updated exe7

Author: GohVh

exe7.ipynb

@@ -599,31 +599,32 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "# 7.17\n",
-    "\"\"\"\n",
-    "In the FavoritesListMTF class, we rely on public methods of the positional list ADT to move an element of a list at position p to become the first element of the list, while keeping the relative order of the remaining elements unchanged. Internally, that combination of operations causes one node to be removed and a new node to be inserted. Augment the PositionalList class to support a new method, move to front(p), that accomplishes this goal more directly, by relinking the existing node.\n",
-    "\"\"\"\n",
     "class FavouriteList:\n",
     "    class _Item:\n",
     "        __slots__ = '_value', '_count'\n",
     "        def __init__(self, e):\n",
-    "            self._value = e\n",
-    "            self._count = 0\n",
+    "            self._value = e #user's element\n",
+    "            self._count = 0 #access count\n",
     "        \n",
     "    def _find_position(self, e):\n",
+    "        #search for e and return its Position, None if not found.\n",
     "        walk = self._data.first()\n",
     "        while walk is not None and walk.element()._value != e:\n",
     "            walk = self._data.after(walk)\n",
     "        return walk\n",
     "    \n",
     "    def _move_up(self, p):\n",
-    "        if p != self._data.first():\n",
+    "        #move item at Position p earlier in the list based on access count.\n",
+    "        if p != self._data.first(): #if not 1st, then consider moving\n",
     "            cnt = p.element()._count\n",
     "            walk = self._data.before(p)\n",
-    "            if cnt > walk.element()._count:\n",
-    "                while (walk != self._data.first() and cnt > self._data.before(walk).element()._count):\n",
+    "            if cnt > walk.element()._count: #if cnt>, then proceed shifting forward\n",
+    "                while (walk != self._data.first() and cnt > self._data.before(walk).element()._count): #if still larger than before(walk), keep looping until walk cnt is larger than cnt\n",
     "                    walk = self._data.before(walk)\n",
-    "                self._data._add_before(walk, self._data.delete(p))\n",
+    "                self._data._add_before(walk, self._data.delete(p)) #after add before p, delete p in ori position\n",
+    "    \n",
+    "    def clear(self):\n",
+    "        pass\n",
     "    \n",
     "    def __init__(self):\n",
     "        self._data = PositionalList()\n",
@@ -635,56 +636,58 @@
     "        return len(self._data)==0\n",
     "    \n",
     "    def access(self, e):\n",
-    "        p = self._find_position(e)\n",
+    "        #user access e, therefore + e's access count\n",
+    "        p = self._find_position(e) #Locate existing e\n",
     "        if p is None:\n",
-    "            p = self._data.add_last(self._Item(e))\n",
+    "            p = self._data.add_last(self._Item(e)) #add as last position, with Item within, that contains e and count\n",
     "        p.element()._count +=1\n",
-    "        self._move_up(p)\n",
+    "        self._move_up(p) #check if p should move forward\n",
     "    \n",
     "    def remove(self, e):\n",
     "        p = self._find_position(e)\n",
     "        if p is not None:\n",
     "            self._data.delete(p)\n",
     "    \n",
     "    def top(self, k):\n",
+    "        #generate sequence of top k elements in terms of access count\n",
     "        if not 1 <= k <= len(self):\n",
     "            raise ValueError('Illegal value for k.')\n",
     "        walk = self._data.first()\n",
     "        for j in range(k):\n",
     "            item = walk.element()\n",
-    "            yield item._value\n",
-    "            walk = self._data.after(walk)\n",
+    "            yield item._value #report user's element\n",
+    "            walk = self._data.after(walk) #loop to next highest k-1 count\n",
     "\n",
     "\n",
     "class FavouriteListMTF(FavouriteList):\n",
     "\n",
+    "    #override _move_up to provide move-to-front heuristics\n",
     "    def _move_up(self, p):\n",
+    "        # move accessed item at Position p to most front\n",
     "        if p != self._data.first():\n",
-    "            self._data.add_first(self._data.delete(p))"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "# 7.22\n",
-    "\"\"\"\n",
-    "Implement a clear( ) method for the FavoritesList class that returns the list to empty\n",
-    "\"\"\""
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "# 7.23\n",
-    "\"\"\"\n",
-    "Implement a reset counts( ) method for the FavoritesList class that resets all elements’ access counts to zero (while leaving the order of the list unchanged).\n",
-    "\"\"\""
+    "            self._data.add_first(self._data.delete(p)) #move to first, delete at ori p.\n",
+    "    \n",
+    "    #override top cause list is no longer sorted.\n",
+    "    def top(self, k):\n",
+    "        if not 1<=k<=len(self):\n",
+    "            raise ValueError('Illegal value for k')\n",
+    "\n",
+    "        #make a copy\n",
+    "        temp = PositionalList()\n",
+    "        for item in self._data:\n",
+    "            temp.add_last(item)\n",
+    "        \n",
+    "        #repeatedly find, report and remove element with largest count.\n",
+    "        for j in range(k):\n",
+    "            highPos = temp.first()\n",
+    "            walk = temp.after(highPos)\n",
+    "            while walk is not None:\n",
+    "                if walk.element()._count>highPos.element()._count:\n",
+    "                    highPos=walk\n",
+    "                walk = temp.after(walk)\n",
+    "            #now have found jth highest count\n",
+    "            yield highPos.element()._value\n",
+    "            temp.delete(highPos) #delete jth highest count, proceed to j+1th highest count (2nd highest count)\n"
    ]
   }
  ],