examples: use std::span (C++20) to plot 2D C-style arrays

Author: amadeus84

CMakeLists.txt

@@ -5,7 +5,6 @@ include(GNUInstallDirs)
 set(PACKAGE_NAME matplotlib_cpp)
 set(INSTALL_CONFIGDIR ${CMAKE_INSTALL_LIBDIR}/${PACKAGE_NAME}/cmake)
 
-
 # Library target
 add_library(matplotlib_cpp INTERFACE)
 target_include_directories(matplotlib_cpp
@@ -100,6 +99,11 @@ if(Python3_NumPy_FOUND)
   add_executable(spy examples/spy.cpp)
   target_link_libraries(spy PRIVATE matplotlib_cpp)
   set_target_properties(spy PROPERTIES RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin")
+
+  add_executable(span examples/span.cpp)
+  target_link_libraries(span PRIVATE matplotlib_cpp)
+  target_compile_features(span PRIVATE cxx_std_20)
+  set_target_properties(span PROPERTIES RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin")
 endif()
 
 

examples/span.cpp

@@ -0,0 +1,34 @@
+//
+// g++ -std=c++20 -g -Wall -o span $(python-config --includes) span.cpp $(python-config --ldflags --embed)
+//
+#include "../matplotlibcpp.h"
+
+#include <span>
+
+using namespace std;
+namespace plt = matplotlibcpp;
+
+int main()
+{
+    // C-style arrays with multiple rows
+    time_t t[]={1, 2, 3, 4};
+    int data[]={
+	3, 1, 4, 5,
+	5, 4, 1, 3,
+	3, 3, 3, 3
+    };
+
+    size_t n=sizeof(t)/sizeof(decltype(t[0]));
+    size_t m=sizeof(data)/sizeof(decltype(data[0]));
+
+    // Use std::span to pass data chunk to plot(), without copying it.
+    // Unfortunately, plt::plot() makes an internal copy of both x and y
+    // before passing them to python.
+    for (size_t offset=0; offset<m; offset+=n)
+	plt::plot(t, std::span {data+offset, n}, "");
+    plt::show();
+
+    plt::detail::_interpreter::kill();
+
+    return 0;
+}