Re: Inserting In a List

Eric Sosman <esosman@comcast-dot-net.invalid>
Tue, 02 Apr 2013 21:03:55 -0400
On 4/2/2013 8:20 PM, Arne Vajh?j wrote:

On 4/2/2013 8:04 PM, Joerg Meier wrote:

On Tue, 2 Apr 2013 22:52:20 +0000 (UTC), Martin Gregorie wrote:

On Tue, 02 Apr 2013 18:22:55 -0400, Eric Sosman wrote:

On 4/2/2013 5:06 PM, Martin Gregorie wrote:

Its also not clear to me whether the OP is expecting some form of
sorted list of filenames. If he is expecting that, it would be best to
use a TreeMap<String> rather than an ArrayList<String> to store the

      Is there a reason to prefer TreeMap (or other SortedMap) over

I think so, yes, because none of File's list() and listFiles() methods
guarantee the order of the returned files. By using TreeMap or
the OP gets the sort for free, should it be required.

For free ? The cost is just distributed amongst the insert calls, and is
likely considerably higher than with an unsorted list that has a single
sort call once it is filled.

It is not that obvious to me that:

n O(1) + 1 O(nlogn) is that much faster than n log(n)

     By design, O() obscures all the coefficients. If you were
to say that n*log(n) and 9999999999999999999999999999*n*log(n)
are both O(n*log(n)) you would be right -- but if you were to
claim the former was not "that much faster" than the latter you
would be wrong by a factor of 9999999999999999999999999999.

     ... and that's the gist of my question: It seems to me likely
that the actual time to sort an ArrayList of large-ish N size will
be less than the time to build a TreeMap (BTW, Martin probably
meant TreeSet) of the same N items. Argument: The TreeSet expends
effort in keeping itself always sorted all the time, while the
ArrayList has the freedom to be disordered up until the end, and
then to impose an ordering just once. The ArrayList is asked to
present one sorted version of N items, while the TreeSet must
offer sorted versions of 1,2,3,...,N items. If the intermediate
orderings are not required, I don't see much reason to compute them.

     By the way, the ArrayList will use less memory -- but both
will use O(n) in all, so who cares? :-)

     For this insight I will charge you only O(1) pfennigs. ;-)

Eric Sosman

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