Added an example for a contraction to a scalar value, e.g. a double contraction of two second order tensors and how you can get the value of the result. I lost one day to get this doen so I think it will help some guys. I also added Eigen:: to the IndexPair and and array in the same example.

1 files changed, 10 insertions, 3 deletions

diff --git a/unsupported/Eigen/CXX11/src/Tensor/README.md b/unsupported/Eigen/CXX11/src/Tensor/README.md
index 4423f81f7..30d553af7 100644
--- a/unsupported/Eigen/CXX11/src/Tensor/README.md
+++ b/unsupported/Eigen/CXX11/src/Tensor/README.md
@@ -1016,13 +1016,20 @@ multidimensional case.
     a.setValues({{1, 2}, {4, 5}, {5, 6}});
 
     // Compute the traditional matrix product
-    array<IndexPair<int>, 1> product_dims = { IndexPair(1, 0) };
+    Eigen::array<Eigen::IndexPair<int>, 1> product_dims = { Eigen::IndexPair(1, 0) };
     Eigen::Tensor<int, 2> AB = a.contract(b, product_dims);
 
     // Compute the product of the transpose of the matrices
-    array<IndexPair<int>, 1> transpose_product_dims = { IndexPair(0, 1) };
+    Eigen::array<Eigen::IndexPair<int>, 1> transpose_product_dims = { Eigen::IndexPair(0, 1) };
     Eigen::Tensor<int, 2> AtBt = a.contract(b, transposed_product_dims);
-
+    
+    // Contraction to scalar value using a ouble contraction 
+    // First coordinate of both tensors are contracted as well as both second coordinates
+    Eigen::array<Eigen::IndexPair<int>, 2> double_contraction_product_dims = { Eigen::IndexPair<int>(0, 0), Eigen::IndexPair<int>(1, 1) };
+    Eigen::Tensor<int, 0> AdoubleontractedA = a.contract(a, double_contraction_product_dims);
+    
+    // Extracting the scalar value of the tensor contraction for further usage
+    int value = AdoublecontractedA(0);
 
 ## Reduction Operations