matplotlib.h: add support for contiguous_range (C++20) and ranges.cpp…

… example
lava · amadeus84 · Mar 10, 2022 · Mar 12, 2022 · Mar 12, 2022 · Mar 14, 2022
commit 3f5b27618cf79c0fbc0219646b298218a3aa9220
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -104,6 +104,12 @@ if(Python3_NumPy_FOUND)
   target_link_libraries(span PRIVATE matplotlib_cpp)
   target_compile_features(span PRIVATE cxx_std_20)
   set_target_properties(span PROPERTIES RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin")
+
+  add_executable(ranges examples/ranges.cpp)
+  target_link_libraries(ranges PRIVATE matplotlib_cpp)
+  target_compile_features(ranges PRIVATE cxx_std_20)
+  set_target_properties(ranges PROPERTIES RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin")
+
 endif()
 
 

diff --git a/examples/ranges.cpp b/examples/ranges.cpp
@@ -0,0 +1,105 @@
+//
+// g++ -std=c++20 -g -Wall -o ranges $(python-config --includes) ranges.cpp $(python-config --ldflags --embed)
 //
+// g++ -std=c++17 -g -Wall -o ranges $(python-config --includes) ranges.cpp $(python-config --ldflags --embed)
+//
+#include "../matplotlibcpp.h"
+
+#include <iostream>
+#include <list>
+#include <map>
+#include <span>
+#include <string>
+#include <vector>
+
+#define UNUSED(x) (void)(x)
+
+using namespace std;
+namespace plt = matplotlibcpp;
+
+int main()
+{
+    plt::detail::_interpreter::get();
+
+
+#if __cplusplus >= CPP20
+
+    // C-style arrays with multiple rows.
+
+    // Care with column-major vs row-major!
+    // C and Python are row-major, but usually a time series is column-major
+    // and we want to plot the columns.
+    // In the example below, these columns are [3,1,4,5] and [5,4,1,3], so
+    // the data must be stored like this:
+    time_t t[]={1, 2, 3, 4};
+    double data [] = {
+	3, 5,
+	1, 4,
+	4, 1,
+	5, 3
+    };
+
+    // Use std::span() to convert to a contiguous range (O(1)).
+    // Data won't be copied, but passed as a pointer to Python.
+    plt::plot(span(t, 4), span(data, 8));
+    plt::grid(true);
+    plt::title("C-arrays, with multiple columns");
+    plt::show();
+
+#else
+
+    cerr << __FILE__ << ": " << __FUNCTION__ << ": " << __LINE__ << ": "
+	 << "No support for contiguous ranges." << endl;
+
+#endif
+
+    // vectors
+
+    // Vectors are also contiguous ranges.
+    // In C++20, as with span, plot resolves to plot(contiguous_range).
+    // In C++ < 20 plot resolves to plot(vector).
+    vector<double> x={1, 2, 3, 4, 5};
+    vector<double> y={0, 1, 0, -1, 0};
+    plt::plot(x, y);
+    plt::grid(true);
+    plt::title("vectors");
+    plt::show();
+
+
+    // lists
+
+    // By contrast, lists are non-contiguous (but iterable) containers, so
+    // plot resolves to plot(iterable).
+    list<double> u { 1, 2, 3, 4, 5};
+    list<double> v { 0, -1, 0, 1, 0};
+    plt::plot(u, v, "");
+    plt::grid(true);
+    plt::title("Lists (non-contiguous ranges)");
+    plt::show();
+
+
+    // All together now
+#if __cplusplus >= CPP20
+
+    // If span is not last, plot resolves to plot(iterable), which copies data.
+    // That's because in the dispatch plot() we have plot_impl() && plot()
+    // (i.e. plot_impl() comes first), and we only have iterable and
+    // callable plot_impl(). That sends us to the iterable plot_impl(),
+    // rather than to plot(contiguous_range).
+    //
+    // TODO: have 3 tags plot_impl(): iterable, callable and contiguous range.
+    plt::plot(span(t, 4), span(data, 8), "", x, y, "b", u, v, "r");
+
+    // This resolves to plot(contiguous_range) and does not copy data.
+    // plt::plot(x, y, "b", u, v, "r", span(t, 4), span(data, 8));
+#else
+    plt::plot(x, y, "b", u, v, "r");
+#endif
+    plt::grid(true);
+    plt::title("Variadic templates recursion");
+    plt::show();
+
+    plt::detail::_interpreter::kill();
+
+    return 0;
+}