Performance of Arrays and Hashes in Ruby

Question

I have a program that will store many instances of one class, let's say up to 10.000 or more. The class instances have several properties that I need from time to time, but their most important one is the ID.

class Document
  attr_accessor :id
  def ==(document)
    document.id == self.id
  end
end

Now, what is the fastest way of storing thousands of these objects?

I used to put them all into an array of Documents:

documents = Array.new
documents << Document.new
# etc

Now an alternative would be to store them in a Hash:

documents = Hash.new
doc = Document.new
documents[doc.id] = doc
# etc

In my application, I mostly need to find out whether a document exists at all. Is the Hash's has_key? function significantly faster than a linear search of the Array and the comparison of Document objects? Are both within O(n) or is has_key? even O(1). Will I see the difference?

Also, sometimes I need to add Documents when it is already existing. When I use an Array, I would have to check with include? before, when I use a Hash, I'd just use has_key? again. Same question as above.

What are your thoughts? What is the fastest method of storing large amounts of data when 90% of the time I only need to know whether the ID exists (not the object itself!)

question from:https://stackoverflow.com/questions/5551168/performance-of-arrays-and-hashes-in-ruby

Answer 1

Hashes are much faster for lookups:

require 'benchmark'
Document = Struct.new(:id,:a,:b,:c)
documents_a = []
documents_h = {}
1.upto(10_000) do |n|
  d = Document.new(n)
  documents_a << d
  documents_h[d.id] = d
end
searchlist = Array.new(1000){ rand(10_000)+1 }

Benchmark.bm(10) do |x|
  x.report('array'){searchlist.each{|el| documents_a.any?{|d| d.id == el}} }
  x.report('hash'){searchlist.each{|el| documents_h.has_key?(el)} }
end

#                user     system      total        real
#array       2.240000   0.020000   2.260000 (  2.370452)
#hash        0.000000   0.000000   0.000000 (  0.000695)