better explanation

content/03_stack.ipynb

@@ -44,7 +44,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": 11,
    "id": "0e704969",
    "metadata": {},
    "outputs": [],
@@ -62,20 +62,26 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 6,
+   "execution_count": 73,
    "id": "102cacf7",
    "metadata": {},
    "outputs": [],
    "source": [
     "class Stack:\n",
     "    def __init__(self): self.items = []\n",
-    "    def __str__ (self): return \"\\n\".join(map(str, self.items[::-1]))\n",
+    "    def __str__ (self): \n",
+    "        ws = []\n",
+    "        for i, item in enumerate(self.items[::-1]):\n",
+    "            if   i == 0                : ws.append(f\"{item} <first\")\n",
+    "            elif i == len(self.items)-1: ws.append(f\"{item} <last\") \n",
+    "            else                       : ws.append(str(item))\n",
+    "        return \"\\n\".join(ws)\n",
     "        \n",
     "    def push(self, value): \n",
     "        if len(self.items) == MAXIMUM_STACK_SIZE-1: raise Exception(\"Stack overflow\")\n",
     "        self.items.append(value)    \n",
     "    \n",
-    "    def pop(self, n=0):\n",
+    "    def pop(self, n=-1):\n",
     "        if len(self.items) < n: raise Exception(\"Stack overflow\")\n",
     "        del self.items[n]"
    ]
@@ -90,7 +96,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 7,
+   "execution_count": 74,
    "id": "aaaf2494",
    "metadata": {},
    "outputs": [],
@@ -108,17 +114,17 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 8,
+   "execution_count": 75,
    "id": "8771e0a0",
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "1\n",
+      "1 <first\n",
       "4\n",
-      "2\n"
+      "2 <last\n"
      ]
     }
    ],
@@ -135,47 +141,39 @@
    "id": "1da65b97",
    "metadata": {},
    "source": [
-    "We pop one of the stack"
+    "We pop one of the stack. Which is `1`."
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 9,
+   "execution_count": 76,
    "id": "41cf0942",
    "metadata": {},
-   "outputs": [
-    {
-     "name": "stdout",
-     "output_type": "stream",
-     "text": [
-      "None\n"
-     ]
-    }
-   ],
+   "outputs": [],
    "source": [
-    "print(stack.pop())"
+    "stack.pop()"
    ]
   },
   {
    "cell_type": "markdown",
    "id": "fb3d028a",
    "metadata": {},
    "source": [
-    "Now only 3 values are left on the stack"
+    "Now only 2 values are left on the stack. Notice how we removed `1`."
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 10,
+   "execution_count": 77,
    "id": "b1612ec9",
    "metadata": {},
    "outputs": [
     {
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "1\n",
-      "4\n"
+      "4 <first\n",
+      "2 <last\n"
      ]
     }
    ],