Feature/Add DMatrixRMaj#get2DData interface

main/ejml-core/src/org/ejml/data/DMatrixRMaj.java

@@ -374,4 +374,14 @@ public void fill( double value ) {
     public void set( double[][] input ) {
         DConvertArrays.convert(input, this);
     }
+
+    /**
+     * Export this matrix using a 2D array representation.
+     *
+     * @return 2D representation of the matrix
+     * @see DMatrixD1#getData() to get a 1D array representation
+     */
+    public double[][] get2DData() {
+        return DConvertArrays.convert(this);
+    }
 }

main/ejml-core/src/org/ejml/ops/DConvertArrays.java

@@ -58,6 +58,24 @@ public static DMatrixRMaj convert( double[][] src, @Nullable DMatrixRMaj dst ) {
         return dst;
     }
 
+    /**
+     * Convert a {@link DMatrixRMaj} to a two-dimensional array,
+     * given DMatrixRMaj can take a double[][] as input to constructor
+     *
+     * @param src is an input DMatrixRMaj
+     * @return a 2D array contains the same elements as the input matrix
+     */
+    public static double[][] convert( DMatrixRMaj src ) {
+        double[][] array = new double[src.numRows][src.numCols];
+        for (int row = 0; row < src.numRows; row++) {
+            for (int column = 0; column < src.numCols; column++) {
+                array[row][column] = src.get(row, column);
+            }
+        }
+
+        return array;
+    }
+
 //    public static DMatrixSparseCSC convert(double[][]src , @Nullable DMatrixSparseCSC dst ) {
 //        int rows = src.length;
 //        if( rows == 0 )

main/ejml-core/test/org/ejml/data/TestDMatrixRMaj.java

@@ -204,4 +204,15 @@ protected DMatrixD1 createMatrix( int numRows, int numCols ) {
             assertTrue(Math.abs(mat.data[i] - orig.data[i]) > UtilEjml.TEST_F64);
         }
     }
+
+    @Test void get2DData() {
+        double[][] expected = {{0, 1}, {2, 3}};
+        double[][] actual = new DMatrixRMaj(expected).get2DData();
+
+        assertArrayEquals(expected, actual);
+        assertEquals(0,actual[0][0], UtilEjml.TEST_F64);
+        assertEquals(1,actual[0][1], UtilEjml.TEST_F64);
+        assertEquals(2,actual[1][0], UtilEjml.TEST_F64);
+        assertEquals(3,actual[1][1], UtilEjml.TEST_F64);
+    }
 }

main/ejml-core/test/org/ejml/ops/TestDConvertArrays.java

@@ -24,6 +24,7 @@
 import org.ejml.data.DMatrixRMaj;
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 public class TestDConvertArrays extends EjmlStandardJUnit {
@@ -37,6 +38,18 @@ public void dd_to_ddrm() {
         assertEquals(3,m.get(1,1), UtilEjml.TEST_F64);
     }
 
+    @Test
+    public void ddrm_to_dd() {
+        double[][] expected = {{0, 1}, {2, 3}};
+        double[][] dd = DConvertArrays.convert(new DMatrixRMaj(expected));
+
+        assertArrayEquals(expected, dd);
+        assertEquals(0,dd[0][0], UtilEjml.TEST_F64);
+        assertEquals(1,dd[0][1], UtilEjml.TEST_F64);
+        assertEquals(2,dd[1][0], UtilEjml.TEST_F64);
+        assertEquals(3,dd[1][1], UtilEjml.TEST_F64);
+    }
+
 //    @Test
 //    public void dd_to_dscc() {
 //        DMatrixSparseCSC m = ConvertDArrays.convert(new double[][]{{0,1},{2,3}},(DMatrixSparseCSC) null);