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

I believe montly_cost should monty_cost, assuming it concerns Monty Python's Flying Circus. — Cary Swoveland
– Cary Swoveland, Commented Jun 11, 2017 at 18:55

Gerry · Accepted Answer · 2017-06-11 12:50:21Z

5

@@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

edited Jun 11, 2017 at 12:50

answered Jun 11, 2017 at 11:52

Gerry

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4 Comments

johnlock1 Over a year ago

I've actually to make both instance variables, before posting, but it didn't worked, I was getting an *': nil can't be coerced into Integer TypeError. Now I see that making Cost a local variable is the reason i'm getting the error. But why? Why @kwh works and @cost doesn't?

Gerry Over a year ago

@johnlock1 How did you set @cost = 0.0946? It should be set inside initialize method (or any other instance method).

johnlock1 Over a year ago

@gery Ok, so in order to use an instance variable I have to set it inside a method? I haven't understood that, thanks for clearing it out for me!

Gerry Over a year ago

@johnlock1 Yes, otherwise it won't be available to the instantiated object.

Cary Swoveland · Accepted Answer · 2017-06-12 01:25:29Z

Try this.

class Device
  singleton_class.send(:attr_accessor, :cost_per_kwh)

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

  def daily_cost(hours_per_day)
    self.class.cost_per_kwh * kwh_per_day(hours_per_day)
  end

  def monthly_cost(hours_per_day)
    30 * daily_cost(hours_per_day)
  end

  private

  def kwh_per_day(hours_per_day)
    hours_per_day * @watts / 1000
  end
end

singleton_class.send(:attr_accessor, :cost_per_kwh) creates a setter and getter for the class instance variable @cost_per_kwh.

First, obtain and save the cost per kwh, which will be used in the calculation of cost for all devices of interest.

puts "Please enter the cost per kwh in $"    
Device.cost_per_kwh = gets.chomp.to_f

Suppose

Device.cost_per_kwh = 0.0946

Calculate the costs for each device of interest.

puts "What is the name of the device?"
name = gets.chomp

puts "How many watts does it draw?"
watts = gets.chomp.to_f

Suppose

name = "chair"
watts = 20000.0

We may now create a class instance.

device = Device.new(name, watts)
  #=> #<Device:0x007f9d530206f0 @name="chair", @watts=20000.0>

Lastly, obtain hours per days, the only variable likely to change in future calculations of costs for the given device.

puts "How many hours do you use the #{name} daily?"
hours_per_day = gets.chomp.to_f

Lastly, suppose

hours_per_day = 0.018

then we may compute the costs.

puts "Daily cost: $#{ device.daily_cost(hours_per_day)}"
Daily cost: $0.034056€

puts "Monthly_cost (30 days/month): $#{ 30 * device.daily_cost(hours_per_day) }"
Monthly_cost (30 days/month): $1.0216800000000001

Suppose circumstances change¹ and use of the device increases. We need only update hours per day. For example,

puts "How many hours do you use the #{name} daily?"
hours_per_day = gets.chomp.to_f

Suppose now

hours_per_day = 1.5

Then

puts "Daily cost: $#{ device.daily_cost(hours_per_day)}"    
Daily cost: $2.838

puts "Monthly_cost (30 days/month): $#{ 30 * device.daily_cost(hours_per_day) }"
Monthly_cost (30 days/month): $85.14

^{1 The election of a new president, for example.}

Nice! I like singleton_class to avoid the (hardcoded) constant.

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