Scope of instance variable on class methods

Question

I'm trying to get the following code to work.

class Animal

end

class Tiger < Animal
    @hunger = 100

    def self.hunger
        @hunger 
    end

    def run
        puts "The Tiger runs"
        @hunger += 10
    end
end

class Statistics
    puts "This tigers hunger is: #{Tiger.hunger}" 
end

tiger = Tiger.new()
tiger.run

So the tiger has a variable called hunger which is by itself at the top of the Tiger class. I want to make it so this variable can be changed throughout the class methods. For example in run the hunger is set to hunger += 10, but when I run this code I get an undefined method '+' for nil:NilClass (NoMethodError). What do I do to make this program work so the variable can be changed and then displayed in the Statistics class?

ndnenkov · Accepted Answer · 2015-08-21 19:20:15Z

2

The problem is that you have completely mushed class and instance variables/methods together.

If you want to use instances (which you should):

class Animal; end

class Tiger < Animal
  attr_reader :hunger

  def initialize
    @hunger = 100
  end

  def run
    puts "The Tiger runs"
    @hunger += 10
  end
end

class Statistics
  def self.show(tiger)
    puts "This tigers hunger is: #{tiger.hunger}" 
  end
end

tiger = Tiger.new
tiger.run
Statistics.show(tiger)

If you want to use class methods/variables:

class Animal; end

class Tiger < Animal
  @hunger = 100

  def self.hunger
    @hunger
  end

  def self.run
    puts "The Tiger runs"
    @hunger += 10
  end
end

class Statistics
  def self.show
    puts "This tigers hunger is: #{Tiger.hunger}" 
  end
end

Tiger.run
Statistics.show

Note that @hunger is now a class instance variable of Tiger. The difference between class instance variables and class variables (ones defined with @@) is that the latter is shared by all descendants of the said class, while the former is only tied to the class where it was defined.

I do not agree with your descision about Statistic in either case though.

edited Aug 21, 2015 at 19:20

answered Aug 21, 2015 at 19:02

ndnenkov

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5 Comments

James Mitchell Over a year ago

Thank you! It seems to work on the sample code so I'll see if I can incorporate it into my bigger project.

Cary Swoveland Over a year ago

Since the OP has a class instance variable (and an instance variable with the same name), and also because class instance variables are generally a better choice than class variables, you might want to say something about the former.

ndnenkov Over a year ago

@CarySwoveland, good point. By the way OP is creating his hierarchy, I was definitely about to make him hit a brick wall.

James Mitchell Over a year ago

@ndn thanks for the update and explanation. I'm going to try out a few of the answers and see what works, then I'll select a best answer. One thing I'm curious about though. What if I only plan to have a single instance of Tiger? Wouldn't it kind of make sense to use @@ or no?

ndnenkov Over a year ago

@JamesMitchell, if you have a single instance of tiger, you should define an instance and not use class variables/methods. But if you mean that you will treat the Tiger class as a single entity - as long as you don't subclass it and the super classes (Animal) don't have it, @@ will be fine. Class variables are weird.

user1943992 · Accepted Answer · 2015-08-21 19:10:34Z

1

Don't you need an initialize method in your Tiger class?

def initialize()
@hunger = 100
end

I am confused about your Statistics class. Shouldn't this be a method in the Tiger class? How does it know what Tiger to access? I would add that puts in a method taking your tiger as a parameter if you want to do that.

class Statistics
def tiger_stat(tiger_name)
puts "This tigers hunger is: #{tiger_name.hunger}" 
end
end

Edit* this is the code I would use:

class Animal
    def initialize(animal_type, hunger_start)
        @animal_type = animal_type
        @hunger = hunger_start
    end

    def hunger_print
        puts "This #{@animal_type}'s hunger is: #{@hunger}."
    end
end

class Tiger < Animal
    def initialize
        super("Tiger", 100)
    end

    def run
        puts "The Tiger runs"
        @hunger += 10
    end
end

This way you can call hunger_print on any animal without worrying about the type or making an extra class. You could do something like.

tiger = Tiger.new
tiger.run
tiger.hunger_print

edited Aug 21, 2015 at 19:10

answered Aug 21, 2015 at 18:58

user1943992

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2 Comments

James Mitchell Over a year ago

All the code I have here is just a sample for a bigger project. I have the stats class because really I have 3 animal classes and then the stats would compare the hunger of all the animals. I tried the initialize and it doesn't work.

user1943992 Over a year ago

Oh I understand. I would recommend using inheritance if you want to do something like that. Are you familiar with the concept?

Tasos Stathopoulos · Accepted Answer · 2015-08-21 19:06:25Z

0

You can use class variable that is access from both instance and class methods.

class Animal
  @@hunger = 100

  def self.hunger
    @@hunger
  end

  def self.increase_hunger
    @@hunger +=1
  end

  def hunger
    @@hunger
  end

  def increase_hunger
    @@hunger += 1
  end
end

Animal.increase_hunger
puts Animal.hunger # => 101
animal = Animal.new
animal.increase_hunger
puts animal.hunger # => 102

answered Aug 21, 2015 at 19:06

Tasos Stathopoulos

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