added example of 2d axis and replaced int with index_type

CMakeLists.txt

@@ -117,6 +117,7 @@ compiled_test(examples/guide_axis_with_labels.cpp)
 compiled_test(examples/guide_axis_with_uoflow_off.cpp)
 compiled_test(examples/guide_custom_modified_axis.cpp)
 compiled_test(examples/guide_custom_minimal_axis.cpp)
+compiled_test(examples/guide_custom_2d_axis.cpp)
 compiled_test(examples/guide_custom_storage.cpp)
 compiled_test(examples/guide_fill_histogram.cpp)
 compiled_test(examples/guide_histogram_operators.cpp)

examples/guide_custom_2d_axis.cpp

@@ -0,0 +1,53 @@
+// Copyright 2018 Hans Dembinski
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+//[ guide_custom_2d_axis
+
+#include <boost/histogram.hpp>
+#include <cassert>
+
+namespace bh = boost::histogram;
+
+int main() {
+  // axis which returns 1 if the input falls inside the unit circle and zero otherwise
+  struct circle_axis {
+    bh::axis::index_type operator()(std::tuple<double, double> point) const {
+      const auto x = std::get<0>(point);
+      const auto y = std::get<1>(point);
+      return x * x + y * y <= 1.0;
+    }
+
+    bh::axis::index_type size() const { return 2; }
+  };
+
+  auto h1 = bh::make_histogram(circle_axis());
+
+  // call looks like for a histogram with N 1d axes if histogram has only one Nd axis
+  h1(0, 0);   // in
+  h1(0, -1);  // in
+  h1(0, 1);   // in
+  h1(-1, 0);  // in
+  h1(1, 0);   // in
+  h1(1, 1);   // out
+  h1(-1, -1); // out
+
+  assert(h1.at(0) == 2); // out
+  assert(h1.at(1) == 5); // in
+
+  auto h2 =
+      bh::make_histogram(circle_axis(), bh::axis::category<std::string>({"red", "blue"}));
+
+  // pass arguments for first axis as std::tuple
+  h2(std::make_tuple(0, 0), "red");
+  h2(std::make_tuple(1, 1), "blue");
+
+  assert(h2.at(0, 0) == 0); // out, red
+  assert(h2.at(0, 1) == 1); // out, blue
+  assert(h2.at(1, 0) == 1); // in, red
+  assert(h2.at(1, 1) == 0); // in, blue
+}
+
+//]

examples/guide_custom_minimal_axis.cpp

@@ -11,21 +11,21 @@
 
 namespace bh = boost::histogram;
 
-// stateless axis which returns 1 if the input is even and 0 otherwise
-struct even_odd_axis {
-  int operator()(int x) const { return x % 2; }
-  int size() const { return 2; }
-};
-
-// threshold axis which returns 1 if the input is above threshold
-struct threshold_axis {
-  threshold_axis(double x) : thr(x) {}
-  int operator()(double x) const { return x >= thr; }
-  int size() const { return 2; }
-  double thr;
-};
-
 int main() {
+  // stateless axis which returns 1 if the input is even and 0 otherwise
+  struct even_odd_axis {
+    bh::axis::index_type operator()(int x) const { return x % 2; }
+    bh::axis::index_type size() const { return 2; }
+  };
+
+  // threshold axis which returns 1 if the input is above threshold
+  struct threshold_axis {
+    threshold_axis(double x) : thr(x) {}
+    bh::axis::index_type operator()(double x) const { return x >= thr; }
+    bh::axis::index_type size() const { return 2; }
+    double thr;
+  };
+
   auto h = bh::make_histogram(even_odd_axis(), threshold_axis(3.0));
 
   h(0, 2.0);

examples/guide_custom_modified_axis.cpp

@@ -12,19 +12,21 @@
 
 namespace bh = boost::histogram;
 
-// custom axis, which adapts builtin integer axis
-struct custom_axis : public bh::axis::integer<> {
-  using value_type = const char*; // type that is fed to the axis
-
-  using integer::integer; // inherit ctors of base
-
-  // the customization point
-  // - accept const char* and convert to int
-  // - then call index method of base class
-  int operator()(value_type s) const { return integer::operator()(std::atoi(s)); }
-};
-
 int main() {
+  // custom axis, which adapts builtin integer axis
+  struct custom_axis : public bh::axis::integer<> {
+    using value_type = const char*; // type that is fed to the axis
+
+    using integer::integer; // inherit ctors of base
+
+    // the customization point
+    // - accept const char* and convert to int
+    // - then call index method of base class
+    bh::axis::index_type operator()(value_type s) const {
+      return integer::operator()(std::atoi(s));
+    }
+  };
+
   auto h = bh::make_histogram(custom_axis(3, 6));
   h("-10");
   h("3");