From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2010-08-31T01:40:40+09:00
Subject: Re: A small problem for arrays

On Mon, Aug 30, 2010 at 5:45 PM, Ruby Users Ruby Users <unc88@mail.ru> wrote:
> Robert Klemme wrote:
>> On 21.08.2010 18:27, Jean-Julien Fleck wrote:
>>> =>  [5, 6, 7]
>>>       Array Difference---Returns a new array that is a copy of the
>>>       original array, removing any items that also appear in other_array.
>>>       (If you need set-like behavior, see the library class Set.)
>>>
>>>          [ 1, 1, 2, 2, 3, 3, 4, 5 ] - [ 1, 2, 4 ]  #=>   [ 3, 3, 5 ]
>>>
>>> Cheers,
>>>
>>
>> Just adding to that: if Arrays are large and / or there are frequent set
>> operations needed then using class Set might yield better performance.
>>
>> Kind regards
>>
>>   robert
>
> -- Just adding to that: if Arrays are large and / or there are frequent
> set
> -- operations needed then using class Set might yield better
> performance.
> I do't much understand what you mean. If not hard can give you an
> example...

It means that there are some operations that are more efficient in Set
than in Array, and that if you need a lot of those, it would be better
to use Set instead. For example, the intersection of two Sets is
faster than the intersection of two Arrays:

require 'benchmark'
require 'set'

n = 1_000

a1 = (1..10_000).map {|x| rand(100)}
a2 = (1..10_000).map {|x| rand(100)}
s1 = Set.new.merge a1
s2 = Set.new.merge a2


Benchmark.bmbm do |x|
    x.report("array minus") do
      n.times {a1 - a2}
    end
    x.report("set &") do
      n.times {s1 & s2}
    end
end

$ ruby set_bm.rb
Rehearsal -----------------------------------------------
array minus   0.900000   0.000000   0.900000 (  0.935476)
set &         0.280000   0.070000   0.350000 (  0.361684)
-------------------------------------- total: 1.250000sec

                  user     system      total        real
array minus   0.880000   0.010000   0.890000 (  0.890552)
set &         0.280000   0.070000   0.350000 (  0.353687)

Jesus.