Sort an array according to the elements of another array

Question

I have an array of ids

a1 = [1, 2, 3, 4, 5]  

and I have another array of objects with ids in random order

a2 = [(obj_w         


        

Answer 1

hash_object = objects.each_with_object({}) do |obj, hash| 
  hash[obj.object_id] = obj
end

[1, 2, 3, 4, 5].map { |index| hash_object[index] }
#=> array of objects in id's order

I believe that the run time will be O(n)

Answer 2

I like the accepted answer, but in ActiveSupport there is index_by which makes creating the initial hash even easier. See Cleanest way to create a Hash from an Array

In fact you could do this in one line since Enumerable supports index_by as well:

a2.index_by(&:id).values_at(*a1)

Answer 3

I'll be surprised if anything is much faster than the obvious way:

a2.sort_by{|x| a1.index x.id}

Answer 4

Inspired by Eric Woodruff's Answer, I came up with the following vanilla Ruby solution:

a2.group_by(&:object_id).values_at(*a1).flatten(1)

Method documentation:

• Enumerable#group_by
• Array#values_at
• Array#flatten