1. Computer Science 112 Fundamentals of Programming II Array-Based Queues

2. Array Implementation I Maintain a rear pointer to the index of the most recently added item The index of the front of the queue is always 0 self._rear = -1 self._items = Array(ArrayQueue.DEFAULT_CAPACITY)

3. Array Implementation I DDDD 0 1 2 3 4 5 Front of queue Rear of queue 3 Rear is always logical size - 1 Resize the array when we run out of room Shift to the left when we pop (yikes, a linear operation!)

4. Array Implementation II Keep separate pointers to front and rear Don ’ t shift items during a pop, but let the front pointer move to the right self._rear = -1 self._front = -1 self._items = Array(ArrayQueue.DEFAULT_CAPACITY)

5. Array Implementation II: Initial State 0 1 2 3 4 5 Front of queue Rear of queue

6. Array Implementation II D 0 1 2 3 4 5 Front of queue Rear of queue 00 Add an item

7. Array Implementation II DD 0 1 2 3 4 5 Front of queue Rear of queue 01 Add an item

8. Array Implementation II DDD 0 1 2 3 4 5 Front of queue Rear of queue 02 Add an item

9. Array Implementation II DDDD 0 1 2 3 4 5 Front of queue Rear of queue 03 Add an item

10. Array Implementation II DDDDD 0 1 2 3 4 5 Front of queue Rear of queue 04 Add an item

11. Array Implementation II DDDD 0 1 2 3 4 5 Front of queue Rear of queue 14 Pop an item

12. Array Implementation II After 5 additions and 2 removals DD 0 1 2 3 4 5 Front of queue Rear of queue 24 D Cells 0 and 1 are unavailable and wasted Adjustments must be made when rear or front reaches capacity

13. Array Implementation III Same as previous version, but manages a circular array When either front or rear reaches the last cell, the pointer is reset to 0 to wrap around the array on the next removal or addition

14. Array Implementation III After 6 additions and 2 removals DD 0 1 2 3 4 5 Front of queue Rear of queue 25 DD Before insertion

15. Array Implementation III Reset the rear pointer to 0 to wrap around the array DD 0 1 2 3 4 5 Front of queue Rear of queue 20 D After insertion D D

16. from arrays import Array from abstractqueue import AbstractQueue class ArrayQueue(AbstractQueue): DEFAULT_CAPACITY = 10 def __init__(self, sourceCollection = None) self._front = self._rear = -1 self._items = Array(ArrayQueue.DEFAULT_CAPACITY) AbstractQueue.__init__(self, sourceCollection) The Array Implementation: __init__ front rear 0 1 2 3 4 5 6 7 8 9 items

17. from arrays import Array from abstractqueue import AbstractQueue class ArrayQueue(AbstractQueue):... def add(self, item): # Resize array if full if self.isEmpty(): self._front = self._rear = 0 elif self._rear == len(self._items) - 1: self._rear = 0 else: self._rear += 1 self._items[self._rear] = item self._size += 1 The Array Implementation: add 0 front rear 4 DDDDD 0 1 2 3 4 5 6 7 8 9 items

18. The Array Implementation: add 1 front rear 9 DDDDDDDDD 0 1 2 3 4 5 6 7 8 9 items from arrays import Array from abstractqueue import AbstractQueue class ArrayQueue(AbstractQueue):... def add(self, item): # Resize array if full if self.isEmpty(): self._front = self._rear = 0 elif self._rear == len(self._items) - 1: self._rear = 0 else: self._rear += 1 self._items[self._rear] = item self._size += 1

19. from arrays import Array from abstractqueue import AbstractQueue class ArrayQueue(AbstractQueue):... def pop(self): # Check precondition here data = self._items[self._front] self._size -= 1 if self.isEmpty(): self._front = self._rear = -1 elif self._front == len(self._items) - 1: self._front = 0 else: self._front += 1 # Resize array here if necessary return data The Array Implementation: pop 1 front rear 0 DDDDDDDDDD 0 1 2 3 4 5 6 7 8 9 items

20. The Array Implementation: pop 2 front rear 0 DDDDDDDDD 0 1 2 3 4 5 6 7 8 9 items from arrays import Array from abstractqueue import AbstractQueue class ArrayQueue(AbstractQueue):... def pop(self): # Check precondition here data = self._items[self._front] self._size -= 1 if self.isEmpty(): self._front = self._rear = -1 elif self._front == len(self._items) - 1: self._front = 0 else: self._front += 1 # Resize array here if necessary return data

21. For Wednesday Modeling and Simulation