INFSCI 0015 Data Structures Lecture 18: Queues

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INFSCI 0015 - Data StructuresLecture 18 Queues

• Peter Brusilovsky
• http//www2.sis.pitt.edu/peterb/0015-011/

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Figure 5-1
Queues in Our Life

• A queue is a FIFO structure Fast In First Out

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Basic Operations with Queues

• Enqueue - Add an item to the end of queue
• Overflow
• Dequeue - Remove an item from the front
• Could be empty
• Queue Front - Who is first?
• Could be empty
• Queue End - Who is last?
• Could be empty

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Figure 5-2
Enqueue
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Figure 5-3
Dequeue
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Figure 5-4
Queue Front
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Figure 5-5
Queue Rear
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Figure 5-6
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Figure 5-15
Queue implmentation with arrays
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Figure 5-16
Queue will overgrow the array

• Should we use VERY L A R G E ARRAYS?

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(No Transcript)
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Array implementation of queues
queueAry
maxsize
count
front
rear
7
4
1
5
front
rear
11
37
22
15
3
-7
1
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Storage for a queue

• struct intqueue
• int queueAry
• int maxSize
• int count
• int front
• int rear

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Create Queue

• struct intqueue createQueue (int maxSize)
• struct intqueue queue
• queue (struct intqueue ) malloc (sizeof
  (struct intqueue))
• if (queue ! NULL)
• / head structure created. Now allocate queue /
• queue-gtqueueAry (int )malloc(maxSize
  sizeof(int))
• queue-gtfront -1
• queue-gtrear -1
• queue-gtcount 0
• queue-gtmaxSize maxSize
• return queue
• / createQueue /

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Enqueue

• int enqueue (struct intqueue queue, int
  datain)
• if (queue-gtcount queue-gtmaxSize)
• return 0 / queue is full /
• (queue-gtrear)
• if (queue-gtrear queue-gtmaxSize)
• / Queue wraps to element 0 /
• queue-gtrear 0
• queue-gtqueueAryqueue-gtrear datain
• if (queue-gtcount 0)
• / Inserting into null queue /
• queue-gtfront 0
• queue-gtcount 1
• / if /
• else (queue-gtcount)
• return 1
• / enqueue /

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Dequeue

• int dequeue (struct intqueue queue, int
  dataOutPtr)
• if (!queue-gtcount)
• return 0
• dataOutPtr queue-gtqueueAryqueue-gtfront
• (queue-gtfront)
• if (queue-gtfront queue-gtmaxSize)
• / queue front has wrapped to element 0 /
• queue-gtfront 0
• if (queue-gtcount 1)
• / Deleted only item in queue /
• queue-gtrear queue-gtfront -1
• (queue-gtcount)--
• return 1
• / dequeue /

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queueFront

• int queueFront (struct intqueue queue,
• int dataOutPtr)
• if (!queue-gtcount)
• return 0
• else
• dataOutPtr queue-gtqueueAryqueue-gtfront
• return 1
• / else /
• / queueFront /

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queueRear

• int queueRear (struct intqueue queue,
• int dataOutPtr)
• if (!queue-gtcount)
• return 0
• else
• dataOutPtr queue-gtqueueAryqueue-gtrear
• return 1
• / else /
• / queueRear /

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emptyQueue and fullQueue

• int emptyQueue (struct intqueue queue)
• / Statements /
• return (queue-gtcount 0)
• / emptyQueue /
• int fullQueue (struct intqueue queue )
• return ( queue-gtcount queue-gtmaxSize)
• / fullQueue /

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destroyQueue

• struct intqueue destroyQueue (struct intqueue
  queue)
• / Statements /
• if (queue)
• free (queue-gtqueueAry)
• free (queue)
• / if /
• return NULL
• / destroyQueue /

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Example 18.1 Driver (1)

• include "QueueIntAr.h"
• include ltstdio.hgt
• void main()
• struct intqueue myqueue
• int i
• int value
• myqueue createQueue(5)
• while(1)
• printf("Queue manipulation\n\n")
• printf("1. Enqueue\t")
• printf("2. Dequeue\n")
• printf("3. Front\t")
• printf("4. Rear\n")
• printf("5. Full\t\t")
• printf("6. Empty\n")
• printf("7. End\n\n")

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Example 18.1 Driver (2)

• if(i 1)
• printf("Value? ")
• scanf("d",value)
• if(enqueue(myqueue, value))
• printf("Value d enqueued in\n", value)
• else
• printf("Queue is Full\n")
• else if (i 2)
• if(dequeue(myqueue, value))
• printf("Value d dequeued\n", value)
• else
• printf("Queue is Empty\n")
• else if (i 3)
• if (queueFront(myqueue, value))
• printf("Front Value is d\n", value)
• else
• printf("Queue is Empty\n")

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Example 18.1 Driver (3)

• else if (i 4)
• if (queueRear(myqueue, value))
• printf("End Value is d\n", value)
• else
• printf("Queue is Empty\n")
• else if (i 5)
• if (fullQueue(myqueue))
• printf("Queue is Full\n")
• else
• printf("Queue is Not Full\n")
• else if (i 6)
• if (emptyQueue(myqueue))
• printf("Queue is Empty\n")
• else
• printf("Queue is Not Empty\n")
• else
• destroyQueue(myqueue)
• return

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Example 19.2 Using Queues

• include "QueueIntAr.h"
• include ltstdio.hgt
• define N 10 / dimension of the array /
• void readarray(int ar, int n)
• void filter_array(int ar, int n)
• void printarray(int ar, int n)
• void main()
• int testarrayN / elements from ar0 to
  arN-1 /
• readarray(testarray, N)
• printf("Before filtering, numbers are ")
• printarray(testarray, N)
• filter_array(testarray, N)
• printf("After filtering, numbers are ")
• printarray(testarray, N)

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Example 19.2

• / filter integer array by removing negative
  numbers /
• void filter_array(int ar, int n)
• int i
• struct intqueue q
• q createQueue(N)
• for (i 0 i lt n i)
• if (ari gt 0)
• enqueue(q, ari)
• for (i 0 i lt n i)
• if (emptyQueue(q))
• ari 0
• else dequeue(q, ari)
• destroyQueue(q)
• return

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How do we work with queues?

• We know a conceptual idea of a queue
• We can use a set of operations
• We do not really care how it is implemented

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Abstract Data Types

• We know a conceptual idea of a data type
• We can work with objects of this type by using a
  set of predefined operations
• We do not really care (and know) how it is
  implemented
• Implementation can be changed - not even a bit in
  the application program will be changed
• Different implementations are possible (could be
  used in different cases)