Re: "Fixing" the O(1) splice / O(1) size std::list problem?

From: (Carl Barron)
Thu, 15 Feb 2007 18:23:22 GMT
In article
Howard Hinnant <> wrote:

In article <>, ("P.J. Plauger") wrote:

I still haven't seen a compelling use case for O(1) "partial
splice", with or without a promised element count.

<g> That is ironic since it is the desire for this very function (with
the O(1) complexity) which has been the basis for a decade-long debate.

Ok, here's a reasonable use case:

I have a master database consisting of a list of Record:

struct Record
    std::string department;
    std::string data;

I want to create separate databases, one for each department, so that I
can send separate smaller lists to each individual department for
parallel processing. An "unmerge" if you will:

std::map<std::string, std::list<Record> >
unmerge(std::list<Record>& c)
    std::map<std::string, std::list<Record> > r;
    for (std::list<Record>::iterator i = c.begin(), e = c.end(); i != e;)
        std::string department = i->department;
        std::list<Record>::iterator j = i;
        std::list<Record>::size_type count = 1;
        for (++j; j != e; ++j, ++count)
            if (j->department != i->department)
        std::list<Record>& t = r[department];
        t.splice(t.end(), c, i, j, count);
        i = j;
    return r;

* This is reasonable code.
* It makes use of splice-some-from-other
* splice-some-from-other is within a loop, so keeping it as efficient as
possible would be nice.
* The count of the number of nodes one wants to splice at a time is
trivially computed without affecting the performance of the entire
* This algorithm purposefully makes a single pass through the master
list in an effort to minimize cache misses.

   Yes but what is the signifigant gain over
      std::string const &department = c.front().department();
      std::list<Record> &t = r[department];
      while(!c.empty() && c.front().department == department)

   The # of compares is the same, the only difference is the # of
internal pointer changes and that is also O(N). so they are both O(N),
where N == c.size(). I suppose if you had 10 million entries in c and
10 distinct departments it might be signifigant but then is a big std::
list a good idea to hold the original data? Seems like a contrived
   I am of no opinion over adding a fourth splice overload, just
wondering what is gained by the addition.

I don't see a case where I know the iterators i,j and distance(i,j)
independent of traversing the list. If I have to traverse the list
little is gained, over splicing to a temp list while I traverse, or in
this case splicing each item to the proper list while traversing the
original list, such as above does,

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