Talk:Sorting algorithms/Quicksort: Difference between revisions
→Fortran-90 code ... pivot choice can cause it to go O(N^2)
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:: The British Computer Society host [http://comjnl.oxfordjournals.org/cgi/content/short/5/1/10 Hoare's original 1962 paper]. In the paper, Hoare emphasizes the speed and memory efficiency of his quicksort algorithm and describes the algorithm explicitly in terms of pointers converging to the middle of the array and "exchanges" of pairs of elements that are in the wrong order and he uses the word "overwrites". In the Conclusion, he says "the data are sorted in situ". I'm sure variations on the theme are welcome but I would expect them to preserve the essence of the original and this bastardized out-of-place "quicksort" does not: it is asymptotically slower in theory (you're supposed to choose the pivot randomly!), hundreds of times slower in practice and consumes asymptotically more memory to the extent that it is practically useless (even the Haskell stdlibs do not use it!). At the very least, I recommend marking the fake quicksorts to avoid confusion. Ideally, demand real quicksorts (preferably parallel and generic) and [http://www.haskell.org/haskellwiki/Introduction/Direct_Translation watch the Haskellers squirm]! --[[User:Jdh30|Jon Harrop]]
::: I already warned you once about vitriol. this site functions as well as it does because of cooperative behavior among participants, regardless of what they think of each others' preferred languages. If you have no intention of non-inflammatory and non-malicious participation, you'll find yourself unwelcome to the point of my exercising admin powers. Last warning on that front. As for the issues with the Quicksort task description, I'll leave that to others to discuss and debate. --[[User:Short Circuit|Michael Mol]] 09:47, 25 May 2010 (UTC)
::I made some measurements for the F# version which physically splits the list to be sorted into 2 new lists for one thousand to ten million elements as follows (x is a random number generator):
<pre>
> let n = List.init 1000 (fun _->x.Next());;
> qsort n;;
Real: 00:00:00.012, CPU: 00:00:00.020, GC gen0: 0, gen1: 0
> let n = List.init 10000 (fun _->x.Next());;
> qsort n;;
Real: 00:00:00.047, CPU: 00:00:00.040, GC gen0: 5, gen1: 0
> let n = List.init 100000 (fun _->x.Next());;
> qsort n;;
Real: 00:00:00.485, CPU: 00:00:00.660, GC gen0: 53, gen1: 2
> let n = List.init 1000000 (fun _->x.Next());;
> qsort n;;
Real: 00:00:05.200, CPU: 00:00:06.990, GC gen0: 602, gen1: 10
> let n = List.init 1000000 (fun _->x.Next());;
> qsort n;;
Real: 00:01:34.239, CPU: 00:02:01.510, GC gen0: 7331, gen1: 20
</pre>
which is a little worse than linear but not too bad, so the situation for Haskell may not be as bad as you think. I have 8 cores available to me and if I wanted to write a parallel version I'd prefer the copied list to the single array with pointers. Even using a single core swapping in place requires three read write operations: read i and write to temp; read j and write to i; read temp and write to i. Whereas copying to new lists can be done in one read and write.
--[[User:Nigel Galloway|Nigel Galloway]] ([[User talk:Nigel Galloway|talk]]) 17:17, 24 April 2018 (UTC)
==Algorithm description==
Would someone care to add some text describing the Quicksort algorithm? I.e., its worst-case completion time, a pseudocode implementation, what the pivot does, etc. --[[User:Short Circuit|Short Circuit]] 03:28, 7 October 2007 (MDT)
: Quicksort is an elegant and powerful algorithm. If you want to learn about it, read the [http://comjnl.oxfordjournals.org/cgi/content/short/5/1/10 Hoare's original 1962 paper on Quicksort]. Note that the task description and code here on Rosetta Code and the Wikipedia article are wrong. --[[User:Jdh30|Jon Harrop]]
::If it wouldn't be too much trouble (since you seem to understand it yourself) could you maybe add a summary to this task then? It would be more convenient to have the proper sort on-site. --[[User:Mwn3d|Mwn3d]] 03:17, 26 May 2010 (UTC)
::: I'm not sure what you mean by a summary but I can certainly fix the task description if you like. The quicksort algorithm is very simple: you randomly choose a pivot and swap it with the last element in the current slice of the array, then you start pointers at either end of the slice and move them towards the middle swapping any pairs that are out of place until the pointers meet and, finally, recursively quicksort the lower and upper slices separated by the point the pointers converged on. Sedgewick gave a nice C++ implementation (without randomization) [http://www.cs.princeton.edu/~rs/talks/QuicksortIsOptimal.pdf here] except for a bug where the array element should only be loaded within the 'if' statement. Here's the fixed code: --[[User:Jdh30|Jon Harrop]]
void quicksort(Item a[], int l, int r) {
if (r > l) {
int i = l-1, j = r;
Item v = a[r];
for (;;) {
while (a[++i] < v) ;
while (v < a[--j]) if (j == l) break;
if (i >= j) break;
exch(a[i], a[j]);
}
exch(a[i], a[r]);
quicksort(a, l, i-1);
quicksort(a, i+1, r);
}
}
== UnixPipes solution ==
Line 48 ⟶ 88:
: The comments say just it's a good pivot choice for already sorted data or for reverse sorted data; I haven't checked it, but I can believe it, as I can believe it's not a so good choice for "symmetric triangle" distribution. --[[User:ShinTakezou|ShinTakezou]] 18:44, 26 June 2009 (UTC)
--[[User:Mecej4|Mecej4]] ([[User talk:Mecej4|talk]]) 01:06, 1 October 2022 (UTC)
The routine Qsort may get stuck in an infinite loop when the input array has two or more elements of equal value. For example:
L=4
A(1)%value = 2.0; A(2)%value = 1.0; A(3)%value = 3.0; A(4)%value=2.0
A(1)%order = 1; A(2)%order = 2; A(3)%order = 3; A(4)%order = 4
call Qsort(A, L)
== IDL ==
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