efficient idiomatic queue?
David Eppstein
eppstein at ics.uci.edu
Thu Jan 24 01:22:56 CET 2002
In article <83it9s4rm2.fsf at panacea.canonical.org>,
Kragen Sitaker <kragen at pobox.com> wrote:
> Even with hacks to make it less likely to hit its worst-case, though,
> quicksort's constant factor makes it about twice as fast as mergesort.
This is either using clever tricks to pick a good pivot, or assuming that
comparisons are fast (probably not a good assumption in Python), right?
Merge sort uses very close to the optimal number of comparisons, single
randomly-chosen pivot quicksort is off by some constant factor.
It also depends on how your data was represented; several years ago when I
needed code (in C++) to sort linked-list data I chose merge sort, since
it's simple, for that representation it does not use any extra memory, and
the lists I was sorting were small enough that I didn't have to worry about
caching effects (as I think someone pointed out earlier, array-based
versions of quick sort and merge sort can be much better than heap sort in
their caching behavior).
> How does heapsort's constant factor compare with mergesort's?
Not as good, but it uses less memory. The problem is, quicksort doesn't
use much more memory than heapsort, and is I think usually much faster, so
heapsort is not so often the best choice.
> Python's built-in sort() uses quicksort (using a linear congruential
> generator to pick pivot elements) to partition the array into small
> chunks, and then uses an algorithm that looks a lot like a bubble sort
> to sort the small chunks. It has special cases for already-sorted
> data, all-the-same data, reverse-sorted data, and each of the above
> followed by some unsorted data. This is the kind of thing you get
> when you aim for practicality.
Yes, theory and practice are not the same thing.
Sometimes (as here) the theoretical algorithms are nice and clean, but
making them practical creates a mess. Other times theoreticians use lots
of complicated tricks to reduce the worst-case complexity while the better
practical solutions are simpler...
--
David Eppstein UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/
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