This C++11 library provides a multi-dimensional histogram class for your statistics needs. It is very customisable through policy classes, but the default policies were carefully designed so that most users won't need to customize anything. In the standard configuration, this library offers a unique safety guarantee not found elsewhere: bin counts cannot overflow or be capped. While being safe to use, the library also has a convenient interface, is memory conserving, and faster than other libraries (see benchmarks).

The histogram class comes in two variants which share a common interface. The static variant uses compile-time information to provide maximum performance, at the cost of runtime flexibility and potentially larger executables. The dynamic variant is a bit slower, but configurable at run-time and may produce smaller executables. Python bindings for the latter are included, implemented with Boost.Python.

The histogram supports value semantics. Histograms can be added and scaled. Move operations and trips over the language boundary from C++ to Python and back are cheap. Histogram instances can be streamed from/to files and pickled in Python. Numpy is supported to speed up operations in Python: histograms can be filled with Numpy arrays at high speed (in most cases several times faster than numpy's own histogram function) and are convertible into Numpy array views without copying data.

My goal is to submit this project to Boost, that's why it uses the Boost directory structure and namespace. The code is released under the Boost Software License.

Check out the full documentation. Highlights are given below.

Features

Multi-dimensional histogram
Simple and convenient interface in C++ and Python
Static and dynamic implementation in C++ with common interface
Counters cannot overflow or be capped (+)
Better performance than other libraries (see benchmarks for details)
Efficient move operations
Efficient conversion between static and dynamic implementation
Efficient use of memory (counter capacity dynamically grows as needed)
Support for many mappings of input values to bin indices (user extensible)
Support for weighted increments
Support for under-/overflow bins (can be disabled individually for each dimension)
Support for variance tracking (++)
Support for addition and scaling of histograms
Support for serialization based on Boost.Serialization
Support for Python 2.x and 3.x
Support for Numpy

(+) In the standard configuration and if you don't use weighted input. (++) If you don't use weighted increments, variance tracking come at zero cost. If you use weighted increments, extra space is reserved internally to keep track of a variance counter per bin. The conversion happens automatically and transparently.

Dependencies

Boost
Optional: CMake Python Numpy

Build instructions

The library can be build with b2 within the boost directory structure, but if you are not a boost developer, use cmake instead.

git clone https://github.com/HDembinski/histogram.git
mkdir build && cd build
cmake ../histogram/build
make # or 'make install'

To run the tests, do make test.

Trouble-shooting when compiling with Python support

If you compile the library with Python support (the default if Python is found) and have several versions of Python installed, cmake will pick the latest Python version by default. Please make sure that this is the same version that Boost.Python and Boost.Numpy were compiled against, otherwise you will get strange errors during compilation and/or at runtime. You can force cmake to pick a specific Python version with the PYTHON_VERSION flag. For example, to force the use of Python2.7, do: cmake -DPYTHON_VERSION=2.7 ../histogram/build

In the future, the build system may be able to pick the right Python version automatically, but right now it has to be done manually.

If you installed Boost with brew on OSX, also have a look at this Stackoverflow question.

Code examples

For the full version of the following examples with explanations, see the Getting started section in the documentation.

Example 1: Fill a 1d-histogram in C++

    #include <boost/histogram.hpp> // proposed for inclusion in Boost
    #include <iostream>
    #include <cmath>

    int main(int, char**) {
        namespace bh = boost::histogram;
        using namespace boost::histogram::literals; // enables _c suffix

        // create 1d-histogram with 10 equidistant bins from -1.0 to 2.0,
        // with axis of histogram labeled as "x"
        auto h = bh::make_static_histogram(bh::axis::regular<>(10, -1.0, 2.0, "x"));

        // fill histogram with data
        h.fill(-1.5); // put in underflow bin
        h.fill(-1.0); // included in first bin, bin interval is semi-open
        h.fill(-0.5);
        h.fill(1.1);
        h.fill(0.3);
        h.fill(1.7);
        h.fill(2.0);  // put in overflow bin, bin interval is semi-open
        h.fill(20.0); // put in overflow bin
        h.fill(0.1, bh::weight(5)); // fill with a weighted entry, weight is 5

