Extending uniq method
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 defuse it to work with complex objects. The current behavior looks like this:
[{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].uniq
#=> [{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}]
Requested request:
[{"three"=>"3"}, {"three"=>"4"}, {"three"=>"3"}].uniq
#=> [{"three"=>"3"}, {"three"=>"4"}]
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To make Array # uniq work for any object, you must override two methods: hash and eql?
All objects have a hash method that calculates the hash value for that object, so for two objects equal to their values when the hash 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"]
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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 that I'm willing to run because this bug seriously breaks a lot of code - in particular, memoizing functions. Just be careful with patches of monkeys that you do not overcome no disturbed 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 are doing something where you manage the deployment environment, just raise an error if the application starts running 1.8.6.
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I've encountered this on my own many times. Hash uniformity is broken in Ruby 1.8.6:
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, does not work in Ruby 1.8.6.
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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 this? just uniq_by hash value or hash key across a block.
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