diff --git a/ipynb/Advent 2017.ipynb b/ipynb/Advent 2017.ipynb
index 0f8750a..b6bec63 100644
--- a/ipynb/Advent 2017.ipynb	
+++ b/ipynb/Advent 2017.ipynb	
@@ -857,8 +857,9 @@
    "source": [
     "def run(program):\n",
     "    memory = list(program)\n",
+    "    mlen = len(memory)\n",
     "    pc = steps = 0\n",
-    "    while pc in range(len(memory)):\n",
+    "    while 0 <= pc < mlen:\n",
     "        steps += 1\n",
     "        oldpc = pc\n",
     "        pc += memory[pc]\n",
@@ -912,8 +913,9 @@
    "source": [
     "def run2(program, verbose=False):\n",
     "    memory = list(program)\n",
+    "    mlen = len(memory)\n",
     "    pc = steps = 0\n",
-    "    while pc in range(len(memory)):\n",
+    "    while 0 <= pc < mlen:\n",
     "        steps += 1\n",
     "        oldpc = pc\n",
     "        pc += memory[pc]\n",