On 7/6/2011 4:40 AM, Olaf Peter wrote:
Hi,
at http://www.boost.org/doc/libs/1_46_1/doc/html/accumulators/user_s_guide.html in the example of section 'Droppable Accumulators' one can read
// This will display "6 2 3" std::cout << sum(acc) << ' ' << count(acc) << ' ' << mean(acc);
The text below says 'Note that at the point at which mean is dropped, sum is 3, count is 2, and therefore mean is 1.5'. After calculating my result is also 1.5 - the comment in the example isn't correct, isn't it?
Probably the typo is in the upcoming release 1.47 too.
No, the comment in the docs is correct. Sum continues to accumulate, but count does not --it was only added to the set to satisfy a dependency from mean, which was dropped. So you end up with sum=6, count=2, and mean=3. Note that if you change the declaration of acc in that example to: accumulator_set< double, features< tag::sum, tag::count, droppable<tag::mean> > > acc; ... the answer you get becomes "6 3 2". That is, once you have explicitly asked for the count statistic, it no longer gets dropped along with mean. HTH, -- Eric Niebler BoostPro Computing http://www.boostpro.com