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# Ruby Variables, Constants and Literals

Variables are the memory locations which holds any data to be used by any program.

There are five types of variables supported by Ruby. You already have gone through a small description of these variables in previous chapter as well. These five types of variables are explained in this chapter.

# Ruby Global Variables:

Global variables begin with $. Uninitialized global variables have the value nil and produce warnings with the -w option.

Assignment to global variables alters global status. It is not recommended to use global variables. They make programs cryptic.

Here is an example showing usage of global variable.

    #!/usr/bin/ruby

    $global_variable = 10
    class Class1
      def print_global
         puts "Global variable in Class1 is #$global_variable"
      end
    end
    class Class2
      def print_global
         puts "Global variable in Class2 is #$global_variable"
      end
    end

    class1obj = Class1.new
    class1obj.print_global
    class2obj = Class2.new
    class2obj.print_global

Here $global_variable is a global variable. This will produce following result:

NOTE: In Ruby you CAN access value of any variable or constant by putting a hash (#) character just before that variable or constant.

    Global variable in Class1 is 10
    Global variable in Class2 is 10

# Ruby Instance Variables:

Instance variables begin with @. Uninitialized instance variables have the value nil and produce warnings with the -w option.

Here is an example showing usage of Instance Variables.

    #!/usr/bin/ruby

    class Customer
       def initialize(id, name, addr)
          @cust_id=id
          @cust_name=name
          @cust_addr=addr
       end
       def display_details()
          puts "Customer id #@cust_id"
          puts "Customer name #@cust_name"
          puts "Customer address #@cust_addr"
        end
    end

    # Create Objects
    cust1=Customer.new("1", "John", "Wisdom Apartments, Ludhiya")
    cust2=Customer.new("2", "Poul", "New Empire road, Khandala")

    # Call Methods
    cust1.display_details()
    cust2.display_details()

Here @cust_id, @cust_name and @cust_addr are instance variables. This will produce following result:

    Customer id 1
    Customer name John
    Customer address Wisdom Apartments, Ludhiya
    Customer id 2
    Customer name Poul
    Customer address New Empire road, Khandala

# Ruby Class Variables:

Class variables begin with @@ and must be initialized before they can be used in method definitions.

Referencing an uninitialized class variable produces an error. Class variables are shared among descendants of the class or module in which the class variables are defined.

Overriding class variables produce warnings with the -w option.

Here is an example showing usage of class variable:

    #!/usr/bin/ruby

    class Customer
       @@no_of_customers=0
       def initialize(id, name, addr)
          @cust_id=id
          @cust_name=name
          @cust_addr=addr
       end
       def display_details()
          puts "Customer id #@cust_id"
          puts "Customer name #@cust_name"
          puts "Customer address #@cust_addr"
        end
        def total_no_of_customers()
           @@no_of_customers += 1
           puts "Total number of customers: #@@no_of_customers"
        end
    end

    # Create Objects
    cust1=Customer.new("1", "John", "Wisdom Apartments, Ludhiya")
    cust2=Customer.new("2", "Poul", "New Empire road, Khandala")

    # Call Methods
    cust1.total_no_of_customers()
    cust2.total_no_of_customers()

Here @@no_of_customers is a class variable. This will produce following result:

    Total number of customers: 1
    Total number of customers: 2

# Ruby Local Variables:

Local variables begin with a lowercase letter or _. The scope of a local variable ranges from class, module, def, or do to the corresponding end or from a block's opening brace to its close brace {}.

When an uninitialized local variable is referenced, it is interpreted as a call to a method that has no arguments.

Assignment to uninitialized local variables also serves as variable declaration. The variables start to exist until the end of the current scope is reached. The lifetime of local variables is determined when Ruby parses the program.

In the above example local variables are id, name and addr.

# Ruby Constants:

Constants begin with an uppercase letter. Constants defined within a class or module can be accessed from within that class or module, and those defined outside a class or module can be accessed globally.

Constants may not be defined within methods. Referencing an uninitialized constant produces an error. Making an assignment to a constant that is already initialized produces a warning.

    #!/usr/bin/ruby

    class Example
       VAR1 = 100
       VAR2 = 200
       def show
           puts "Value of first Constant is #{VAR1}"
           puts "Value of second Constant is #{VAR2}"
       end
    end

    # Create Objects
    object=Example.new()
    object.show

Here VAR1 and VAR2 are constant. This will produce following result:

    Value of first Constant is 100
    Value of second Constant is 200

# Ruby Pseudo-Variables:

They are special variables that have the appearance of local variables but behave like constants. You can not assign any value to these variables.

