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This clarifies how to use histogram::fill efficiently with multi-dimensional histograms, and discusses a workaround for the design choice that Axis::index cannot be overloaded. And since I haven't build the documentation locally for a long time, and a new computer where I needed to set things up again from scratch, I also added a README.md on how to do that (which is not trivial for boostbook).
72 lines
2.8 KiB
C++
72 lines
2.8 KiB
C++
// Copyright 2015-2018 Hans Dembinski
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//
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE_1_0.txt
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// or copy at http://www.boost.org/LICENSE_1_0.txt)
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//[ guide_fill_histogram
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#include <boost/histogram.hpp>
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#include <boost/core/span.hpp>
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#include <cassert>
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#include <functional>
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#include <numeric>
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#include <utility>
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#include <vector>
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int main() {
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using namespace boost::histogram;
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auto h = make_histogram(axis::integer<>(0, 3), axis::regular<>(2, 0.0, 1.0));
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// fill histogram, number of arguments must be equal to number of axes,
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// types must be convertible to axis value type (here integer and double)
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h(0, 0.2); // increase a cell value by one
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h(2, 0.5); // increase another cell value by one
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// fills from a tuple are also supported; passing a tuple of wrong size
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// causes an error at compile-time or an assertion at runtime in debug mode
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auto xy = std::make_tuple(1, 0.3);
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h(xy);
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// chunk-wise filling is also supported and more efficient, make some data...
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std::vector<double> x = {0, 2, 5};
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std::vector<double> y = {0.8, 0.4, 0.7};
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// fill accepts an iterable over iterables, to avoid copying data we use
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// std::vector<boost::span<double>> and not std::vector<std::vector<double>>
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std::vector<boost::span<double>> xy2 = {boost::make_span(x), boost::make_span(y)};
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// alternatively, if the number of axes is known at compile-time,
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// it is better to use an array of spans, which avoids the heap allocation
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// std::array<boost::span<double>, 2> xy2 = {boost::make_span(x), boost::make_span(y)};
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// ... and call fill method
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h.fill(xy2);
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// once histogram is filled, access individual cells using operator[] or at(...)
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// - operator[] can only accept a single argument in the current version of C++,
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// it is convenient when you have a 1D histogram
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// - at(...) can accept several values, so use this by default
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// - underflow bins are at index -1, overflow bins at index `size()`
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// - passing an invalid index triggers a std::out_of_range exception
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assert(h.at(0, 0) == 1);
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assert(h.at(0, 1) == 1);
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assert(h.at(1, 0) == 1);
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assert(h.at(1, 1) == 0);
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assert(h.at(2, 0) == 1);
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assert(h.at(2, 1) == 1);
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assert(h.at(-1, -1) == 0); // underflow for axis 0 and 1
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assert(h.at(-1, 0) == 0); // underflow for axis 0, normal bin for axis 1
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assert(h.at(-1, 2) == 0); // underflow for axis 0, overflow for axis 1
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assert(h.at(3, 1) == 1); // overflow for axis 0, normal bin for axis 1
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// iteration over values works, but see next example for a better way
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// - iteration using begin() and end() includes under- and overflow bins
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// - iteration order is an implementation detail and should not be relied upon
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const double sum = std::accumulate(h.begin(), h.end(), 0.0);
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assert(sum == 6);
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}
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//]
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