From: Ezra Zygmuntowicz <ezra@...>
Date: 2005-12-14T08:39:35+09:00
Subject: Re: stable sort_by?


On Dec 13, 2005, at 3:54 AM, Kroeger, Simon (ext) wrote:

>
>
>> -----Original Message-----
>> From: list-bounce@example.com
>> [mailto:list-bounce@example.com] On Behalf Of Frederick Ros
>> Sent: Tuesday, December 13, 2005 12:37 PM
>> To: ruby-talk ML
>> Subject: Re: stable sort_by?
>>
>> Patrick Gundlach wrote:
>>> Hi,
>>>
>>> I have to sort a list using a number of criterions (a,b,c,d) where d
>>> is the least important criterion and a is the most
>> important one. All
>>> comparisons should be 'higher number: better position'.
>> This is what I
>>> have tried:
>>>
>>> --------------------------------------------------
>>> require 'pp'
>>>
>>> h= {"Team A"=>{:a=>3,   :b=>2, :c=>6, :d=>114 },
>>>     "Team B"=>{:a=>0,  :b=>-2, :c=>4, :d=>112 },
>>>     "Team C"=>{:a=>3,   :b=>4, :c=>4, :d=>110 },
>>>     "Team D"=>{:a=>3,   :b=>4, :c=>4, :d=>108 },
>>> }
>>>
>>> tmp=h.to_a
>>>
>>> [:d,:c,:b,:a].each do |criterion|
>>>   tmp=tmp.sort_by { |a| a[1][criterion]  }.reverse
>>> end
>>>
>>> pp tmp
>>>
>>> --------------------------------------------------
>>>
>>> [["Team D", {:d=>108, :a=>3, :b=>4, :c=>4}],
>>>  ["Team C", {:d=>110, :a=>3, :b=>4, :c=>4}],
>>>  ["Team A", {:d=>114, :a=>3, :b=>2, :c=>6}],
>>>  ["Team B", {:d=>112, :a=>0, :b=>-2, :c=>4}]]
>>>
>>>
>>> but as far as I can see 'Team C' should be better than 'Team D',
>>> because of criterion d. Is it possible that sort_by is not
>> stable? Or
>>> is there something I did wrong?
>>
>>
>> Wouldn't this be simpler ? :
>>
>>  h.sort_by {|k,v| [v[:a],v[:b],v[:c],v[:d]]}.reverse
>>
>> Best Regards,
>> Fred
>
> Full Ack,
>
> obviously sort_by isn't stable:
>
> test = [[1, 'b'], [1, 'c'], [0, 'a']]
> p test.sort_by{|t|t[0]}
>
> => [[0, "a"], [1, "c"], [1, "b"]]
> (ruby 1.8.2 (2004-12-25) [i386-mswin32])
>
> cheers
>
> Simon

  class Array
    def stable_sort
      n = 0
      sort_by {|x| n+= 1; [x, n]}
    end
  end

 >> test = [[1, 'b'], [1, 'c'], [0, 'a']]
=> [[1, "b"], [1, "c"], [0, "a"]]
 >> p test.stable_sort{|t|t[0]}
[[0, "a"], [1, "b"], [1, "c"]]

-Ezra Zygmuntowicz
WebMaster
Yakima Herald-Republic Newspaper
ezra@yakima-herald.com
509-577-7732