diff --git a/src/extra/use_eigen_ex.cpp b/src/extra/use_eigen_ex.cpp
new file mode 100644
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--- /dev/null
+++ b/src/extra/use_eigen_ex.cpp
@@ -0,0 +1,52 @@
+/********************************************************
+ *  ██████╗  ██████╗████████╗██╗
+ * ██╔════╝ ██╔════╝╚══██╔══╝██║
+ * ██║  ███╗██║        ██║   ██║
+ * ██║   ██║██║        ██║   ██║
+ * ╚██████╔╝╚██████╗   ██║   ███████╗
+ *  ╚═════╝  ╚═════╝   ╚═╝   ╚══════╝
+ * Geophysical Computational Tools & Library (GCTL)
+ *
+ * Copyright (c) 2023  Yi Zhang (yizhang-geo@zju.edu.cn)
+ *
+ * GCTL is distributed under a dual licensing scheme. You can redistribute 
+ * it and/or modify it under the terms of the GNU Lesser General Public 
+ * License as published by the Free Software Foundation, either version 2 
+ * of the License, or (at your option) any later version. You should have 
+ * received a copy of the GNU Lesser General Public License along with this 
+ * program. If not, see <http://www.gnu.org/licenses/>.
+ * 
+ * If the terms and conditions of the LGPL v.2. would prevent you from using 
+ * the GCTL, please consider the option to obtain a commercial license for a 
+ * fee. These licenses are offered by the GCTL's original author. As a rule, 
+ * licenses are provided "as-is", unlimited in time for a one time fee. Please 
+ * send corresponding requests to: yizhang-geo@zju.edu.cn. Please do not forget 
+ * to include some description of your company and the realm of its activities. 
+ * Also add information on how to contact you by electronic and paper mail.
+ ******************************************************/
+
+// Declare the marco to enable eigen wrappers before include the head file.
+// To compile the code, use: g++ use_eigen_ex.cpp -I<path-to-eigen3>
+#define GCTL_USE_EIGEN3
+
+#include "gctl/core/eigen_wrapper.h"
+
+using namespace gctl;
+
+int main(int argc, char const *argv[])
+{
+    _1d_array a(10, 1, 0.5);
+    a.show();
+
+    Eigen::VectorXd b;
+    array2VectorXType(a, b);
+    std::cout << b << "\n";
+    std::cout << b.mean() << "\n";
+
+    b.setRandom();
+
+    _1d_array c;
+    VectorXType2array(b, c);
+    c.show();
+    return 0;
+}