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Commit d84860f

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updated exe7
1 parent 946e584 commit d84860f

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‎exe7.ipynb‎

Lines changed: 43 additions & 40 deletions
Original file line numberDiff line numberDiff line change
@@ -599,31 +599,32 @@
599599
"metadata": {},
600600
"outputs": [],
601601
"source": [
602-
"# 7.17\n",
603-
"\"\"\"\n",
604-
"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",
605-
"\"\"\"\n",
606602
"class FavouriteList:\n",
607603
" class _Item:\n",
608604
" __slots__ = '_value', '_count'\n",
609605
" def __init__(self, e):\n",
610-
" self._value = e\n",
611-
" self._count = 0\n",
606+
" self._value = e #user's element\n",
607+
" self._count = 0 #access count\n",
612608
" \n",
613609
" def _find_position(self, e):\n",
610+
" #search for e and return its Position, None if not found.\n",
614611
" walk = self._data.first()\n",
615612
" while walk is not None and walk.element()._value != e:\n",
616613
" walk = self._data.after(walk)\n",
617614
" return walk\n",
618615
" \n",
619616
" def _move_up(self, p):\n",
620-
" if p != self._data.first():\n",
617+
" #move item at Position p earlier in the list based on access count.\n",
618+
" if p != self._data.first(): #if not 1st, then consider moving\n",
621619
" cnt = p.element()._count\n",
622620
" walk = self._data.before(p)\n",
623-
" if cnt > walk.element()._count:\n",
624-
" while (walk != self._data.first() and cnt > self._data.before(walk).element()._count):\n",
621+
" if cnt > walk.element()._count: #if cnt>, then proceed shifting forward\n",
622+
" 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",
625623
" walk = self._data.before(walk)\n",
626-
" self._data._add_before(walk, self._data.delete(p))\n",
624+
" self._data._add_before(walk, self._data.delete(p)) #after add before p, delete p in ori position\n",
625+
" \n",
626+
" def clear(self):\n",
627+
" pass\n",
627628
" \n",
628629
" def __init__(self):\n",
629630
" self._data = PositionalList()\n",
@@ -635,56 +636,58 @@
635636
" return len(self._data)==0\n",
636637
" \n",
637638
" def access(self, e):\n",
638-
" p = self._find_position(e)\n",
639+
" #user access e, therefore + e's access count\n",
640+
" p = self._find_position(e) #Locate existing e\n",
639641
" if p is None:\n",
640-
" p = self._data.add_last(self._Item(e))\n",
642+
" p = self._data.add_last(self._Item(e)) #add as last position, with Item within, that contains e and count\n",
641643
" p.element()._count +=1\n",
642-
" self._move_up(p)\n",
644+
" self._move_up(p) #check if p should move forward\n",
643645
" \n",
644646
" def remove(self, e):\n",
645647
" p = self._find_position(e)\n",
646648
" if p is not None:\n",
647649
" self._data.delete(p)\n",
648650
" \n",
649651
" def top(self, k):\n",
652+
" #generate sequence of top k elements in terms of access count\n",
650653
" if not 1 <= k <= len(self):\n",
651654
" raise ValueError('Illegal value for k.')\n",
652655
" walk = self._data.first()\n",
653656
" for j in range(k):\n",
654657
" item = walk.element()\n",
655-
" yield item._value\n",
656-
" walk = self._data.after(walk)\n",
658+
" yield item._value #report user's element\n",
659+
" walk = self._data.after(walk) #loop to next highest k-1 count\n",
657660
"\n",
658661
"\n",
659662
"class FavouriteListMTF(FavouriteList):\n",
660663
"\n",
664+
" #override _move_up to provide move-to-front heuristics\n",
661665
" def _move_up(self, p):\n",
666+
" # move accessed item at Position p to most front\n",
662667
" if p != self._data.first():\n",
663-
" self._data.add_first(self._data.delete(p))"
664-
]
665-
},
666-
{
667-
"cell_type": "code",
668-
"execution_count": null,
669-
"metadata": {},
670-
"outputs": [],
671-
"source": [
672-
"# 7.22\n",
673-
"\"\"\"\n",
674-
"Implement a clear( ) method for the FavoritesList class that returns the list to empty\n",
675-
"\"\"\""
676-
]
677-
},
678-
{
679-
"cell_type": "code",
680-
"execution_count": null,
681-
"metadata": {},
682-
"outputs": [],
683-
"source": [
684-
"# 7.23\n",
685-
"\"\"\"\n",
686-
"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",
687-
"\"\"\""
668+
" self._data.add_first(self._data.delete(p)) #move to first, delete at ori p.\n",
669+
" \n",
670+
" #override top cause list is no longer sorted.\n",
671+
" def top(self, k):\n",
672+
" if not 1<=k<=len(self):\n",
673+
" raise ValueError('Illegal value for k')\n",
674+
"\n",
675+
" #make a copy\n",
676+
" temp = PositionalList()\n",
677+
" for item in self._data:\n",
678+
" temp.add_last(item)\n",
679+
" \n",
680+
" #repeatedly find, report and remove element with largest count.\n",
681+
" for j in range(k):\n",
682+
" highPos = temp.first()\n",
683+
" walk = temp.after(highPos)\n",
684+
" while walk is not None:\n",
685+
" if walk.element()._count>highPos.element()._count:\n",
686+
" highPos=walk\n",
687+
" walk = temp.after(walk)\n",
688+
" #now have found jth highest count\n",
689+
" yield highPos.element()._value\n",
690+
" temp.delete(highPos) #delete jth highest count, proceed to j+1th highest count (2nd highest count)\n"
688691
]
689692
}
690693
],

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