Hi Morgan:

Thanks for the info although I have to admit that I don't understand how your solutions work.

I also needed my sort to return a modified flag to update the file if changed so I wrote my own bubble sort.
I haven't test this yet:

def sort_children # Don't trust Ruby sort to maintain sequence, also need set_modified
return if @children.empty?


arr = @children.map{ | child | [ child.sequence, child ]
modified = false


while true # bubble sort
change = false
new_arr = []


arr.each_with_index do | new_child, index |
if index.zero?
prior_child = new_child
new_arr[ 0 ] = new_child


elsif new_child.first < prior_child.first # OOO
change = true
new_arr.insert( index - 1, new_child )


else
new_arr[ index ] = new_child
prior_child = new_child
end
end
break unless change


modified = true
arr = new_arr
end
return unless modified


@children = arr.map{ | child | child.last }
set_modified()
end

Thanks,
Bob Rice

On Jan 30, 2011, at 7:19 PM, Morgan Schweers wrote:

Greetings,
Ruby's sort algorithm is quicksort, last I checked, and quicksort is not stable (which is the property you're looking for in a sort).  There are a bunch of ways around this, including writing your own, but one cute, quick, but possibly performance-impairing, approach I've seen (Matz's suggestion) is:

  n = 0 
  ary.sort_by {|x| [x, n += 1]}

Apparently it's also possible in 1.9.x (and thus MacRuby) to do:

  ary.sort_by.with_index {|x, i| [x, i]}

It's not much faster, though.  In the end, I'd probably suggest writing your own, if the performance of this is too poor.  (One person claimed this was on the order of 50 times slower; I haven't benchmarked it myself.)  Mergesort is stable, for example.

This is a common problem; most systems don't need a stable sort, so they use Quicksort as a 'best general purpose' algorithm.

--  Morgan

On Sun, Jan 30, 2011 at 12:03 PM, Robert Rice <rice.audio@pobox.com> wrote:
Hi:

Does the Ruby Array sort algorithm maintain the relative position for children returning the same value for the comparison? I had an instance where two children having the compare value were interchanged.

Thanks,
Bob Rice

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