add a missing operator= for copying a matrix into an expression.

src/MatrixBase.h

@@ -211,6 +211,16 @@ class MatrixBase
     MatrixBase() {};
 };
 
+template<typename Content>
+template<typename Derived>
+void MatrixXpr<Content>::operator=(const MatrixBase<Derived>& matrix)
+{
+  assert(rows() == matrix.rows() && cols() == matrix.cols());
+  for(int i = 0; i < rows(); i++)
+    for(int j = 0; j < cols(); j++)
+      this->operator()(i, j) = matrix(i, j);
+}
+
 template<typename Derived>
 std::ostream & operator <<
 ( std::ostream & s,

src/MatrixXpr.h

@@ -105,6 +105,9 @@ template<typename Content> class MatrixXpr
           this->operator()(i, j) = other(i, j);
     }
     
+    template<typename Derived>
+    void operator=(const MatrixBase<Derived>& matrix);
+    
     MatrixXpr<MatrixRow<MatrixXpr<Content> > > row(int i);
     MatrixXpr<MatrixCol<MatrixXpr<Content> > > col(int i);
     MatrixXpr<MatrixMinor<MatrixXpr<Content> > > minor(int row, int col);