gaehlerm
diff --git a/‎object_oriented_programming.ipynb
Lines changed: 23 additions & 15 deletions b/‎object_oriented_programming.ipynb
Lines changed: 23 additions & 15 deletions
diff --git a/‎tutorial/tests/test_object_oriented_programming.py
Lines changed: 14 additions & 12 deletions b/‎tutorial/tests/test_object_oriented_programming.py
Lines changed: 14 additions & 12 deletions
@@ -1404,7 +1404,7 @@
     "\n",
     "2. To determine the sign of the velocity change, consider the moons' positions. For example, if `G` stands for Ganymede and `C` for Callisto:\n",
     "\n",
-    "    * If `Gx = 5` (the `x` position of Ganymede) and `Cx = 3`, then Ganymede's `x` velocity changes by `+1` (because `Gx > Cx`), and Callisto's `x` velocity must change by `-1` (because `Cx < Gx`).\n",
+    "    * If `Gx = 3` (the `x` position of Ganymede) and `Cx = 5`, then Ganymede's `x` velocity changes by `+1` (because `5 > 3`), and Callisto's `x` velocity must change by `-1` (because `3 < 5`).\n",
     "    * If the positions on a given axis **are the same**, then the velocity on that axis doesn't change at all.\n",
     "    \n",
     "3. Once the gravity has been calculated and the velocity updated, we should also update the position: simply **add the velocity** of each moon to its current position. For example, if Europa's position is `x=1, y=2, z=3` and its velocity `x=-2, y=0, z=3`, then the new position would be `x=-1, y=2, z=6`."
@@ -1434,8 +1434,8 @@
     "After simulating **10 time steps**, the system's configurations is\n",
     "```\n",
-    "Ganymede: x= 2, y= 1, z=-3, vx=-3, vy=-2, vz=1\n",
-    "Io: x= 1, y=-8, z= 0, vx=-1, vy= 1, vz=3\n",
+    "Ganymede: x= 2, y= 1, z=-3, vx=-3, vy=-2, vz= 1\n",
+    "Io: x= 1, y=-8, z= 0, vx=-1, vy= 1, vz= 3\n",
     "Europa: x= 3, y=-6, z= 1, vx= 3, vy= 2, vz=-3\n",
     "Callisto: x= 2, y= 0, z= 4, vx= 1, vy=-1, vz=-1\n",
     "```\n",
@@ -1452,17 +1452,6 @@
     "Sum of total energy: `36 + 45 + 80 + 18 = 179`"
    ]
   },
-  {
-   "cell_type": "markdown",
-   "id": "024c6e45-4204-4930-9e1c-47a7a472236c",
-   "metadata": {},
-   "source": [
-    "<div class=\"alert alert-block alert-warning\">\n",
-    "<h4><b>Question</b></h4>\n",
-    "    What is the <b>total energy of the system</b> after simulating the moons for <b>1000 time steps</b>?\n",
-    "</div>"
-   ]
-  },
   {
    "cell_type": "markdown",
    "id": "ccb911c1-625a-45e9-b21d-7057bcc2672e",
@@ -1485,7 +1474,7 @@
    "source": [
     "class Universe:\n",
     "    \"\"\"A class for a universe\"\"\"\n",
-    "    # Write here your implementation here of the Universe class \n"
+    "    # Write here your implementation here of the Universe class "
    ]
   },
   {
@@ -1504,6 +1493,17 @@
     "</div>"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "17a227ba-6c7a-4df1-b635-1c6f95ef8bc6",
+   "metadata": {},
+   "source": [
+    "<div class=\"alert ale
10000
rt-block alert-warning\">\n",
+    "<h4><b>Question</b></h4>\n",
+    "    What is the <b>average</b> of the total energy of the system after simulating the universe for <b>1000 time steps</b>?\n",
+    "</div>"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
@@ -1519,6 +1519,14 @@
     "    # Write your solution here\n",
     "    pass"
    ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "21e23134-7fc3-42ce-8024-9f9583abfc9f",
+   "metadata": {},
+   "outputs": [],
+   "source": []
   }
  ],
  "metadata": {
 
@@ -3,6 +3,7 @@
 from typing import List
 
 import pytest
+from numpy import average
 
 
 #
@@ -61,7 +62,7 @@ def read_data(name: str, data_dir: str = "data") -> pathlib.Path:
     """Read input data"""
     current_module = sys.modules[__name__]
     return (
-        pathlib.Path(current_module.__file__).parent / f"{data_dir}/{name}"
+            pathlib.Path(current_module.__file__).parent / f"{data_dir}/{name}"
     ).resolve()
 
 
@@ -120,15 +121,15 @@ def update_velocities(self, moon: "Moon") -> None:
         """Update the velocity of the moon"""
         for n, position in enumerate(self.positions):
             if position > moon.positions[n]:
-                delta = 1
-            elif position < moon.positions[n]:
                 delta = -1
+            elif position < moon.positions[n]:
+                delta = 1
             else:
                 delta = 0
 
             if delta:
-                self.velocities[n] -= delta
-                moon.velocities[n] += delta
+                self.velocities[n] += delta
+                moon.velocities[n] -= delta
 
     def update_positions(self) -> None:
         """Update the position of the moon"""
@@ -165,18 +166,20 @@ def test_moons(moons: str, function_to_test):
 class Universe:
     """A class for a universe"""
 
-    def __init__(self, moons: str) -> None:
-        self.moons = [Moon(moon) for moon in moons.splitlines()]
+    def __init__(self, universe_start: str) -> None:
+        self.moons = [Moon(moon) for moon in universe_start.splitlines()]
 
-    def evolve(self) -> None:
+    def evolve(self) -> "Universe":
         """Evolve the universe"""
         for n, moon_i in enumerate(self.moons[:-1]):
-            for moon_j in self.moons[n + 1 :]:
+            for moon_j in self.moons[n + 1:]:
                 moon_i.update_velocities(moon_j)
 
         for moon in self.moons:
             moon.update_positions()
 
+        return self
+
     @property
     def energy(self) -> int:
         """Return the total energy of the universe"""
@@ -194,6 +197,5 @@ def __repr__(self) -> str:
 @pytest.mark.parametrize("universe_start", universes)
 def test_n_body(universe_start: str, function_to_test) -> None:
     universe = Universe(universe_start)
-    for _ in range(1000):
-        universe.evolve()
-    assert function_to_test(universe) == universe.energy
+    energy = [universe.evolve().energy for _ in range(1000)]
+    assert function_to_test(universe_start) == pytest.approx(average(energy))