Ruby - fastest way to select items that belong to two arrays by nested attribute -

my ruby query slow. i'm looking faster way select items belong 2 arrays

lista list of objecta attribute name

listb list of objectb attribute name

there can multiple items in lista have same name, nothing in listb same name. there items in lista names not in listb. want function (loop) return items in lista name in listb.

this current solution, it's slow. how can make faster ?

z = 0 new = [] while z < listb.length      hold = lista.select{|obj| obj.name == listb[z].name}     new += hold     z += 1 end 

you want maximise lookup speed. best speed can reach lookup o(1) using hashes. method described below 1000x faster code , 350x faster loop include? n=10000

edit 2 : this link explains more in detail why hash lookup fast.

names = {} listb.each{|item| names[item.name] = true} result = lista.select{|item| names[item.name]} 

edit : updates benchmark.

• 03/08/2015 - added user12341234 set method

---

benchmark code

class myobject   def initialize(name)     @name = name   end    def name     @name   end end  n=10000 k=n/10  lista = (1..n).map{ myobject.new(('a'..'z').to_a.shuffle[0,8].join)} listb = lista.sample(k) listb += (1..(n-k)).map{ myobject.new(('a'..'z').to_a.shuffle[0,8].join)}  require 'benchmark' require 'set'  benchmark.bm |x|     x.report("hash")         names = {}         listb.each{|item| names[item.name] = true}         result = lista.select{|item| names[item.name]}     end      x.report("op's code")         z = 0         new = []         while z < listb.length              hold = lista.select{|obj| obj.name == listb[z].name}             new += hold             z += 1         end     end      x.report("include")       names = listb.map(&:name)       lista.select{|obj| names.include?(obj.name) }     end      x.report("set")          names = listb.map(&:name).to_set         lista.select{|item| names.include?(item.name)}     end end 

---

benchmark

specs :

• intel core i7 920 @ 2.67 ghz
• 13 gb ram
• windows 8 x64
• ruby 21 x64

results :

algorithm       user     system      total        real hash         0.015000   0.000000   0.015000 (  0.013002) op's code   26.053000   0.016000  26.069000 ( 26.161283) include      9.219000   0.000000   9.219000 (  9.244161) set          0.016000   0.000000   0.016000 (  0.013001)