Class Methods

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 12 of 57.

A class method belongs to the class, not to one instance. In the class body self is the class, so def self.name defines a method on the class. You call it on the class itself:

class Temperature
  def self.to_fahrenheit(celsius)
    celsius * 9 / 5.0 + 32
  end
end

puts Temperature.to_fahrenheit(100)
puts Temperature.to_fahrenheit(-40)

Output:

212.0
-40.0

Class methods and instance methods live in different places. An instance cannot call a class method directly; it reaches it through self.class or the class name:

class Circle
  def self.area_for(r)
    (3.14159 * r * r).round(2)
  end

  def initialize(r)
    @r = r
  end

  def area
    self.class.area_for(@r)
  end
end

puts Circle.area_for(1)
puts Circle.new(2).area
begin
  Circle.new(2).area_for(2)
rescue NoMethodError => e
  puts e.message
end

Output:

3.14
12.57
undefined method `area_for' for an instance of Circle

class << self opens a section of the class body in which every def is a class method. It saves writing self. when a class has several of them:

class Units
  class << self
    def km_to_miles(km)
      (km * 0.621371).round(1)
    end

    def miles_to_km(miles)
      (miles / 0.621371).round(1)
    end
  end
end

puts Units.km_to_miles(10)
puts Units.miles_to_km(26.2)

Output:

6.2
42.2

Use a class method for work that belongs to the kind of thing rather than to one object: converting units, parsing text into an object or keeping a count of all instances. Work that needs one object's data stays an instance method:

class Invoice
  def self.tax_rate      # the same for every invoice
    0.2
  end

  def tax                # needs this invoice's amount
    @amount * Invoice.tax_rate
  end
end

Challenge

Easy

Complete the class methods of Temperature. to_fahrenheit(c) and to_celsius(f) convert a value. feel(celsius) returns freezing below 0, cold below 15, mild below 25 and hot otherwise. convert(text) takes text such as 20 C or 68 F and returns 20.0 C = 68.0 F (mild) or 68.0 F = 20.0 C (mild), with one decimal and the feel of the Celsius value; it uses the other class methods.

The supplied code calls Temperature.convert(line) for every input line and prints the result. No instance of Temperature is created.

Your code goes in temperature.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'temperature'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
input.each { |line| puts Temperature.convert(line) }