Question

How would I pass an attribute name to method sort_by, and use it within? For example, say I wanted to create a FrequencyCounter with an array of singleton objects Foo. (It’s important that they’re singletons because it means that there will be a certain number of identical Foos).

class Foo
  attr_accessor :arbitrary_sorter
  def initialize arbitrary_sorter
    @arbitrary_sorter = arbitrary_sorter
  end
end

class FrequencyCounter
  def initialize ary
    #multiset will create a hash of frequency like {3=>obj, 2->obj, 2=>obj, etc}
    @hash = Multiset.new(ary).hash
  end

  def sort_by params={}
    Hash[@hash.sort_by {|k,v| [-1 * v, -1 * k]}]
  end
end

And when I create Frequency object:

@fc = FrequencyCounter.new([Foo.get(5), Foo.get(4), Foo.get(5), Foo.get(1)])

I’d like to tell @fc what to order on:

@fc.sort_by(:arbitrary_sorter)

Is this possible?

Answer 1

on any enumerator, you can tell it what to sort by. So, let’s take a simple example:

class Animal
  attr_accessor :name, :leg_count
  def initialize(name, leg_count)
    @name = name
    @leg_count = leg_count
  end
  def to_s
    @name
  end
end

animals = [Animal.new('human', 2), Animal.new('dog', 4), Animal.new('snake', 0)]
 => [human, dog, snake] 

# sort by an attribute or method on the model
animals.sort_by{|a| a.leg_count} 
 => [snake, human, dog] 

Same as:

animals.sort_by(&:leg_count)
 => [snake, human, dog]

You can event do something like:

sorter_lambda = lambda {|animal| animal.name == "human" ? 0 : 999}
animals.sort_by(&sorter_lambda)
 => [human, dog, snake] 

The lambda may work best for you with the complicated logic I saw.