Class variable confusion

ruby-talk
1

Hi,

Could someone explain how i get this to work please:

module Keith
  def self.greeting
    @greeting ||= "hello"
  end
  def self.greeting=(value)
    puts "updating knowledge"
    @greeting = value
  end
  def self.speak
    puts greeting
  end
  def self.learn_ruby
    self.greeting = "ruby"
  end
  def self.learn_french
    learn_language(greeting, "bonjour")
  end
  def self.learn_language(prop, value)
    puts prop.inspect
    prop = value
    puts prop.inspect
    #p =
    value
  end
end

Keith.speak
Keith.learn_ruby
Keith.learn_french
Keith.speak

This outputs:

hello
updating knowledge
"ruby"
"bonjour"
ruby

I can't for the life of me get it to work...

many thanks
keith

···

--
Posted via http://www.ruby-forum.com/.

2

Or more simply, i would like this to update the variable @greeting, when
i call Keith.learn_french:

module Keith
  def self.greeting
    @greeting ||= "hello"
  end
  def self.greeting=(value)
    puts "updating knowledge"
    @greeting = value
  end
  def self.speak
    puts greeting
  end
  def self.learn_french
    learn_language(greeting, "bonjour")
  end
  def self.learn_language(prop, value)
    prop = value
  end
end

Keith.speak
Keith.learn_french
Keith.speak

But the only way i can get it to work is using this:

module Keith
  def self.greeting
    @greeting ||= "hello"
  end
  def self.greeting=(value)
    puts "updating knowledge"
    @greeting = value
  end
  def self.speak
    puts greeting
  end
  def self.learn_french
    self.greeting = learn_language(greeting, "bonjour")
  end
  def self.learn_language(prop, value)
    prop = value
  end
end

Keith.speak
Keith.learn_french
Keith.speak

Which is basically the same as:

module Keith
  def self.greeting
    @greeting ||= "hello"
  end
  def self.greeting=(value)
    puts "updating knowledge"
    @greeting = value
  end
  def self.speak
    puts greeting
  end
  def self.learn_french
    self.greeting = "bonjour"
  end
end

Keith.speak
Keith.learn_french
Keith.speak

···

--
Posted via http://www.ruby-forum.com/.

3

Or more simply, i would like this to update the variable @greeting, when
i call Keith.learn_french:

module Keith
def self.greeting
@greeting ||= "hello"
end
def self.greeting=(value)
puts "updating knowledge"
@greeting = value
end
def self.speak
puts greeting
end
def self.learn_french
learn_language(greeting, "bonjour")

Here, you are calling the greeting method (which returns "hello")

end
def self.learn_language(prop, value)
prop = value

Here you are reassigning a local variable, which goes out of scope
when the method ends.
Try this:

jesus@jesus-laptop:~/temp/ruby$ ruby class_variables.rb
hello
bonjour

jesus@jesus-laptop:~/temp/ruby$ cat class_variables.rb
module Keith
def self.greeting
   @greeting ||= "hello"
end
def self.greeting=(value)
   puts "updating knowledge"
   @greeting = value
end
def self.speak
   puts greeting
end
def self.learn_french
   learn_language(:@greeting, "bonjour")
end
def self.learn_language(prop, value)
   instance_variable_set(prop, value)
end
end

Keith.speak
Keith.learn_french
Keith.speak

or if you want to call the self.greeting= method, try this variation:

def self.learn_french
   learn_language(:greeting, "bonjour")
end
def self.learn_language(prop, value)
   send("#{prop}=", value)
end

Hope this helps,

Jesus.

···

On Thu, Apr 23, 2009 at 4:01 PM, Keith Salisbury <keithsalisbury@gmail.com> wrote:

4

Or more simply, i would like this to update the variable @greeting, when
i call Keith.learn_french:

module Keith
def self.greeting
   @greeting ||= "hello"
end
def self.greeting=(value)
   puts "updating knowledge"
   @greeting = value
end
def self.speak
   puts greeting
end
def self.learn_french
   learn_language(greeting, "bonjour")
end
def self.learn_language(prop, value)
   prop = value
end
end

Keith.speak
Keith.learn_french
Keith.speak

But the only way i can get it to work is using this:

module Keith
def self.greeting
   @greeting ||= "hello"
end
def self.greeting=(value)
   puts "updating knowledge"
   @greeting = value
end
def self.speak
   puts greeting
end
def self.learn_french
   self.greeting = learn_language(greeting, "bonjour")
end
def self.learn_language(prop, value)
   prop = value
end
end

Keith.speak
Keith.learn_french
Keith.speak

Which is basically the same as:

module Keith
def self.greeting
   @greeting ||= "hello"
end
def self.greeting=(value)
   puts "updating knowledge"
   @greeting = value
end
def self.speak
   puts greeting
end
def self.learn_french
   self.greeting = "bonjour"
end
end

Keith.speak
Keith.learn_french
Keith.speak
--
Posted via http://www.ruby-forum.com/\.

Keeping the other methods the same:

module Keith
   def self.learn_french
     learn_language(:greeting, "bonjour")
   end
   def self.learn_language(prop, value)
     self.send("#{prop}=", value)
   end
end

Keith.speak

hello
=> nil

Keith.learn_french

updating knowledge
=> "bonjour"

Keith.speak

bonjour
=> nil

Although I'd think that you'd be better with a Speaker class and do:
Keith = Speaker.new
Then you'd be dealing with instance variables on instances of the Speaker class rather than instance variables on the Keith module. Contrary to what you might think, you don't have class variables in your code. (Those would be @@greeting)

-Rob

Rob Biedenharn http://agileconsultingllc.com
Rob@AgileConsultingLLC.com

···

On Apr 23, 2009, at 10:01 AM, Keith Salisbury wrote:

5

Jesús Gabriel y Galán wrote:

Hope this helps,

Jesus.

Legend

Thankyou!!

···

--
Posted via http://www.ruby-forum.com/\.

6

Hi --

Keith Salisbury wrote:

Jesús Gabriel y Galán wrote:

Hope this helps,

Jesus.

Legend

Thankyou!!

It's worth mentioning that none of the code in this thread contains any class variables. What you've got are instance variables (belonging to a class object). Class variables look like @@this.

David

···

--
David A. Black / Ruby Power and Light, LLC
Ruby/Rails consulting & training: http://www.rubypal.com
Now out in PDF: The Well-Grounded Rubyist (http://manning.com/black2\)
"Ruby 1.9: What You Need To Know" Envycasts with David A. Black
http://www.envycasts.com