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Computer Science 112
Fundamentals of Programming IIArray-Based Queues
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 = -1self._items = Array(ArrayQueue.DEFAULT_CAPACITY)
Array Implementation I
D D D D
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!)
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 = -1self._front = -1self._items = Array(ArrayQueue.DEFAULT_CAPACITY)
Array Implementation II: Initial State
0 1 2 3 4 5
Front of queue Rear of queue-1 -1
Array Implementation II
D
0 1 2 3 4 5
Front of queue Rear of queue0 0
Add an item
Array Implementation II
D D
0 1 2 3 4 5
Front of queue Rear of queue0 1
Add an item
Array Implementation II
D D D
0 1 2 3 4 5
Front of queue Rear of queue0 2
Add an item
Array Implementation II
D D D D
0 1 2 3 4 5
Front of queue Rear of queue0 3
Add an item
Array Implementation II
D D D D D
0 1 2 3 4 5
Front of queue Rear of queue0 4
Add an item
Array Implementation II
D D D D
0 1 2 3 4 5
Front of queue Rear of queue1 4
Pop an item
Array Implementation II
• After 5 additions and 2 removals
D D
0 1 2 3 4 5
Front of queue Rear of queue2 4
D
• Cells 0 and 1 are unavailable and wasted
• Adjustments must be made when rear or front reaches capacity
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
Array Implementation III
• After 6 additions and 2 removals
D D
0 1 2 3 4 5
Front of queue Rear of queue2 5
D DBeforeinsertion
Array Implementation III
• Reset the rear pointer to 0 to wrap around the array
D D
0 1 2 3 4 5
Front of queue Rear of queue2 0
DAfterinsertion D D
from arrays import Arrayfrom 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__
-1frontrear -1 0 1 2 3 4 5 6 7 8 9
items
from arrays import Arrayfrom 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
0frontrear 4
D D D D D0 1 2 3 4 5 6 7 8 9
items
The Array Implementation: add
1frontrear 9
D D D D D D D D D0 1 2 3 4 5 6 7 8 9
items
from arrays import Arrayfrom 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
from arrays import Arrayfrom 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
1frontrear 0
D D D D D D D D D D0 1 2 3 4 5 6 7 8 9
items
The Array Implementation: pop
2frontrear 0
D D D D D D D D D0 1 2 3 4 5 6 7 8 9
items
from arrays import Arrayfrom 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
For Wednesday
Modeling and Simulation
