Add a nullary-functor example performing index-based sub-matrices.

doc/CustomizingEigen_NullaryExpr.dox

@@ -53,6 +53,33 @@ showing that the program works as expected:
 
 This implementation of \c makeCirculant is much simpler than \ref TopicNewExpressionType "defining a new expression" from scratch.
 
+
+\section NullaryExpr_Indexing Example 2: indexing rows and columns
+
+The goal here is to mimic MatLab's ability to index a matrix through two vectors of indices referencing the rows and columns to be picked respectively, like this:
+
+\snippet nullary_indexing.out main1
+
+To this end, let us first write a nullary-functor storing references to the input matrix and to the two arrays of indices, and implementing the required \c operator()(i,j):
+
+\snippet nullary_indexing.cpp functor
+
+Then, let's create an \c indexing(A,rows,cols) function creating the nullary expression:
+
+\snippet nullary_indexing.cpp function
+
+Finally, here is an example of how this function can be used:
+
+\snippet nullary_indexing.cpp main1
+
+This straightforward implementation is already quite powerful as the row or column index arrays can also be expressions to perform offsetting, modulo, striding, reverse, etc.
+
+\snippet nullary_indexing.cpp main2
+
+and the output is:
+
+\snippet nullary_indexing.out main2
+
 */
 
 }

doc/examples/CMakeLists.txt

@@ -14,3 +14,8 @@ foreach(example_src ${examples_SRCS})
   )
   add_dependencies(all_examples ${example})
 endforeach(example_src)
+
+check_cxx_compiler_flag("-std=c++11" EIGEN_COMPILER_SUPPORT_CPP11)
+if(EIGEN_COMPILER_SUPPORT_CPP11)
+ei_add_target_property(nullary_indexing COMPILE_FLAGS "-std=c++11")
+endif()

doc/examples/nullary_indexing.cpp

@@ -0,0 +1,66 @@
+#include <Eigen/Core>
+#include <iostream>
+
+using namespace Eigen;
+
+// [functor]
+template<class ArgType, class RowIndexType, class ColIndexType>
+class indexing_functor {
+  const ArgType &m_arg;
+  const RowIndexType &m_rowIndices;
+  const ColIndexType &m_colIndices;
+public:
+  typedef Matrix<typename ArgType::Scalar,
+                 RowIndexType::SizeAtCompileTime,
+                 ColIndexType::SizeAtCompileTime,
+                 ArgType::Flags&RowMajorBit?RowMajor:ColMajor,
+                 RowIndexType::MaxSizeAtCompileTime,
+                 ColIndexType::MaxSizeAtCompileTime> MatrixType;
+
+  indexing_functor(const ArgType& arg, const RowIndexType& row_indices, const ColIndexType& col_indices)
+    : m_arg(arg), m_rowIndices(row_indices), m_colIndices(col_indices)
+  {}
+
+  const typename ArgType::Scalar& operator() (Index row, Index col) const {
+    return m_arg(m_rowIndices[row], m_colIndices[col]);
+  }
+};
+// [functor]
+
+// [function]
+template <class ArgType, class RowIndexType, class ColIndexType>
+CwiseNullaryOp<indexing_functor<ArgType,RowIndexType,ColIndexType>, typename indexing_functor<ArgType,RowIndexType,ColIndexType>::MatrixType>
+indexing(const Eigen::MatrixBase<ArgType>& arg, const RowIndexType& row_indices, const ColIndexType& col_indices)
+{
+  typedef indexing_functor<ArgType,RowIndexType,ColIndexType> Func;
+  typedef typename Func::MatrixType MatrixType;
+  return MatrixType::NullaryExpr(row_indices.size(), col_indices.size(), Func(arg.derived(), row_indices, col_indices));
+}
+// [function]
+
+
+int main()
+{
+  std::cout << "[main1]\n";
+  Eigen::MatrixXi A = Eigen::MatrixXi::Random(4,4);
+  Array3i ri(1,2,1);
+  ArrayXi ci(6); ci << 3,2,1,0,0,2;
+  Eigen::MatrixXi B = indexing(A, ri, ci);
+  std::cout << "A =" << std::endl;
+  std::cout << A << std::endl << std::endl;
+  std::cout << "A([" << ri.transpose() << "], [" << ci.transpose() << "]) =" << std::endl;
+  std::cout << B << std::endl;
+  std::cout << "[main1]\n";
+
+  std::cout << "[main2]\n";
+  B =  indexing(A, ri+1, ci);
+  std::cout << "A(ri+1,ci) =" << std::endl;
+  std::cout << B << std::endl << std::endl;
+#if __cplusplus >= 201103L
+  B =  indexing(A, ArrayXi::LinSpaced(13,0,12).unaryExpr([](int x){return x%4;}), ArrayXi::LinSpaced(4,0,3));
+  std::cout << "A(ArrayXi::LinSpaced(13,0,12).unaryExpr([](int x){return x%4;}), ArrayXi::LinSpaced(4,0,3)) =" << std::endl;
+  std::cout << B << std::endl << std::endl;
+#endif
+  std::cout << "[main2]\n";
+}
+