. 

For example: newline(enter), tab, question mark etc.

In order to use these characters, escape sequences are used.

Escape Sequences	Character
\b	Backspace
\f	Form feed
\n	Newline
\r	Return
\t	Horizontal tab
\v	Vertical tab
\\	Backslash
\'	Single quotation mark
\"	Double quotation mark
\?	Question mark
\0	Null character

For example: \n is used for a newline. The backslash \ causes escape from the normal way the characters are handled by the compiler.

5. String Literals : A string literal is a sequence of characters enclosed in double-quote marks. 

For example :

     "program"                // string constant

     "earth is round\n"    // print string with a newline

Constants : If you want to define a variable whose value cannot be changed, you can use the const keyword. This will create a constant.

For example:  const double PI=3.14;

String Literals : String literals or constants are enclosed in double quotes "". A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters.

We can break a long line into multiple lines using string literals and separating them using white spaces.

For example : 

       "hello,world"

       "hello, \

        world"

Defining Constants : There are two simple ways in C to define constants.

Using   #define preprocessor.

Using   const keyword.

The #define Preprocessor : Given below is the form to use #define preprocessor to define a constant.

Syntax:    #define identifier value

Example:   #define PI 3.14

The const Keyword : we can use const prefix to declare constants with a specific type as follows 

Syntax:

const type variable=value;

Example:  const int MAX=10;