From: "s.ross" <cwdinfo@...>
Date: 2009-07-20T04:43:26+09:00
Subject: Re: [Q] removing array duplicates where a subset is unique

Hi---

On Jul 17, 2009, at 6:14 PM, David A. Black wrote:

> Hi --
>
> On Sat, 18 Jul 2009, brabuhr@gmail.com wrote:
>
>> On Fri, Jul 17, 2009 at 7:51 PM, Chuck  
>> Remes<cremes.devlist@mac.com> wrote:
>>> (And Joel, I have presorted the array prior to removing the
>>> dupes so I have already taken care of the ordering issue.)
>>
>> I think what Joel was referring to was that in Ruby 1.8 a Hash  
>> doesn't
>> maintain insertion order when traversed (Ruby 1.9 does maintain
>> insertion order):
>>
>> ruby 1.8.2 (2004-12-25) [powerpc-darwin8.0]:
>> irb(main):001:0> h = {}
>> => {}
>> irb(main):002:0> 5.times{|n| h[n] = n}
>> => 5
>> irb(main):003:0> h
>> => {0=>0, 1=>1, 2=>2, 3=>3, 4=>4}
>> irb(main):004:0> h["sadf"] = 3
>> => 3
>> irb(main):005:0> h
>> => {0=>0, 1=>1, "sadf"=>3, 2=>2, 3=>3, 4=>4}
>
> If you wanted to maintain order you could (for some but probably not
> much performance penalty) do something like:
>
>  def dedup(ary)
>    uniq = {}
>    res = []
>    ary.each do |line|
>      key = line[0..2]
>      next if uniq[key]
>      res << (uniq[key] = line)
>    end
>    res
>  end
>
>
> David

I missed the beginning of this thread, but here is an implementation  
I've used successfully:

def uniq_by(subject, &block)
   h = {}
   a = []
   subject.each do |s|
     comparator = yield(s)
     unless h[comparator]
       a.push(s)
       h[comparator] = s
     end
   end
   a
end

Usage:

u = uniq_by(ary|{ |item| item.element }

Basically, what this allows you to do is specify what exactly about an  
array item must be unique. It also preserves the original array order,  
with a "first entry wins" approach to duplicate elimination.

Hope this is useful.