From: dblack@...
Date: 2007-08-09T20:26:59+09:00
Subject: Re: Collect objects from an array based on one unique parame

Hi --

On Thu, 9 Aug 2007, Milo Thurston wrote:

> Robert Klemme wrote:
>> selection = things.select {|x| x.name = "foo"}
>
> That is closer, but still not quite it.
> A slight change of my previously posted code may make it clearer. In
> this case I start with an array of "Thing" objects called "things", and
> would like an array called "uniquely_named_things" containing Things
> where the name is unique.
>
>
> check = Hash.new
> uniquely_named_things = Array.new
> things.each do |s|
>  if check[s.name].nil?
>   uniquely_named_things << s
>  end
>  check[s.name] = 1
> end
>
> Basically, I wonder if anything that does the same as this has already
> been included in Ruby.

I don't think so.  It might be handy to generalize it:

   module UniqBy
     def uniq_by
       res = []
       count = Hash.new(0)
       each do |item|
         y = yield(item)
         if y
           count[y] += 1
           if count[y] == 1
             res << item
           else
             res.delete(item)
           end
         end
       end
       res
     end
   end

   Thing = Struct.new(:name)

   a = Thing.new("David")
   b = Thing.new("John")
   c = Thing.new("David")
   d = Thing.new("Mary")
   e = Thing.new("Joe")

   things = [a,b,c,d,e].extend(UniqBy)

   p things.uniq_by {|thing| thing.name }


Another way to do this, which I imagine is much slower, is:

   things.select do |thing|
     things.select {|other| other.name == thing.name }.size == 1
   end


David

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