Hi Alex, Sorry for the miswording of my question. The think I am interested in the first couple in the descending list (which would be last few in the ascending list) Thanks Sunil. On Sun, Aug 21, 2011 at 2:23 PM, Alexander Batischev wrote:

Hi!

On Sun, Aug 21, 2011 at 01:48:05PM +0530, Sunil S Nandihalli wrote:

...

 Is there a more efficient way to convert a Set to a descending list? I was looking for a function similar to Set.toAscList something like Set.toDecList . I feel first converting to a ascending list and then reversing may be in-efficient given that I actually don't need all the elements only a last couple of elements..

Why don't you use something like

 take N $ Set.toAscList s

then? "A last couple of elements" in descending list would be "a first couple of elements" in ascending list. That approach may even be *faster* than building descending list, because to get your elements you'll build only beginning of the list, while with descending list, you'll need to build whole list in order to get last elements.

-- Regards, Alexander Batischev

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Hi Felipe, Thanks a lot for your response. I thought I should describe what I want in a little more detail. You may have a better suggestion to solve the problem. I currently have a Set I want to take a couple of elements on either side of a given key. How can I do this? Here is a description of the subseq function in clojure. http://clojuredocs.org/clojure_core/clojure.core/subseq Thanks, Sunil. On Sun, Aug 21, 2011 at 6:00 PM, Felipe Almeida Lessa wrote:

On Sun, Aug 21, 2011 at 5:18 AM, Sunil S Nandihalli wrote:

...

 Is there a more efficient way to convert a Set to a descending list? I was looking for a function similar to Set.toAscList something like Set.toDecList . I feel first converting to a ascending list and then reversing may be in-efficient given that I actually don't need all the elements only a last couple of elements..

Implementing toDecList should be easy, but it's not implemented.  So I can see a couple of ideas:

a) Instead of using a 'Set X', where X is your datatype, use 'Set (Rev X)' where

 newtype Rev t = Rev t  instance Eq t => Eq (Rev t) where Rev x == Rev y = x == y  instance Ord t => Ord (Rev t) where compare (Rev x) (Rev y) = compare y x

Then you would just need to use toAscList =).

b) If you need both the few first and few last elements to toAscList, then option a doesn't work.  But if you really need just a few elements, you may use maxKey:

 toDecList s =    case maxView s of      Nothing -> []      Just (x, s') -> x : toDecList s'

 The problem is that 'take k . toDecList' take O(k log n) instead of O(k).

c) Data.Set.fold says that the order is unspecified.  But the current implementation does the fold in ascending order.  You may cheat and implement:

 toDecList s = fold (\a b -> b . (a:)) id s []

Cheers, =)

-- Felipe.