Queues

1
Queues
Chapter 5
2
Linear Lists

• Operations are
• Insertion
• Deletion
• Retrieval
• Traversal (exception for restristed lists).

Figure 3-2
3
Queue
A queue is a linear list. Data can be inserted at
one end (rear) and deleted from the other end
(front).
First In First Out - FIFO
Figure 5-1
4
Queue Operations

• There are four basic queue operations.
• Data can be inserted at the rear and processed
  from the front.
• Enqueue inserts an element at the rear of the
  queue.
• Dequeue deletes an element at the front of the
  queue.
• Queue Front examines the element at the front of
  the queue.
• Queue Rear examines the element at the rear of
  the queue.

5
Queue Operations
Figure 5-2
6
Queue Operations
Figure 5-3
7
Queue Operations
If there are no data in the queue, then the queue
is in an underflow state.
Figure 5-4
8
Queue Operations
If there are no data in the queue, then the queue
is in an underflow state.
Figure 5-5
9
Figure 5-6
10
Queue Linked List Design
Figure 5-7
11
Queue Data Structure
Figure 5-8
12
Figure 5-9, Part I
13
Queue Algorithms - Create Queue

• algorithm createQueue
• Allocates memory for a queue head node from
  dynamic memory and returns its address to the
  caller.
• Pre Nothing
• Post head has been allocated and initialized
• Return heads address if successful, null if
  memory owerflow.
• if (memory available)
• allocate (newPtr)
• newPtr?front null pointer
• newPtr?rear null pointer
• newPtr?count 0
• return newPtr
• else
• return null pointer
• end createQueue

14
Queue Algorithms - Enqueue
Figure 5-10
15
Queue Algorithms - Enqueue

• algorithm enqueue(val queue lthead pointergt, val
  item ltdataTypegt)
• This algorithm inserts data into queue.
• Pre queue has been create
• Post item data have been inserted
• Return boolean, true if successful, false if
  overflow.
• if (queue full)
• return false
• allocate (newPtr)
• newPtr?data item
• newPtr?next null pointer
• if (queue?count zero) //inserting into null queue
• queue?front newPtr
• else // insert data and adjust meta data
• queue?rear?next newPtr
• queue?rear newPtr
• queue?count queue?count 1
• return true
• end enqueue

16
Figure 5-9, Part II
17
Figure 5-11
18
Queue Algorithms - Dequeue

• algorithm dequeue(val queue lthead pointergt, ref
  item ltdataTypegt)
• This algorithm deletes a node from a queue.
• Pre queue has been create
• Post data at front of the queue returned to user
  through item and front element deleted and
  recycled.
• Return boolean, true if successful, false if
  overflow.
• if (queue?count is 0)
• return false
• item queue?front?data
• deleteLoc queue?front
• if (queue?count is 1) // deleting only item in
  queue
• queue?rear null pointer
• queue?front queue?front?next
• queue?count queue?count 1
• recycle (deleteLoc)
• return true
• end dequeue

19
Queuing Theory

• A single server queue can provide service to
  only one customer at a time.
• Multiserver queues, can provide service to many
  customers at a time.
• A customer is any person or thing needing
  service.
• The service is any activity needed to accomplish
  the required result.
• The rate at which customers arrive in the queue
  for service is known as the
• arrival rate.
• Service time is the avarage time required to
  complete the processing of a
• customer request.

Figure 5-12
20
Queuing Theory

• Queuing theory attempts to predict some patterns,
  such as
• Queuing time, which is defined the avarage
  length of time customers wait in the queue,
• Avarage and maximum size of queue,
• Response time is an important statistical tool
  for online computer systems
• There are two factors that affect the performance
  of queues
• Arrival rate
• Service time

21
Queue Data Structures
Figure 5-13
22
Queues Array Implementation
Figure 5-15
23
Queues Array Implementation
Figure 5-16
24

• Store the address of the queue array.
• Store the max. number of elements in array.

Figure 5-17
25
Exercise

• Imagine the contents of queue Q1 and Q2 are as
  shown. What would be the content of Q3 after the
  following code is executed? The queue contents
  are shown front (left) to rear (right).
• Q1 42 30 41 31 19 20 25 14 10
  11 12 15
• Q2 1 4 5 4 10 13
• 1 Q3 createQueue
• 2 count 0
• 3 loop (not empty Q1 and not empty Q2)
• 1 count count 1
• 2 dequeue (Q1, x)
• 3 dequeue (Q2, y)
• 4 if (y equal count)
• 1 enqueue (Q3, x)

26
Exercise

• Imagine the contents of queue Q1 and Q2 are as
  shown. What would be the content of Q3 after the
  following code is executed? The queue contents
  are shown front (left) to rear (right).
• Q1 42 30 41 31 19 20 25 14 10
  11 12 15
• Q2 1 4 5 4 10 13
• 1 Q3 createQueue
• 2 count 0
• 3 loop (not empty Q1 and not empty Q2)
• 1 count count 1
• 2 dequeue (Q1, x)
• 3 dequeue (Q2, y)
• 4 if (y equal count)
• 1 enqueue (Q3, x)

Step count Q1 Q2 Q3 x y
1 0,1 42,30,.. 1,4,5,4,.. 42 42 1
2 2 30,41,.. 4,5,4,..   30 4
3 3 42,31,19,.. 5,4,10,13   42 5
4 4 31,19,20,.. 4,10,13 42,31 31 4
5 5 19,20,25,.. 10,13   19 10
6 6 20,25,14,.. 13   20 13
7   25,14,10,.. NULL      
Q3 42, 31