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Bug #9340

Document order related behavior in Array#uniq

Added by agrimm (Andrew Grimm) over 6 years ago. Updated over 6 years ago.

Status:
Closed
Priority:
Normal
Target version:
ruby -v:
ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-linux]
[ruby-core:59457]

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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