How can I avoid using class variables in Ruby

Question

I have a piece of code that uses class variables. I've read that class variables should generally be avoided in Ruby.

The class variables are @@cost and @@kwh.

How can I rewrite the following without using class variables?

class Device
 attr_accessor :name, :watt

 @@cost = 0.0946

 def initialize(name, watt)
   @name = name
   @watt = watt
 end

  def watt_to_kwh(hours)
    @@kwh = (watt / 1000) * hours
  end

  def cost_of_energy
    puts "How many hours do you use the #{self.name} daily?"
  hours = gets.chomp.to_i
    self.watt_to_kwh(hours)
    daily_cost = @@kwh * @@cost
    montly_cost = daily_cost * 30
    puts "Dayly cost: #{daily_cost}€"
    puts "montly_cost: #{montly_cost}€"
  end
end

Answer 1

@@cost behaves more like a constant (i.e. it won't change during runtime), so you should use one instead:

COST = 0.0946

@@kwh should be an instance variable, since it is used only within the instantiated object, so you could use @kwh instead:

@kwh = (watt / 1000) * hours

And daily_cost = @@kwh * @@cost will become:

daily_cost = @kwh * COST

---

That will avoid the use of class variables, but you could also eliminate @kwh altogether since you don't use it anywhere else.

So, instead of:

def watt_to_kwh(hours)
  @kwh = (watt / 1000) * hours
end

You could just do:

def watt_to_kwh(hours)
  (watt / 1000) * hours
end

And use it like this in cost_of_energy method:

def cost_of_energy
  puts "How many hours do you use the #{self.name} daily?"
  hours = gets.chomp.to_i
  daily_cost = watt_to_kwh(hours) * COST
  montly_cost = daily_cost * 30
  puts "Dayly cost: #{daily_cost}€"
  puts "montly_cost: #{montly_cost}€"
end