ceph/src/boost/libs/histogram/examples/guide_histogram_reduction.cpp

   1 // Copyright 2015-2018 Hans Dembinski
   2 //
   3 // Distributed under the Boost Software License, Version 1.0.
   4 // (See accompanying file LICENSE_1_0.txt
   5 // or copy at http://www.boost.org/LICENSE_1_0.txt)
   6
   7 //[ guide_histogram_reduction
   8
   9 #include <boost/histogram.hpp>
  10 #include <cassert>
  11
  12 int main() {
  13   using namespace boost::histogram;
  14   // import reduce commands into local namespace to save typing
  15   using algorithm::rebin;
  16   using algorithm::shrink;
  17   using algorithm::slice;
  18
  19   // make a 2d histogram
  20   auto h = make_histogram(axis::regular<>(4, 0.0, 4.0), axis::regular<>(4, -2.0, 2.0));
  21
  22   h(0, -0.9);
  23   h(1, 0.9);
  24   h(2, 0.1);
  25   h(3, 0.1);
  26
  27   // reduce takes positional commands which are applied to the axes in order
  28   // - shrink is applied to the first axis; the new axis range is 0.0 to 3.0
  29   // - rebin is applied to the second axis; pairs of adjacent bins are merged
  30   auto h2 = algorithm::reduce(h, shrink(0.0, 3.0), rebin(2));
  31
  32   assert(h2.axis(0) == axis::regular<>(3, 0.0, 3.0));
  33   assert(h2.axis(1) == axis::regular<>(2, -2.0, 2.0));
  34
  35   // reduce does not change the total count if the histogram has underflow/overflow bins
  36   assert(algorithm::sum(h) == 4 && algorithm::sum(h2) == 4);
  37
  38   // One can also explicitly specify the index of the axis in the histogram on which the
  39   // command should act, by using this index as the the first parameter. The position of
  40   // the command in the argument list of reduce is then ignored. We use this to slice only
  41   // the second axis (axis has index 1 in the histogram) from bin index 2 to 4.
  42   auto h3 = algorithm::reduce(h, slice(1, 2, 4));
  43
  44   assert(h3.axis(0) == h.axis(0)); // unchanged
  45   assert(h3.axis(1) == axis::regular<>(2, 0.0, 2.0));
  46 }
  47
  48 //]