Re: STL set question

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 20 Feb 2008 09:42:47 -0500

Message-ID:

<fphe98$iqu$1@news.datemas.de>

James Kanze wrote:

On Feb 20, 12:10 am, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

Stefan Istrate wrote:

How can I find the k-th position in a STL set? I want to do
this in O(logN) time, where N is the dimension of the set.

I don't think it's possible. The only way I know (without
going into the implementation) is to 'advance' the 'begin()'
iterator 'k' times. That's O(k) time, most likely amortized.
To make it better you have to rely on a certain implementation
of the list, which isn't specified (although could be deduced,
probably).

From time to time, it is suggested to use a sorted vector
instead of a set. Depending on what he's doing with it, it may
even be faster as well. Generally, insertions and deletions
are slower, lookups, using lower_bound, are faster. But there
are exceptions to even this rule. And of course, finding the
nth element is clearly O(1), with a very, very small constant
term as well:-).

One of the ways we combined the two in our practice is to form
the collection in a set and then copy it into a vector. The
copying is O(N). Of course this means huge savings iff the
"formation" (insertions) happens before the "use" (lookups).
Not all algorithms can be massaged into that scheme, but those
that can gain significantly.

V
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