Re: Did the sort do anything?

From:
Tom Anderson <twic@urchin.earth.li>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 11 May 2011 21:32:19 +0100
Message-ID:
<alpine.DEB.2.00.1105112104320.27211@urchin.earth.li>
On Tue, 10 May 2011, Roedy Green wrote:

I am interested in simple algorithms to detect whether the sort actually
did anything.


Write a Comparator that keeps track of how many times it returns >0, and
keep your fingers crossed that the sort algorithm always passes it
parameters in index order? A quick try seems to indicate that this does
work with Arrays.sort, although i don't believe the spec makes it
something you could rely on.

You could do something with a decorator that associates each element with
its index in the list, then use that information to do the order check in
the comparator. I'm not explaining this well, but it's a pretty bad idea,
so i won't.

If you wrote your own implementation, you could simply count the number of
times you swap elements, and if it's nonzero, you know the list was out of
order. This would be a good opportunity to implement a good sorting
algorithm - something that is, unlike mergesort, adaptive and in-place
(although not necessarily stable), perhaps Smoothsort:

http://www.keithschwarz.com/smoothsort/

You could also half-implement your own algorithm; do a linear scan through
your list, checking that each consecutive pair of elements is in order. As
soon as you hit a pair that isn't, stop, ask Collections.sort to sort the
remainder of the list, then go back and merge that half with the sorted
prefix you scanned through. Then return false. If you made it to the end
of the list without finding anything out of order, return true.

tom

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