diff --git a/16_reinforcement_learning.ipynb b/16_reinforcement_learning.ipynb
index 30f2ab2..b8aea98 100644
--- a/16_reinforcement_learning.ipynb
+++ b/16_reinforcement_learning.ipynb
@@ -1576,6 +1576,23 @@
     "replay_memory_size = 500000\n",
     "replay_memory = deque([], maxlen=replay_memory_size)\n",
     "\n",
+    "\n",
+    "class ReplayMemory:\n",
+    "    \n",
+    "    def __init__(self, size):\n",
+    "        self.size = size\n",
+    "        self.buf= [None] * self.size\n",
+    "        self.index = 0\n",
+    "        \n",
+    "    def append(self, data):\n",
+    "        self.buf[self.index] = data\n",
+    "        self.index = (self.index + 1) % self.size\n",
+    "    \n",
+    "    def __getitem__(self, idx):\n",
+    "        return self.buf[idx]\n",
+    "\n",
+    "    \n",
+    "\n",
     "def sample_memories(batch_size):\n",
     "    indices = np.random.permutation(len(replay_memory))[:batch_size]\n",
     "    cols = [[], [], [], [], []] # state, action, reward, next_state, continue\n",