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 1:01 PM
>> To: ruby-talk ML
>> Subject: Re: stable sort_by?
>> 
>> Kroeger, Simon (ext) wrote:
>> 
>>> 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])
>> 
>> Hummm not sure to understand ... You asked for a sort on the
>> first item
>> ... So, in this case [1,"c"] and [1,"b"] are equivalent, and
>> their order
>> is un-important ...
> 
> From http://en.wikipedia.org/wiki/Stable_sort :
> 
> Stability: stable sorting algorithms maintain the relative order
> of records with equal keys (i.e. values). That is, a sorting algorithm
> is stable if whenever there are two records R and S with the same
> key and with R appearing before S in the original list, R will appear
> before S in the sorted list.

It's pretty easy to transform every sorting operation into a stable one:

orig:
enum.sort_by {|x| calculate_key(x)}

stable:
i=0
enum.sort_by {|x| [calculate_key(x), i+=1]}

Kind regards

    robert