Extending uniq method

Question

This is Ruby 1.8 Question:

We all know how to use Array#uniq :

[1,2,3,1].uniq #=> [1,2,3]

However I'm wondering if we can monkey patch it in a way to work with complex objects. The current behavior is like this:

[{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].uniq 
#=> [{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}]

The requested one is:

[{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].uniq 
#=> [{"three"=>"3"}, {"three"=>"4"}]

Answer 1

It already works for me in 1.8.7.

1:~$ ruby -v
ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
1:~$ irb -v
irb 0.9.5(05/04/13)
1:~$ irb
>> [{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].uniq 
=> [{"three"=>"3"}, {"three"=>"4"}]

Answer 2

To make Array#uniq work for any object you must override two methods: hash and eql?

All objects have a hash method which calculates the hash value for that object, so for two objects to be equal their values when hashed must also be equal.

Example--a user is unique when their email address is unique:

class User
  attr_accessor :name,:email

  def hash
    @email.hash
  end

  def eql?(o)
    @email == o.email
  end
end

>> [User.new('Erin Smith','roo@example.com'),User.new('E. Smith','roo@example.com')].uniq 
=> [#<User:0x1015a97e8 @name="Erin Smith", @email="maynurd@example.com"]

Answer 3

The problem is that Hash#hash and Hash#eql? both give bogus results in Ruby 1.8.6. This is one of the very rare monkey patches I've been willing to perform, because this bug seriously breaks a lot of code — in particular memoizing functions. Just be careful with monkey patches that you don't override non-broken behavior.

So:

class Hash
  if {}.hash != {}.hash
    def hash
      # code goes here
    end
  end
  if !{}.eql?({})
    def eql?(other)
      # code goes here
    end
  end
end

But if you're doing something where you control the deploy environment, just raise an error if the app gets started with 1.8.6.

Answer 4

How about this?

h={}
[{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].select {|e| need=!h.key?(e) ; h[e]=1 ; need} 
#=> [{"three"=>"3"}, {"three"=>"4"}]

Answer 5

I've run into this myself many times. Hash equality in Ruby 1.8.6 is broken:

require 'test/unit'

class TestHashEquality < Test::Unit::TestCase
  def test_that_an_empty_Hash_is_equal_to_another_empty_Hash
    assert({}.eql?({}), 'Empty Hashes should be eql.')
  end
end

Passes in Ruby 1.9 and Ruby 1.8.7, fails in Ruby 1.8.6.

Answer 6

1.8.7 :039 > [{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].uniq {|x|x.values} 
=> [{"three"=>"3"}, {"three"=>"4"}] 
1.8.7 :040 > [{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].uniq {|x|x.keys}
=> [{"three"=>"3"}] 

How about something like that? just uniq_by the hash value or hash key via the block.