SparseQR: Add toSparse method in matrixQ return type to extract the related Eigen SparseMatrix

Author: Lucas-Haubert

include/eigenpy/decompositions/sparse/SparseQR.hpp

@@ -64,6 +64,7 @@ struct SparseQRMatrixQReturnTypeVisitor
   typedef Eigen::SparseQRMatrixQTransposeReturnType<SparseQRType>
       QTransposeType;
   typedef Eigen::SparseQRMatrixQReturnType<SparseQRType> QType;
+  typedef typename SparseQRType::QRMatrixType QRMatrixType;
   typedef Eigen::VectorXd VectorXd;
   typedef Eigen::MatrixXd MatrixXd;
 
@@ -92,9 +93,21 @@ struct SparseQRMatrixQReturnTypeVisitor
             +[](const QType& self) -> QTransposeType { return self.adjoint(); })
 
         .def(
-            "transpose", +[](const QType& self) -> QTransposeType {
+            "transpose",
+            +[](const QType& self) -> QTransposeType {
               return self.transpose();
-            });
+            })
+
+        .def(
+            "toSparse",
+            +[](QType& self) -> QRMatrixType {
+              Eigen::Index m = self.rows();
+              MatrixXd I = MatrixXd::Identity(m, m);
+              MatrixXd Q_dense = self * I;
+              return Q_dense.sparseView();
+            },
+            bp::arg("self"),
+            "Convert the Q matrix to a sparse matrix representation.");
   }
 
   static void expose() {

unittest/python/decompositions/sparse/test_SparseQR.py

@@ -72,3 +72,24 @@
 QtAP = Qt @ A_permuted
 R_dense = spqr.matrixR().toarray()
 assert eigenpy.is_approx(QtAP, R_dense)
+
+Q_sparse = Q.toSparse()
+R_sparse = R
+
+assert Q_sparse.shape == (dim, dim)
+
+QtQ_sparse = Q_sparse.T @ Q_sparse
+QQt_sparse = Q_sparse @ Q_sparse.T
+I_sparse = spa.identity(dim, format="csc")
+
+assert eigenpy.is_approx(QtQ_sparse.toarray(), I_sparse.toarray())
+assert eigenpy.is_approx(QQt_sparse.toarray(), I_sparse.toarray())
+
+Q_sparse_test_vec = Q_sparse @ test_vec
+assert eigenpy.is_approx(Qv, Q_sparse_test_vec)
+
+Q_sparse_test_matrix = Q_sparse @ test_matrix
+assert eigenpy.is_approx(QM, Q_sparse_test_matrix)
+
+QR_sparse = Q_sparse @ R_sparse.toarray()
+assert eigenpy.is_approx(QR_sparse, A_permuted)