# [Haskell-cafe] Sorting efficiency

David Feuer david.feuer at gmail.com
Sun Aug 5 11:37:19 CEST 2012

```Unfortunately, I doubt it can. That algorithm reduces the number of
comparisons a good bit, but the asymptotic complexity of its other
operations remains the same, with apparently bad constant factors
(according to the article). Also, that article describes the algorithm
in terms of sorting [a+b| a<-A, b<-A], whereas I'm actually dealing
(in the more substantial phase) with [a+b | a<-A, b<-B], where B is
much larger than A. Using the merging approach I described in my last
email, I can reduce the complexity from mn log(mn) in the naive
algorithm to mn log (min {m,n}).  Since m=100 and n~=10,000, I should
be able to get nearly double the speed of the naive approach, as long
as my multi-merge is fast enough.

On Sun, Aug 5, 2012 at 3:59 AM, Heinrich Hördegen
<hoerdegen at funktional.info> wrote:
>
> Hi David,
>
> can this help you?
>
> http://www.scribd.com/doc/81029312/5/Sorting-pairwise-sums
>
> Heinrich
>
> Am 04.08.2012 20:47, schrieb David Feuer:
>>
>> I realized my algorithm is insane. The correct way to sort [a*b|a<-A,
>> b<-B] is clearly to sort A and B, then for each a in A construct
>> either map (a*) B or map (a*) (reverse B), depending on the sign of a,
>> then merge all these results together with a merge that collapses
>> duplicates. I was multiplying and then sorting, which is way worse.
>> The same (modulo sign) goes for adding lists.
>> On Aug 4, 2012 1:55 PM, "Clark Gaebel" <cgaebel at uwaterloo.ca [6]>
>> wrote:
>>
>>> Its generally not advisable to use Data.List for
>>>
>>> performance-sensitive parts of an application.
>>>
>>> Try using Data.Vector instead:
>>> http://hackage.haskell.org/package/vector [4]
>>>
>>>
>>> On Sat, Aug 4, 2012 at 11:23 AM, David Feuer <david.feuer at gmail.com
>>> [5]> wrote:
>>>
>>>> Im writing a toy program (for a SPOJ problem--see
>>>> https://www.spoj.pl/problems/ABCDEF/ [1] ) and the profiler says
>>>> my
>>>> performance problem is that Im spending too much time sorting. Im
>>>> using Data.List.sort on [Int32] (its a 32-bit architecture).
>>>>
>>>> Others,
>>>> using other languages, have managed to solve the problem within
>>>> the
>>>> time limit using the same approach Ive taken (I believe), but
>>>>
>>>> mine is
>>>> taking too long. Any suggestions? Do I need to do something
>>>> insane
>>>> like sorting in an STUArray?
>>>>
>>>> David Feuer
>>>>
>>>> _______________________________________________
>>>> Haskell-Cafe mailing list
>>>> Haskell-Cafe at haskell.org [2]
>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe [3]
>>
>>
>>
>> Links:
>> ------
>> [1] https://www.spoj.pl/problems/ABCDEF/
>> [2] mailto:Haskell-Cafe at haskell.org
>> [3] http://www.haskell.org/mailman/listinfo/haskell-cafe
>> [4] http://hackage.haskell.org/package/vector
>> [5] mailto:david.feuer at gmail.com
>> [6] mailto:cgaebel at uwaterloo.ca
>
>
>
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```

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