Advanced Indexing - Update Exercise solution (#348)

* Advanced Indexing - Update Exercise solution

The small exercise at the bottom of the page declared two DataArrays, but only used one. The solution was still correct but confusing, so I reduced the needed variable declaration to the required one and renamed.

* Advanced Indexing - Update exercise

Vary the showcase x and y coords as suggested by @ianhi

Author: jomey

intermediate/indexing/advanced-indexing.ipynb

@@ -366,18 +366,19 @@
     "::::{admonition} Exercise\n",
     ":class: tip\n",
     "\n",
-    "In the simple 2D 5x5 Xarray data array above, select the sub-array containing (0,0),(2,2),(4,4):\n",
+    "In the simple 2D 5x5 Xarray data array above, select the sub-array containing (0,4),(2,2),(4,0):\n",
     "\n",
     ":::{admonition} Solution\n",
     ":class: dropdown\n",
     "```python\n",
     "\n",
-    "indices = np.array([0, 2, 4])\n",
+    "x_indices = np.array([0, 2, 4])\n",
+    "y_indices = np.array([4, 2, 0])\n",
     "\n",
-    "xs_da = xr.DataArray(indices, dims=\"points\")\n",
-    "ys_da = xr.DataArray(indices, dims=\"points\")\n",
+    "da_x = xr.DataArray(x_indices, dims=\"points\")\n",
+    "da_y = xr.DataArray(y_indices, dims=\"points\")\n",
     "\n",
-    "subset_da = da.sel(x=xs_da, y=xs_da)\n",
+    "subset_da = da.sel(x=da_x, y=da_y)\n",
     "subset_da\n",
     "```\n",
     ":::\n",