From: alexg <alexg@...>
Date: 2010-05-14T23:19:11+09:00
Subject: Re: Comparing elements within an array

On Fri, 14 May 2010 22:03:21 +0900, Toby Rodwell <trodwell@iee.org> wrote:
> Robert Dober wrote:
> Many thanks for this, and you ahve exactly described what I am wanting 
> to do.  Unfortunately it is throwing up 'undefined method `tap' for 
> nil:NilClass (NoMethodError)'.  I tried figuring this out for myself but

> no joy :(  Any ideas?
> 
>> On Fri, May 14, 2010 at 12:47 PM, Toby Rodwell <trodwell@iee.org>
wrote:
>> 
>> ah a #unique_by, I therefore conclude that you want to keep one
>> element with the given property.
>> 
>> module Enumerable
>>   def unique_by &blk
>>     keys = {}
>>     reject{ | ele |
>>       key = blk[ele]
>>       keys[key].tap{ keys[key]=true }
>>     }
>>   end
>> end
>> 
>> p [*1..10].unique_by{ |x| x % 3 }
>> 
>> Could'nt come up with something more concise :(
>> HTH
>> R.

The code is ruby 1.9 only I think. Another way for 1.8.6:

module Enumerable
  def unique_by &blk
    keys = {}
    find_all{ | ele |
      key = blk[ele]
      keys.has_key?(key) ? false : keys[key] = true
    }
  end
end

p [1,2,3,4,5].unique_by{ |x| x % 3 }

Or you can temporarily redefine eql? and hash:

class Foo
  attr_reader :a
  def initialize(a)
    @a = a
  end
end

bar = Foo.new(1)
baz = Foo.new(2)
boz = Foo.new(2)

puts [bar,baz,boz].uniq.size

class Foo
  alias_method :old_eql?, :eql?
  alias_method :old_hash, :hash
  def eql?(baz)
    @a == baz.a
  end
  def hash
    @a.hash
  end
end

puts [bar,baz,boz].uniq.size

class Foo
  alias_method :eql?, :old_eql?
  alias_method :hash, :old_hash
end

puts [bar,baz,boz].uniq.size

Alex Gutteridge