Here's a sample scenario:
Base class: Product
Sub-classes that inherit from base class: Electronic, Clothing, MusicalInstrument, Food
local variables
- does not get set to nil prior to initialisation
- to declare one, just put a underscore in front of the name OR just the name itself (no "@" or "@@").
- they come to live only when they are assigned with a value (initialisation)
- its scope is limited to the block which it is in
- helper method: defined?( expression/argument ). Thanks to Shane Hanna for passing me this link for a better understanding of the method :)
- it will return the type of expression or argument is otherwise the value, nil.
p defined?( exchange_rate = 2.5; end )>> expression
- if given an expression, please note that defined DOES NOT RUN the expression. It just tries to figure out what it actually is
p defined?(exhange_rate = 2.5); end)>> expression
p exchange_rate
>> nil
- the reason why this is returning nil is because exchange_rate was a local variable within the expression but given that defined never runs any expression, the value of the 2.5 was in the scope of the defined expression only
- the argument is usually a single identifier. For example, a variable
def store_name; end;puts defined.store_name;
>> "method"
- Refererence: http://www.rubyist.net/~slagell/ruby/localvars.html
instance variables
- gets set to nil before it's used for the first time
- i would call them "private" or "attribute variables" of a class
- if you've done C++ or java, think of instance variables as variables with a private scope for which class accessor and mutator methods are used to interact with these variables (encapsulation virtue of object oriented programming)
- example: name of each Electronic, MusicalInstrument, Clothing, Food class. Say if stratocaster and telecaster were objects which instantiated from the Electronic class, its name and description attributes are instance variables. ** Telecaster and stratocasters are musical instruments
- Reference: http://www.rubyist.net/~slagell/ruby/instancevars.html
class variables
- gets set to nil before it's used for the first time
- scope is limited to the class and all its instances hence they are shared between all instances of the class
- to declare one, just prepend a "@@" symbol in front of the variable's name
- example: @@government_tax variable that is the same across all products being sold in a store. For Australia, the government tax (aka gst) is 10% across the board
global variables
- append a "$" to the name of the variable
- scope - available to the whole application/program
- it gets initialised to nil automaticall
- Reference: http://www.rubyist.net/~slagell/ruby/globalvars.html
constants
- begins with a letter within A-Z
- it can be reassigned but ruby will issue a warning. In perl, doing so is impossible as the interpreter restricts further assignments after the initial assignment
Variable names - ruby 1.9 and above now support utf 8
- this means that we are no longer restricted to a-zA-Z characters for variable names
- one way to explicitly have ruby select utf-8 for encoding is to have the comment, "# encoding: utf-8" at the top of the script (usually after "#!/usr/bin/ruby")
- prior to ruby 1.9, local variable names have to begin with a letter within a-z
- with ruby 1.9 onwards, utf 8 encoding is supported
- Credits to Shane Hanna's example and Ruby 1.9's Three Default Encodings
Good reference(s):
