Chapter 58. Boost.Accumulators

Boost.Accumulators provides classes to process samples. For example, you can find the largest or smallest sample, or calculate the total of all samples. While the standard library supports some of these operations, Boost.Accumulators also supports statistical calculations, such as mean and standard deviation.

The library is called Boost.Accumulators because the accumulator is an essential concept. An accumulator is a container that calculates a new result every time a value is inserted. The value isn’t necessarily stored in the accumulator. Instead the accumulator continuously updates intermediary results as it is fed new values.

Boost.Accumulators contains three parts:

All classes and functions provided by Boost.Accumulators are defined in boost::accumulators or nested namespaces. For example, all accumulators are defined in boost::accumulators::tag.

#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics.hpp>
#include <iostream>

using namespace boost::accumulators;

int main()
{
  accumulator_set<int, features<tag::count>> acc;
  acc(4);
  acc(-6);
  acc(9);
  std::cout << count(acc) << '\n';
}

Example 58.1 uses boost::accumulators::tag::count, a simple accumulator that counts the number of values passed to it. Thus, since three values are passed, this example writes 3 to standard output. To use an accumulator, you access the class boost::accumulators::accumulator_set, which is a template that expects as its first parameter the type of the values that will be processed. Example 58.1 passes int as the first parameter.

The second parameter specifies the accumulators you want to use. You can use multiple accumulators. The class name boost::accumulators::accumulator_set indicates that any number of accumulators can be managed.

Strictly speaking, you specify features, not accumulators. Features define what should be calculated. You determine the what, not the how. There can be different implementations for features. The implementations are the accumulators.

Example 58.1 uses boost::accumulators::tag::count to select an accumulator that counts values. If several accumulators exist that can count values, Boost.Accumulators selects the default accumulator.

Please note that you can’t pass features directly to boost::accumulators::accumulator_set. You need to use boost::accumulators::features.

An object of type boost::accumulators::accumulator_set can be used like a function. Values can be passed by calling operator(). They are immediately processed. The values passed must have the same type as was passed as the first template parameter to boost::accumulators::accumulator_set.

For every feature, there is an identically named extractor. An extractor receives the current result of an accumulator. Example 58.1 uses the extractor boost::accumulators::count(). The only parameter passed is acc. boost::accumulators::count() returns 3.

#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics.hpp>
#include <iostream>

using namespace boost::accumulators;

int main()
{
  accumulator_set<double, features<tag::mean, tag::variance>> acc;
  acc(8);
  acc(9);
  acc(10);
  acc(11);
  acc(12);
  std::cout << mean(acc) << '\n';
  std::cout << variance(acc) << '\n';
}

Example 58.2 uses the two features boost::accumulators::tag::mean and boost::accumulators::tag::variance to calculate the mean and the variance of five values. The example writes 10 and 2 to standard output.

The variance is 2 because Boost.Accumulators assigns a weight of 0.2 to each of the five values. The accumulator selected with boost::accumulators::tag::variance uses weights. If weights are not set explicitly, all values are given the same weight.

#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics.hpp>
#include <iostream>

using namespace boost::accumulators;

int main()
{
  accumulator_set<double, features<tag::mean, tag::variance>, int> acc;
  acc(8, weight = 1);
  acc(9, weight = 1);
  acc(10, weight = 4);
  acc(11, weight = 1);
  acc(12, weight = 1);
  std::cout << mean(acc) << '\n';
  std::cout << variance(acc) << '\n';
}

Example 58.3 passes int as a third template parameter to boost::accumulators::accumulator_set. This parameter specifies the data type of the weights. In this example, weights are assigned to every value.

Boost.Accumulators uses Boost.Parameter to pass additional parameters, such as weights, as name/value pairs. The parameter name for weights is weight. You can treat the parameter like a variable and assign a value. The name/value pair is passed as an additional parameter after every value to the accumulator.

In Example 58.3, the value 10 has a weight of 4 while all other values have a weight of 1. The mean is still 10 since weights don’t matter for means. However, the variance is now 1.25. It has decreased compared to the previous example because the middle value has a higher weight than the other values.

Boost.Accumulators provides many more accumulators. They are used like the accumulators introduced in this chapter. The documentation of the library contains an overview on all available accumulators.