Ruby: Variable initialization within classes

Question

Having some trouble when it comes to initializing variables within a class (instance variables etc.) and I was wondering if anybody could clarify the proper syntax for me.

Sample code:

Class Pets
    attr_accessor :name

    def initialize(name)
        @name=name
    end

    def name=(name)
        @name = name
        #I believe this is where I change @name instance variable
    end
    #in this space I could create more <methods> for Class.new.<method>
end

My question is do I need to have attr_accessor as well as def initialize and def name=?

In addition, if I have multiple attr_accessors do I need to add them as arguments to def initialize, e.g.:

Class Pets
    attr_accessor :name :age :color

    def initialize(name, age, color)
        @name = name
        @age = age
        @color = color

 #and if this is the case do I need methods for each (name= age= color= etc.)
end

One last thing: If someone could confirm or deny my thought process on the name= age= and color= type of methods within the classes. Am I correct in thinking method= is necessary to change the instance variable? I am a bit unsure about what the method= is for and why I cannot change the instance variable within initialize.

BroiSatse · Accepted Answer · 2013-09-16 22:52:59Z

1

attr_accessor :symbol do the same as attr_writer :symbol and attr_reader :symbol, i.e. it creates both reader (def symbol; @symbol; end) and writer (def symbol=(value); @symbol = value; end).

Initialize is a method called every time new instance of the class is being created. It is not the same as new method as some classes may have its own custom factory methods. You don't need to define your initialize method, only problem is that then symbol reader would return nil, as the local variable would not been set.

In ruby everything is a method. In case of objects, object.attr = value is just a short for object.attr=(value) where attr= is just another method. (Similarly << operator is defined as a method on Array class, attr_accessor is a method defined on class "Class").

answered Sep 16, 2013 at 22:52

BroiSatse

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2 Comments

Sai Dandamudi Over a year ago

Ah ok that makes sense. I have seen some examples where the code does not include a method= value but instead something along the lines of def method (argument) /n @instance variable so I was confused as to why I needed to use the method=. For clarification, I am doing an rspec tutorial and when I took the = out of the method= the tests did not pass. I, therefore, assumed that it was always necessary. How would I go about defining multiple instance variables (as in the second section of code)?

BroiSatse Over a year ago

You just go with attr_accessor :attr1, :attr2, :attr 3 ...

Lorena Nicole · Accepted Answer · 2013-11-21 04:19:33Z

To piggy back on what what said earlier, recall that if you want your attributes to be accessible outside your class (you want to write over the attribute value or you want to read it) you will need to use the attr_accessor (or attr_writer or attr_reader).

If I had a class like ...

class Calendar
  attr_reader :event_name, :all_events

  def initialize
    @event_name = event_name
    @all_events = [] 
  end

 def create_event(event_name)
   puts "#{event_name} has been added to your calendar."
   @all_events << event_name
   p @all_events
 end

 def see_all_events
   puts "Here are your events --"
   @all_events.each {|event| puts "- #{event}"}
 end

end

my_calendar=Calendar.new
my_calendar.create_event("interview")
my_calendar.see_all_events
my_calendar.all_events

I can read all my events either with the method see_all_events or by calling all_events on my class Calendar object. If for some reason I did not want a see_all_events method but instead only wanted it to be seen by calling all_events on my object I can only do this because of attr_reader.

Basically the point here is to remember exactly how you want your users to interact with your object attributes. If it needs to be private and only accessed via methods then you should be weary of using attr_accessor or attr_writer or attr_reader (depending on the situation).

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