Add mutable row access method and corresponding tests for Matrix

src/matrix/mat.rs

@@ -198,6 +198,18 @@ impl<T: Clone> Matrix<T> {
         row_data
     }
 
+    #[inline]
+    pub fn row_mut(&mut self, r: usize) -> &mut [T] {
+        assert!(
+            r < self.rows,
+            "row index {} out of bounds for {} rows",
+            r,
+            self.rows
+        );
+        let start = r;
+        &mut self.data[start..start + self.cols]
+    }
+
     /// Deletes a row from the matrix. Panics on out-of-bounds.
     /// This is O(N) where N is the number of elements, as it rebuilds the data vec.
     pub fn delete_row(&mut self, row: usize) {
@@ -1172,6 +1184,25 @@ mod tests {
         assert_eq!(ma.row(2), &[3, 6, 9]);
     }
 
+    #[test]
+    fn test_row_mut() {
+        let mut ma = static_test_matrix();
+        let row1_mut = ma.row_mut(1);
+        row1_mut[0] = 20;
+        row1_mut[1] = 50;
+        row1_mut[2] = 80;
+
+        assert_eq!(ma.row(1), &[20, 50, 80]);
+        assert_eq!(ma.data(), &[1, 2, 3, 20, 50, 80, 7, 8, 9]);
+    }
+
+    #[test]
+    #[should_panic(expected = "row index 3 out of bounds for 3 rows")]
+    fn test_row_mut_out_of_bounds() {
+        let mut ma = static_test_matrix();
+        ma.row_mut(3);
+    }
+
     #[test]
     fn test_shape() {
         let ma = static_test_matrix_2x4();