[accumulators] Empty histogram

[Robert Kubrick <robertkubrick@…>]

When the number of samples entered is lower than the cache_size parameter, histogram is not initialized. Example:

  typedef iterator_range<std::vector<std::pair<double, double> >::iterator > histogram_type;
    accumulator_set<double, features<tag::density> > acc(tag::density::num_bins = 20, tag::density::cache_size = 10);

    // push in some data ...
    acc(0.1);
    acc(1.2);
    acc(2.3);
    acc(3.4);
    acc(4.5);
    acc(5.4);
    acc(5.5);

    // Display the results ...
    histogram_type hist = density(acc);
    for( int i = 0; i < hist.size(); i++ )
      std::cout << "Bin lower bound: " << hist[i].first << ", Value: " << hist[i].second << std::endl;

    return 0;

[Eric Niebler]

According to Matthias Troyer:

It actually makes sense, since the density estimator first needs to collect a certain number of samples before it can make an educated guess about the locations and widths of histogram bins. It does not make sense to inquire the density before that.

Therefore, this is not a bug by by design. I have now documented the precondition on density result extraction, both in the code and in the users' guide (see changelist [53405]).