How do I add 'each' method to Ruby object (or should I extend Array)?

I have an object Results that contains an array of result objects along with some cached statistics about the objects in the array. I'd like the Results object to be able to behave like an array. My first cut at this was to add methods like this

 def <<(val)
    @result_array << val
 end

This feels very c-like and I know Ruby has better way.

I'd also like to be able to do this

 Results.each do |result|   
    result.do_stuff   
 end

but am not sure what the each method is really doing under the hood.

Currently I simply return the underlying array via a method and call e开发者_运维技巧ach on it which doesn't seem like the most-elegant solution.

Any help would be appreciated.

For the general case of implementing array-like methods, yes, you have to implement them yourself. Vava's answer shows one example of this. In the case you gave, though, what you really want to do is delegate the task of handling each (and maybe some other methods) to the contained array, and that can be automated.

require 'forwardable'

class Results
  include Enumerable
  extend Forwardable
  def_delegators :@result_array, :each, :<<
end

This class will get all of Array's Enumerable behavior as well as the Array << operator and it will all go through the inner array.

Note, that when you switch your code from Array inheritance to this trick, your << methods would start to return not the object intself, like real Array's << did -- this can cost you declaring another variable everytime you use <<.

each just goes through array and call given block with each element, that is simple. Since inside the class you are using array as well, you can just redirect your each method to one from array, that is fast and easy to read/maintain.

class Result
    include Enumerable

    def initialize
        @results_array = []
    end

    def <<(val)
        @results_array << val
    end

    def each(&block)
        @results_array.each(&block)
    end
end

r = Result.new

r << 1
r << 2

r.each { |v|
   p v
}

#print:
# 1
# 2

Note that I have mixed in Enumerable. That will give you a bunch of array methods like all?, map, etc. for free.

BTW with Ruby you can forget about inheritance. You don't need interface inheritance because duck-typing doesn't really care about actual type, and you don't need code inheritance because mixins are just better for that sort of things.

Your << method is perfectly fine and very Ruby like.

To make a class act like an array, without actually inheriting directly from Array, you can mix-in the Enumerable module and add a few methods.

Here's an example (including Chuck's excellent suggestion to use Forwardable):

# You have to require forwardable to use it
require "forwardable"

class MyArray
  include Enumerable
  extend Forwardable

  def initialize
    @values = []
  end

  # Map some of the common array methods to our internal array
  def_delegators :@values, :<<, :[], :[]=, :last

  # I want a custom method "add" available for adding values to our internal array
  def_delegator :@values, :<<, :add

  # You don't need to specify the block variable, yield knows to use a block if passed one
  def each
    # "each" is the base method called by all the iterators so you only have to define it
    @values.each  do |value| 
      # change or manipulate the values in your value array inside this block
      yield value
    end
  end

end

m = MyArray.new

m << "fudge"
m << "icecream"
m.add("cake")

# Notice I didn't create an each_with_index method but since 
# I included Enumerable it knows how and uses the proper data.
m.each_with_index{|value, index| puts "m[#{index}] = #{value}"}

puts "What about some nice cabbage?"
m[0] = "cabbage"
puts "m[0] = #{m[0]}"

puts "No! I meant in addition to fudge"
m[0] = "fudge"
m << "cabbage"
puts "m.first = #{m.first}"
puts "m.last = #{m.last}"

Which outputs:

m[0] = fudge
m[1] = icecream
m[2] = cake
What about some nice cabbage?
m[0] = cabbage
No! I meant in addition to fudge
m.first = fudge
m.last = cabbage

This feels very c-like and I know Ruby has better way.

If you want an object to 'feel' like an array, than overriding << is a good idea and very 'Ruby'-ish.

but am not sure what the each method is really doing under the hood.

The each method for Array just loops through all the elements (using a for loop, I think). If you want to add your own each method (which is also very 'Ruby'-ish), you could do something like this:

def each
  0.upto(@result_array.length - 1) do |x|
    yield @result_array[x]
  end
end

If you create a class Results that inherit from Array, you will inherit all the functionality.

You can then supplement the methods that need change by redefining them, and you can call super for the old functionality.

For example:

class Results < Array
  # Additional functionality
  def best
    find {|result| result.is_really_good? }
  end

  # Array functionality that needs change
  def compact
    delete(ininteresting_result)
    super
  end
end

Alternatively, you can use the builtin library forwardable. This is particularly useful if you can't inherit from Array because you need to inherit from another class:

require 'forwardable'
class Results
  extend Forwardable
  def_delegator :@result_array, :<<, :each, :concat # etc...

  def best
    @result_array.find {|result| result.is_really_good? }
  end

  # Array functionality that needs change
  def compact
    @result_array.delete(ininteresting_result)
    @result_array.compact
    self
  end
end

In both of these forms, you can use it as you want:

r = Results.new
r << some_result
r.each do |result|
  # ...
end
r.compact
puts "Best result: #{r.best}"

Not sure I'm adding anything new, but decided to show a very short code that I wish I could have found in the answers to quickly show available options. Here it is without the enumerator that @shelvacu talks about.

class Test
   def initialize
     @data = [1,2,3,4,5,6,7,8,9,0,11,12,12,13,14,15,16,172,28,38]
   end

   # approach 1
   def each_y
     @data.each{ |x| yield(x) }
   end

   #approach 2
   def each_b(&block)
     @data.each(&block)
   end  
end

Lets check performance:

require 'benchmark'
test = Test.new
n=1000*1000*100
Benchmark.bm do |b|
  b.report { 1000000.times{ test.each_y{|x| @foo=x} } }
  b.report { 1000000.times{ test.each_b{|x| @foo=x} } }
end

Here's the result:

       user     system      total        real
   1.660000   0.000000   1.660000 (  1.669462)
   1.830000   0.000000   1.830000 (  1.831754)

This means yield is marginally faster than &block what we already know btw.

UPDATE: This is IMO the best way to create an each method which also takes care of returning an enumerator

class Test
  def each
    if block_given?
      @data.each{|x| yield(x)}  
    else    
      return @data.each
    end  
  end  
end

If you really do want to make your own #each method, and assuming you don't want to forward, you should return an Enumerator if no block is given

class MyArrayLikeClass
  include Enumerable

  def each(&block)
    return enum_for(__method__) if block.nil?
    @arr.each do |ob|
      block.call(ob)
    end
  end
end

This will return an Enumerable object if no block is given, allowing Enumerable method chaining