Why this instance variable is not incrementing?

Question

This short code is working when I'm using class variable @@points instead of @points. I wonder why it's happening like this? Someone can explain me? It looks like @points is always nil.

class Game

  @points = 0

  def start

    until @points == 10
      puts "Guess number between 0 and 10:" 
      num = gets.chomp.to_i
      break if @points == 0
      guess_number(num)
      puts "Your score is: #{@points}"

    end

  end

  def guess_number(num)
    @points += 1 if num == rand(0..10)
  end

end

game = Game.new
game.start

A minuscule point: gets.chomp.to_i is often written without chomp: gets.to_i. That's because "123".to_i, "123\n".to_i and "123X456abc".to_i all return 123. See String#to_i, which includes, "Extraneous characters past the end of a valid number are ignored. ". — Cary Swoveland
– Cary Swoveland, Commented Oct 24, 2016 at 22:11

Andrey Deineko · Accepted Answer · 2016-10-25 06:47:34Z

2

Because @points is a class instance variable, and to access it from within the instance method's scope you'd have to either do

self.class.instance_variable_get(:@points)

or define an attr_accessor in Game's singleton_class

class Game; class << self; attr_accessor :points; end; end

and then you'd be able to do

self.class.points

But neither of these is what you really want.

code is working when I'm using class variable @@points instead of @points

It is working, because you do have access to class variable from within a scope of an instance methods.

It looks like @points is always nil

It is always nil, because you never defined an instance variable @points, but, as said, class instance variable.

So these three things are different (you could read up something about Ruby scoping - do not mix with AR scopes):

class variable
class instance variable
instance variable

To solve it there are many ways, but if you want to keep it on the instance level, wrap @points into a method:

def points
  @points ||= 0
end

And then use it as points - now it'll work as you are expecting.

edited Oct 25, 2016 at 6:47

answered Oct 24, 2016 at 18:46

Andrey Deineko

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3 Comments

pjs Over a year ago

If OP didn't want to expose the points outside the class, wouldn't it just be easier to implement a constructor that sets @points = 0?

Andrey Deineko Over a year ago

@pjs as i said, there's quit a few ways to do it, so it's up to OP to choose, but of course, it's one of them :)

Cary Swoveland Over a year ago

It may be misleading to say that the value of a class instance variable cannot be obtained within an instance's scope, since that can be done with self.class.instance_variable_get(:@points) (as you well-know).

Andrey Deineko · Accepted Answer · 2016-10-25 18:11:49Z

0

Thanks to answer by Andrey Deineko. I came up with such solution to use instance variable here.

class Game

  def initialize
    @points = 0
  end

  def start

    until points == 10
      puts "Guess number between 0 and 10:" 
      num = gets.chomp.to_i
      break if points == 10
      guess_number(num)
      puts "Your score is: #{points}"

    end

  end

  private

  def guess_number(num)
    if num == rand(0..10)
      increment_points
    end
  end

  def points
    @points
  end

  def increment_points
    @points += 1
  end

end

game = Game.new
game.start

edited Oct 25, 2016 at 18:11

Andrey Deineko

52.5k11 gold badges119 silver badges151 bronze badges

answered Oct 24, 2016 at 19:05

Hubert Jakubiak

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4 Comments

pjs Over a year ago

The hazard of this approach is that a user who pokes into your class will find they can call increment_points directly without playing the game. If you want to use an instance variable, add an initialize method to the class where the @points get initialized to 0 when you create a Game.new instance.

Hubert Jakubiak Over a year ago

It seems to be good idea. I edited my anwser and also I made some of methods private.

Dave Newton Over a year ago

You're also subverting canonical Ruby standards by naming the getter something other than what attr_reader would name it. Just something to consider.

Hubert Jakubiak Over a year ago

I changed it to just points method not get_points. Thanks for your advice.

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