How can I avoid using class variables in Ruby - 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

Answer 2

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.