Basics of queue ADT with array/linked list implementation, operations (enqueue & dequeue), and applications.

1. VELAMMAL ENGINEERING COLLEGE
An Autonomous Institution, Affiliated to Anna University Chennai, & Approved by AICTE Delhi
Department of Computer Science and
Engineering
UNIT 2 -QUEUE

2. Queue ADT
Mrs. G. Sumathi
Assistant Professor
Velammal Engineering College
Chennai
VELAMMAL ENGINEERING COLLEGE
An Autonomous Institution, Affiliated to Anna University Chennai, & Approved by AICTE Delhi

3. Dept of CSE, Velammal Engineering College,
Chennai
3
Agenda
• Queue ADT
• Array implementation of queue
• Linked list implementation of queue
• Types
– Circular Queue
– Double Ended Queue
– Priority Queue
• Applications of Queue
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4. Dept of CSE, Velammal Engineering College,
Chennai
4
Queue ADT
• Linear data structure
• Insertion occurs only at one end called rear
• Deletion occurs only at one end called front
• Element that is inserted first is deleted (FIFO
principle)
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5. Dept of CSE, Velammal Engineering College,
Chennai
5
Operations in queue:
• Enqueue: Inserting an element into the queue (at
rear)
• Dequeue: Deleting an element from the queue (at
front)
Exceptional Conditions:
• Overflow: Attempting to insert an element into
queue when it is full
• Underflow: Attempting to delete an element from
queue when it is empty
Queue ADT
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6. Dept of CSE, Velammal Engineering College,
Chennai
6
Array Implementation of Queue
Enqueue operation
Void Enqueue (int num) //Let num = 12 and max=5
{
if (rear==max-1)
print(“Queue Overlfow”);
else if (front == -1 && rear == -1)
{
front = rear = 0;
Q[rear] = num;
}
else
{
rear++;
Q[rear] = num;
}
}
12 25 18
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7. Dept of CSE, Velammal Engineering College,
Chennai
7
Dequeue Operation
Void Dequeue()
{
int val;
if (front == -1 || front > rear)
print(“Queue Underflow”);
else
{
val= Q[front];
front++;
}
}
10 20 30
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8. Dept of CSE, Velammal Engineering College,
Chennai
8
Display Operation
void Display ()
{
int i;
if (front == -1 || front > rear)
print (“Queue Underflow”);
else
{
for(i=front; i<=rear; i++)
print(Q[i]);
}
}
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9. Dept of CSE, Velammal Engineering College,
Chennai
9
Linked List Implementation of Queue
Enqueue operation
void Enqueue (int num) // Insert at end of linked list
{
n=(struct node *) malloc (sizeof (struct node));
ndata=num;
nnext=NULL;
if(rear == NULL)
front = rear= n;
else
{
rearnext = n;
rear = n;
}
}
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10. Dept of CSE, Velammal Engineering College,
Chennai
10
Dequeue Operation
Void Dequeue() // Delete at beginning of linked list
{
struct node *t;
if (front == NULL)
print(“Underflow”);
else
{
t = front;
front = front  next;
free(t);
}
}
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11. Dept of CSE, Velammal Engineering College,
Chennai
11
Circular Queue
• Type of queue
• Last position of the queue is connected to the first
position in a circular fashion
• Overcome the disadvantage of wastage of front end
memory space after performing dequeue operation
in linear queue
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12. Dept of CSE, Velammal Engineering College,
Chennai
12
• In linear queue, after performing dequeue, the
memory space at the front end remains
unused
• This situation does not occur in circular queue
Need for Circular Queue
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13. Dept of CSE, Velammal Engineering College,
Chennai
13
Enqueue Operation
void Cir_Enqueue (int num) // Let max=6
{
if((front == 0 && rear == max-1) || (rear ==(front-1)))
print(“Queue Overflow”);
else if (front == -1 && rear == -1)
{ front = rear = 0;
Q[rear] = num;
}
else if (rear == max-1 && front != 0)
{ rear = 0;
Q[rear] = num;
}
else
{ rear ++;
Q[rear] = num;
}
}
30
40
50 60
70
50
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14. Dept of CSE, Velammal Engineering College,
Chennai
14
Dequeue Operation
Void Cir_Dequeue()
{
int val;
if (front ==-1)
print (“Queue Underflow”);
val = Q[front];
if (front == rear)
front = rear = – 1;
else if (front == max – 1)
front = 0;
else
front ++;
}
20
40
20
30
20
30
40
front
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15. Dept of CSE, Velammal Engineering College,
Chennai
15
Double Ended Queue (DEQUE)
DEQUE is of two types
• Input restricted DEQUE
– Insertion at only one end
– Deletion at both ends
• Output restricted DEQUE
– Deletion at only one end
– Insertion at both ends
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16. Dept of CSE, Velammal Engineering College,
Chennai
16
Priority Queue
• Every item has a priority associated with it
• Element with higher priority is served first
• If two elements have same priority then they served
according to the order in the queue
• Generally, the value of the element itself is
considered as higher priority
• Applications:
– CPU scheduling
– Graph algorithms like Dijkstra’s shortest path, Prim’s
minimum spanning tree, etc,.
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17. Dept of CSE, Velammal Engineering College,
Chennai
17
Applications of Queue
• Data getting transferred between the IO Buffers
(Input Output Buffers).
• CPU scheduling and Disk scheduling.
• Managing shared resources between various
processes.
• Job scheduling
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18. Dept of CSE, Velammal Engineering College,
Chennai
18
• Traffic light functioning is the best example
for circular queues . The colors in the traffic light
follow a circular pattern.
• In page replacement algorithms, a circular list of
pages is maintained and when a page needs to be
replaced, the page in the front of the queue will be
chosen.
Applications of Circular Queue
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19. Dept of CSE, Velammal Engineering College,
Chennai
19
Queries??
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20. Dept of CSE, Velammal Engineering College,
Chennai
20
Thank You !!!
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