Bug #9340
closed
Document order related behavior in Array#uniq
Description
The behavior of Array#uniq without a block in Ruby 2.1.0 is inconsistent with the behavior of Array#uniq in Ruby 2.0.0, and with the behavior of Array#uniq with a block in Ruby 2.1.0.
Array#uniq without a block in Ruby 2.1.0 returns the last item that is duplicate, while everything else returns the first item that is duplicate.
class Person
attr_reader :name
def initialize(id, name)
# @id is not used by hash or eql?
@id = id
@name = name
end
def hash
[@name].hash
end
def eql?(other)
[@name].eql?([other.name])
end
end
p1 = Person.new(1, "Bob")
p2 = Person.new(2, "Bob")
p3 = Person.new(3, "Bob")
arr = [p1, p2, p3]
arr2 = arr.uniq {|p| p}
arr3 = arr.uniq
p RUBY_VERSION
p arr2
p arr3
Gives
"2.1.0"
[#<Person:0x007f1b47ef7720 @id=1, @name="Bob">]
[#<Person:0x007f1b47ef7680 @id=3, @name="Bob">]
"2.0.0"
[#<Person:0x007ffa9203bf30 @id=1, @name="Bob">]
[#<Person:0x007ffa9203bf30 @id=1, @name="Bob">]
This behavior does not contradict what RDoc says about Array#uniq, so people ought not to rely upon which duplicate item is chosen. However, at the very least, different behavior for Array#uniq with and without a block would be somewhat unexpected.
Should the documentation explicitly warn that uniq is not guaranteed to choose the first duplicate item?
As Ruby 2.1.0 has already been released, I assume it is too late to change the behavior of Array#uniq .
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