Question

Here’s a classic fizzbuzz in Ruby:

class PrimeChecker
  def print_em
    1.upto 100 do |fizzbuzz|
      if (fizzbuzz % 2) == 0 && (fizzbuzz % 5) == 0
        puts "fizzbuzz: " + fizzbuzz.to_s
      elsif (fizzbuzz % 5) == 0
        puts "fizz: "+fizzbuzz.to_s
      elsif (fizzbuzz % 2) == 0
        puts 'buzz: ' + fizzbuzz.to_s
      else
        puts "-" + fizzbuzz.to_s
      end
    end
  end
end
PrimeChecker.print_em

When I execute this, I get this error:

undefined method ‘print_em’.

I change the method to self.print_em and it works. Does this mean it’s a class method (I think so)? Was the method “not found” before because I can only call such methods in a class on actual instances of the object? If I wanted it to be a instance method what is the syntax for that? I’m trying to understand Ruby, classes and methods better.

Answer 1

Class methods are just that: called on the class. Whereas instance methods are called on an instance of that class. An example is more useful:

class Foo
  def self.bar
    "This is a class method!"
  end

  def bar
    "This is an instance method!"
  end
end

Foo.bar        # => "This is a class method!"
foo = Foo.new  # This creates "foo" to be a new instance of Foo
foo.bar        # => "This is an instance method!"

Note that “class methods” in Ruby are actually methods on the class object’s singleton. This is a rather difficult concept to explain, and you can read more about it if you’d like.