- self: The receiver object of the current method.

- true: Value representing true.

- false: Value representing false.

- nil: Value representing undefined.

- __FILE__: The name of the current source file.

- __LINE__: The current line number in the source file.

# Ruby Basic Literals:

The rules Ruby uses for literals are simple and intuitive. This section explains all basic Ruby Literals.

## Integer Numbers:

Ruby supports integer numbers. An integer number can range from -230 to 230-1 or -262 to 262-1. Integers with-in this range are objects of class Fixnum and integers outside this range are stored in objects of class Bignum.

You write integers using an optional leading sign, an optional base indicator (0 for octal, 0x for hex, or 0b for binary), followed by a string of digits in the appropriate base. Underscore characters are ignored in the digit string.

You can also get the integer value corresponding to an ASCII character or escape sequence by preceding it with a question mark.

Example:

    123                  # Fixnum decimal
    1_234                # Fixnum decimal with underline
    -500                 # Negative Fixnum
    0377                 # octal
    0xff                 # hexadecimal
    0b1011               # binary
    ?a                   # character code for 'a'
    ?\n                  # code for a newline (0x0a)
    12345678901234567890 # Bignum

NOTE: Class and Objects are explained in a separate chapter of this tutorial.

## Floating Numbers:

Ruby supports integer numbers. They are also numbers but with decimals. Floating-point numbers are objects of class Float and can be any of the following:

Example:

    123.4                # floating point value
    1.0e6                # scientific notation
    4E20                 # dot not required
    4e+20                # sign before exponential

## String Literals:

Ruby strings are simply sequences of 8-bit bytes and they are objects of class String. Double-quoted strings allow substitution and backslash notation but single-quoted strings don't allow substitution and allow backslash notation only for \\ and \'

Example:

    #!/usr/bin/ruby -w

    puts 'escape using "\\"';
    puts 'That\'s right';

This will produce following result:

    escape using "\"
    That's right

You can substitute the value of any Ruby expression into a string using the sequence #{ expr }. Here expr could be any ruby expression.

    #!/usr/bin/ruby -w

    puts "Multiplication Value : #{24*60*60}";

This will produce following result:

    Multiplication Value : 86400

## Backslash Notations:

Following is the list of Backslash notations supported by Ruby:

Notation | Character represented
--- | ---
\n | Newline (0x0a)
\r | Carriage return (0x0d)
\f | Formfeed (0x0c)
\b | Backspace (0x08)
\a | Bell (0x07)
\e | Escape (0x1b)
\s | Space (0x20)
\nnn | Octal notation (n being 0-7)
\xnn | Hexadecimal notation (n being 0-9, a-f, or A-F)
\cx, \C-x | Control-x
\M-x | Meta-x (c | 0x80)
\M-\C-x | Meta-Control-x
\x | Character x

For more detail on Ruby Strings, go through Ruby Strings.

## Ruby Arrays:

Literals of Ruby Array are created by placing a comma-separated series of object references between square brackets. A trailing comma is ignored.

## Example:

    #!/usr/bin/ruby

    ary = [  "fred", 10, 3.14, "This is a string", "last element", ]
    ary.each do |i|
       puts i
    end

This will produce following result:

    fred
    10
    3.14
    This is a string
    last element

For more detail on Ruby Arrays, go through Ruby Arrays.

## Ruby Hashes:

A literal Ruby Hash is created by placing a list of key/value pairs between braces, with either a comma or the sequence => between the key and the value. A trailing comma is ignored.

## Example:

    #!/usr/bin/ruby

    hsh = colors = { "red" => 0xf00, "green" => 0x0f0, "blue" => 0x00f }
    hsh.each do |key, value|
       print key, " is ", value, "\n"
    end

This will produce following result:

    green is 240
    blue is 15
    red is 3840

For more detail on Ruby Hashes, go through Ruby Hashes.

## Ruby Ranges:

A Range represents an interval.a set of values with a start and an end. Ranges may be constructed using the s..e and s...e literals, or with Range.new.

Ranges constructed using .. run from the start to the end inclusively. Those created using ... exclude the end value. When used as an iterator, ranges return each value in the sequence.

A range (1..5) means it includes 1, 2, 3, 4, 5 values and a range (1...5) means it includes 1, 2, 3, 4 values.

## Example:

    #!/usr/bin/ruby

    (10..15).each do |n|
       print n, ' '
    end

This will produce following result:

    10 11 12 13 14 15

For more detail on Ruby Ranges, go through Ruby Ranges.

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