        // iterate over bins, loop skips under- and overflow bin
        for (auto it = h.axis(0_c).begin(); it; ++it) {
            std::cout << "bin " << it.idx()
                      << " x in [" << it->lower() << ", " << it->upper() << "): "
                      << h.bin(it).value() << " +/- " << std::sqrt(h.bin(it).variance())
                      << std::endl;
        }

        /* program output:

        bin 0 x in [-1, -0.7): 1 +/- 1
        bin 1 x in [-0.7, -0.4): 1 +/- 1
        bin 2 x in [-0.4, -0.1): 0 +/- 0
        bin 3 x in [-0.1, 0.2): 5 +/- 5
        bin 4 x in [0.2, 0.5): 1 +/- 1
        bin 5 x in [0.5, 0.8): 0 +/- 0
        bin 6 x in [0.8, 1.1): 0 +/- 0
        bin 7 x in [1.1, 1.4): 1 +/- 1
        bin 8 x in [1.4, 1.7): 0 +/- 0
        bin 9 x in [1.7, 2): 1 +/- 1

        */
    }

Example 2: Fill a 2d-histogram in Python with data in Numpy arrays

    import histogram as bh
    import numpy as np

    # create 2d-histogram over polar coordinates, with
    # 10 equidistant bins in radius from 0 to 5 and
    # 4 equidistant bins in polar angle
    h = bh.histogram(bh.axis.regular(10, 0.0, 5.0, "radius", uoflow=False),
                     bh.axis.circular(4, 0.0, 2*np.pi, "phi"))

    # generate some numpy arrays with data to fill into histogram,
    # in this case normal distributed random numbers in x and y,
    # converted into polar coordinates
    x = np.random.randn(1000)             # generate x
    y = np.random.randn(1000)             # generate y
    radius = (x ** 2 + y ** 2) ** 0.5
    phi = np.arctan2(y, x)

    # fill histogram with numpy arrays; call looks as
    # if radius and phi are numbers instead of arrays
    h.fill(radius, phi)

    # access histogram counts (no copy)
    count_matrix = np.asarray(h)

    print count_matrix

    # program output:
    #
    # [[37 26 33 37]
    #  [60 69 76 62]
    #  [48 80 80 77]
    #  [38 49 45 49]
    #  [22 24 20 23]
    #  [ 7  9  9  8]
    #  [ 3  2  3  3]
    #  [ 0  0  0  0]
    #  [ 0  1  0  0]
    #  [ 0  0  0  0]]

Benchmarks

Thanks to meta-programming and dynamic memory management, this library is not only safer, more flexible and convenient to use, but also faster than the competition. In the plot below, its speed is compared to classes from the GNU Scientific Library, the ROOT framework from CERN, and to the histogram functions in Numpy. The orange to red items are different compile-time configurations of the histogram in this library. More details on the benchmark are given in the documentation

Rationale

There is a lack of a widely-used free histogram class in C++. While it is easy to write a one-dimensional histogram, writing a general multi-dimensional histogram is not trivial. Even more so, if you want the histogram to be serializable and have Python-bindings and support Numpy. In high-energy physics, the ROOT framework from CERN is widely used. This histogram class is designed to be more convenient, more flexiable, and faster than the equivalent ROOT histograms. This library comes in a clean and modern C++ design which follows the advice given in popular C++ books, like those of Meyers and Sutter and Alexandrescu.

Read more about the design choices in the documentation

State of project

The histogram is feature-complete. More than 500 individual tests make sure that the implementation works as expected. Full documentation is available. User feedback is appreciated!

As we are finalising the interface in the review process, code breaking changes of the interface are not unlikely to happen. If you want to use the library in production code, please use the latest release instead of the master. After the library is accepted as part of Boost, the interface will be kept stable, of course.

Review of the library is planned to happen in March 2018.