From: Frederick Ros <sl33p3r@...>
Date: 2005-12-13T20:36:56+09:00
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

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