ruby class collections

how does this work?

in irb:

>&g开发者_如何学Pythont; class A
>>   b = [1, 2,3]
>> end
=> [1, 2, 3]

Is b an instance variable? class variable? how would I access b from outside the class? Is it used for meta-programming?

Is b an instance variable? class variable?

No, it's a local variable inside the class ... end scope.

how would I access b from outside the class?

You wouldn't. It goes out of scope (and is thus inaccessible) once it reaches the end.

Is it used for meta-programming?

It can be. Example:

class A
  b = [1,2,3]
  b.each do |i|
    define_method("foo#{i}") do end
  end
end

I've now defined the methods foo1, foo2 and foo3.

Of course this wouldn't behave any differently if I didn't create the variable b and just did [1,2,3].each directly. So creating a local variable by itself does nothing, it allows you to write cleaner code (the same as using local variables in a method).

b is a simple block variable, you cannot access it from outside of the block.

You can use class like this :

class Building
  @@count=0                        #This is a class variable
  MIN_HEIGHT=50                    #This is a constant
  attr_accessor :color, :size      #grant access to instance variables
  def initialize options
    @color=options[:color]         #@color is an instance variable
    @size=options[:size]           #@size too
    @@count=@@count+1
  end

  def self.build options           #This is a class method
    # Adding a new building
    building=Building.new options
  end
end
#[...]
Building.build({:color=>'red', :size=>135})
blue_building=Building.new({:color=>'blue', :size=>55})
puts blue_building.color          # How to use an instance variable
#                          => 'blue'
puts "You own  #{Building.count.to_s} buildings !"     # How to use a class variable
#                          => 'You own 2 buildings !'
puts Building::MIN_HEIGHT          # How to use a constant
#                          => 50