From: Robert Klemme <shortcutter@...>
Date: 2010-06-05T21:35:10+09:00
Subject: Re: comparing objects

On 05.06.2010 00:35, Anderson Leite wrote:
>   Before
>> we can provide solutions we have to know what problem must be solved.

> I have a list of objects that came from database and another list of
> objects extracted from a xml. I need the elements who are in both lists.
>
>
> Then...I thought to compare objects overriding the == method like Marcin
> Wolski wrote. There is another solution ?

If all objects you are dealing with implement #eql? and #hash in a way 
to be suitable for that comparison then using #eql? is the most 
straightforward approach.

If they do not and you want to do the comparison based on other criteria 
then you need to pick a different solution.  For example:

in_both = obj_from_db.select do |u1|
   obj_from_xml.any? do |u2|
     u1.email == u2.email && u1.age == u2.age
   end
end

The larger set should be used for the outer iteration.  If both sets are 
really large then you probably rather want to use a different strategy 
by speeding up access via a Hash.  For that you need a specific Hash key 
(remember, we had assumed #eql? and #hash cannot be used).  That key 
must implement #eql? and #hash; the easiest way to get such a Key class 
is to use Struct:

Key = Struct.new :email, :age

db_index = {}

obj_from_db.each do |u1|
   db_index[Key[u1.email, u1.age]] = u1
end

in_both = obj_from_xml.select do |u2|
   db_index[Key[u2.email, u2.age]]
end

Kind regards

	robert

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