Queue - Data Structure - Notes

Definition of Queue
An ordered collection of items from which items may be deleted from one end called the front and into which items
may be inserted from other end called rear is known as Queue.
It is a linear data structure.
It is called First In First Out (FIFO) list. Since in queue, first element will be the first element out.
1
Deletion
Front Rear
Insertion
(a) Insertion and deletion of elements in Queue

Queue (Example)
2
Front Rear
(b) Queue
A B C D
Front Rear
A B C D E
(c) Front and near pointer after insertion
Front Rear
(d) Front and rear pointer after deletion
B C D E
Element E will be added at the rear end.Queue with four elements A, B, C and D.
A is at front and D is at rear.
Element can be deleted only from the front.
Thus, A will be the first to be removed from the queue.

Difference between Stack & Queue.
3
SR Stack Queue
1 Stack is a LIFO (Last in first out)
data structure.
Queue is a FIFO (first in first out)
data structure.
2 In a stack, all insertion and
deletions are permitted only at one
end of stack called top of stack.
In a queue items are inserted at
one end called the rear and
deleted from the other end called
the front.
3 Stack is widely used in recursion. Queue is more useful in many of
the real life applications.
4 Example: A stack of plates, a stack
of coin etc.
Example: A queue of people
waiting to purchase tickets, in a
computer system process waiting
for line printer etc.

Operations on a Queue.
1. Create: Creates empty queue.
2. Insert: Add new element at rear.
3. Delete: Delete element at front.
4. Isempty: Checks queue is empty or not.
5. Isfull: Checks queue is full or not.
4

Algorithm to insert and delete element in the Queue.
1. Create an empty queue by giving name and
2. Initially assign Rear = -1, front = -1.
3. If choice == Insert then
if Rear == max-1 then
print “queue is full”
else
Rear = Rear +1
q [Rear] = element
4. If choice == Delete then
If front == -1 then
print “queue is empty”
else
front = front +1
5. Stop
5

Queue as an abstract data type
(1) Initialize the queue to be empty.
(2) Check if the queue is empty or not.
(3) Check if the queue is full or not.
(4) Insert a new element in the queue if it is not full.
(5) Retrieve the value of first element of the queue, if the queue is not empty.
(6) Delete the first element from the queue if it is not empty.
Queue abstract data type = Queue elements + Queue operations.
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Representation of an Queue as an Array
7
Deletion
Front Rear
Deletion
(a) Queue with array
Front = -1 Rear = -1
0 1 2 3 4
0 1 2 3 4 0 1 2 3 4
0 1 2 3 4
(b) Empty Queue
(c) Queue after adding an element U (d) Queue after adding 5 element
Front = 0
Rear = 0
Front = 0 Rear = 4
Front = 1
U V X Y ZU
V W X Y
Rear = 4
(E) Queue after adding U element

Types of Queue
1. Linear Queue.
2. Circular Queue.
3. Double ended Queue.
4. Priority Queue.
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Circular Queue full (overflow)
9
60
30
60
50
40
20
10
Q[5]
Q[1]
Q[4]
Q[0]
Q[3] Q[2]
Circular queue full

10
Circular Queue Empty (underflow)
60
Q[5]
Q[1]
Q[4]
Q[0]
Q[3] Q[2]
Circular queue empty

Circular Queue Diagram & Algorithm (to insert)
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Algorithm to insert an element into circular Queue.
Step 1: If front = 0 and rear = MAX -1 then
write ‘circular queue is overflow’
else if front = -1 and rear =-1 then set
front = rear = 0
else if rear = MAX -1 and front !=0
set rear = 0
else
set rear = rear +1
Step 2: Set queue[rear] = val
Step 3: Exit
60 60
Q[0]
Q[1]
Q[4]
Q[2]Q[3]
Q[4]
Q[3] Q[2]
Q[1]
Q[0]
Rear
Rear
Front Front
20
5
10
5
20
10
30
(a) (b)
• If a new element is to be inserted in the queue,
the position of the element to be inserted will be
calculated by the relation as follows:
Rear = (rear +1) % MAXSIZE
• If we add an element 30 to the queue. The rear is calculated as follows:
rear = (rear +1) % MAXSIZE
=(2+1) % 5
=3

Circular Queue Diagram & Algorithm (to delete)
12
Algorithm to delete an element into circular Queue.
Step 1: If front = -1 the write ‘queue underflow’
Step 2: Set val = queue[front]
Step 3: if front = rear
set front = rear = -1
else
if front = MAX – 1
set front = 0
else
set front = front = -1
Step 4: Exit
60
Q[0]
Q[1]
Q[2]Q[3]
Q[4]
Rear
Front
20
5
10
(c)
The front is calculated as follow:
front = (0 + 1) % 5 = 1
4

Double ended Queue & Type
13
A deque is a list in which the elements can be inserted or deleted at either end.
In a dequeue two pointers are maintained LEFT and RIGHT which point to either end of the dequeue.
Front Rear
Insertion
Deletion Insertion
Deletion
Left = 3 Right = 7
0 1 2 3 4 5 6 7 8 9
29 37 45 54 63
(a) Double-ended queue
(1) Input Restricted dequeue: In this dequeue, insertion can be done only at one of the dequeue while deletions can
be done from both ends.
Insertion
Deletion
Deletion
(b) Input restricted dequeue

Double Ended Queue Algorithm to Insert at the Rear & Front End
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Algorithm to insert at the Rear End.
Step 1: if (Rear = (max -1)) then
print “Queue is full”
Step 2: set Rear = Rear +1
Step 3: Queue[Rear] = element
Step 4: Exit
Algorithm to insert at the Front End.
Step 1: if front == 0 then
print “Queue is full”
rerun
Step 2: front = front -1
Step 3: Queue[front] = element
Step 4: Exit

Double Ended Queue Algorithm to Delete from the Rear & Front End
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Algorithm to delete at the Rear End.
Step 1: if (front == rear) then
print “Queue is empty”
return
Step 2: Rear = Rear -1
Step 3: element = Queue[Rear]
Step 4: Exit
Algorithm to insert at the Front End.
Step 1: if (front == Rear) then
print “Queue is empty”
rerun
Step 2: front = front +1
Step 3: element = Queue[front]
Step 4: Exit

Priority Queue
In a priority queue ( i ) each element is assign a priority ( ii ) The elements are processed according to
• Highest priority element is processed before lower priority element.
• Two elements of same priority are processed according to the order of insertion.
A priority queue contains a collection of items that are waiting to be processed. In queue, items are removed for
processing in the same order in which they are added to the queue.
In a priority queue, however, each item, has a priority, and when its time to select an item, the item with the highest
priority is the one that is order.
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Types of Priority Queue
1. Ascending Priority Queue
2. Descending Priority Queue

Application of Queue
• Queue are used in computer for scheduling of resources to application . These
resources are CPU, Printer etc.
• In batch Programming, multiple jobs are combined into a batch and the first program is
executed first, the second is executed next and so on in a sequential manners.
• A queue is used in break first search traversal of a graph & level wise traversal of tree.
• Computer simulation, Airport simulation.
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Queue - Data Structure - Notes

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Queue - Data Structure - Notes