Queue as data_structure

Queue www.eshikshak.co.in

Data Structure

Introduction
● It is linear data structure
● It is collection of items – List
● Queue means line of items waiting for
their turn
● Queue has two ends
○ Elements are added at one end.
○ Elements are removed from the other end.

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Queue
● Removal of data item is restricted at one
end known as FRONT
● Insertion of data item is restricted at other
end known as REAR
● The FRONT and REAR are used in
describing a linear list, when queue is
implemented
● First element inserted in list, will be the
first to be removed - FIFO
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Queue as FIFO
● The element inserted first will be removed
first from the Queue
● Thus, Queue is known as FIFO (First In-
First Out) or FCFS (First Come First
Serve)

● Examples of Queue
○ People waiting in Queue to purchase tickets at
railway station or cinema hall, where the first person
in the queue will be served first
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Representation of Queue
● It has two pointer varaibles
○ FRONT : Containing the location of the front
element of the queue
○ REAR : Containing the location of the rear
element of the queue
● When queue is empty
FRONT = -1 and REAR = -1

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Operations of Queue
● Insertion
○ Adding an element in queue will increased
value of REAR by 1
■ REAR = REAR + 1
● Removal
○ Removing an element from queue will
increased value of FRONT by 1
■ FRONT = FRONT + 1

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Queue FRONT = -1, REAR =
Empty -1
ADD(Q,10) FRONT = 0 ,REAR =
0
ADD(Q,20) FRONT = 0 ,REAR
=1
=2
=3
=4
Remove(Q) FRONT = 1 ,REAR
=4
=4
=4
ADD(Q, FRONT = 3 ,REAR
60) =1 www.eshikshak.co.in

Types of Queue
● Queue as Array
● Circular Queue
● Priority Queue
● Input Restricted Queue
● Output Restricted Queue
● Dqueue

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Queue as Array : Insert
Initially when the QUEUE is empty, set FRONT = NULL and REAR = 0

Step 1: start
Step2: [check for queue is over flow or not]
If (REAR >n) or (REAR==FRONT)
Print “queue is overflow”
else
go to step 3
Step 3: [enter the item]
QUEUE[REAR]=value
REAR=REAR+1
Step 4:[ check condition]
If(FRONT==null)
FRONT=0
Step 5:end

Queue as Array : Delete

Step 1: start
Step 2: [check for queue is under flow or not]
If front>N or front==Null
Print ”queue is underflow”
else
goto step 3
Step 3: [check condition]
If front==rear
Front==null
Rear=0
else
goto step 4
Step 4: [delete element]
Queue[front]=null
Step 5: front=front+1
Step 6: end

Circular Queue
● To solve this problem, queues implement
wrapping around. Such queues are called
Circular Queues.
● Both the front and the rear pointers wrap
around to the beginning of the array.
● It is also called as “Ring buffer”.
● Items can inserted and deleted from a queue in
O(1) time.

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Circular Queue
● When a new item is inserted at the rear, the
pointer to rear moves upwards.
● Similarly, when an item is deleted from the
queue the front arrow moves downwards.
● After a few insert and delete operations the rear
might reach the end of the queue and no more
items can be inserted although the items from
the front of the queue have been deleted and
there is space in the queue.

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QINSERT(Queue, N, FRONT, REAR, ITEM)
1 [Queue already filled ?]
if FRONT = 0 and REAR = N-1, or if FRONT=REAR + 1 then
write : Overflow and Return
2 [Find new value of REAR]
if FRONT=-1 then [Queue initially empty]
Set FRON T = 0 and REAR = 0
else if REAR = N-1 then
Set REAR = 1
else
Set REAR = REAR + 1
[End of If structure]
3 Set QUEUE[REAR] = ITEM [This inserts new element]
4 Return

QDELETE(Queue, N, FRONT, REAR,
ITEM)
1 [Queue already empty]
If FRONT = -1, then Write : Underflow and Return
2 Set ITEM = Queue[FRONT]
3 [Find new value of FRONT]
If FRONT = REAR, then [Queue has only one element to start]
Set FRONT = -1 and REAR = -1
Else if FRONT = N-1, then
Set FRONT = 0
Else
Set FRONT = FRONT + 1
[End of If Structure]
4 Return

Priority Queue

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Priority Queue
● It is collection of elements where elements are
stored according to the their priority levels
● Inserting and removing of elements from queue
is decided by the priority of the elements
● The two fundamental methods of a priority
queue P:
○ insertItem(k,e): Insert an element e with key k into P.
○ removeMin(): Return and remove from P an element
with the smallest key.

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Priority Queue
● When you want to determine the priority
for your assignments, you need a value
for each assignment, that you can
compare with each other.

● key: An object that is assigned to an
element as a specific attribute for that
element, which can be used to identify,
rank, or weight that element.

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Example: Student records
Any of the attributes, Student Name, Student Number, or Final Score
Student Name ID Final Score (out
can be used as keys. of 450)
Note: Keys may not be unique (Final Score).
Ashwin 09BCA08 310
Payal 09BCA80 311
Darshika 09BCA24 380
Nilkamal 09BCA75 400
Nikunj 09BCA74 440
Mori 09BCA102 400

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Rules to maintain a Priority Queue
● The elements with the higher priority will
be processed before any element of lower
priority
● If there are elements with the same
priority, then the element added first in
the queue would get processed

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Priority Queue Insert
PQInsert (M, Item)
Step 1 Find the Row M
Step2 [Reset the Rear Pointer]
If Rear[M] = N-1 then Rear[M] = 0
Else
Rear[M] = Rear[M]+1
Step 3 [Overflow]
If Front[M] = Rear[M] then Write (“This Priority Queue is full”)
Return
Step 4 [Insert Element]
Q[M] [Rear[M]] = then
Step 5 [Is Front Pointer Properly Set]
If Front[M] = -1 then Front[m] = 0
Return
Step 6 Exit

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Priority Queue Delete
PQDelete (K, Item)
Step 1 Initialize K = 0
Step 2 while (Front[K] = -1)
K = K+1
[To find the first non empty queue]
Step 3 [Delete Element]
Item = Q[K] [Front[K]
Step 4 [Queue Empty]
If Front[K] = N-1 then Front[K] = 0
Else
Front[K] = Front[K]+1
Return Item
Step 6 Exit

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Deque
● Deque stands for double-end queue

● A data structure in which elements can be
added or deleted at either the front or rear

● But no changes can be made in the list

● Deque is generalization of both stack and
queue

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Removing Element from Deque
● There are two variations of a deque.
These are
○ Input Restricted Deque
■ An input restricted deque restricts the insertion of
elements at one end only, but the deletion of
elements can perform at both the ends.

○ Output Restricted Deque
■ An output restricted queue, restricts the deletion
of elements at one end only, and allows insertion
to be done at both the ends of deque

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Possibilities
● The two possibilities that must be
considered while inserting or deleting
elements into the queue are :
○ When an attempt is made to insert an
element into a deque which is already full, an
overflow occurs.
○ When an attempt is made to delete an
element from a deque which is empty,
underflow occurs.

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Representation of Deque

Deletio Insertion
n
Insertion Deletio
n

Front Rea
r

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Inserting Element in Deque
InsertQatEnd(dq,7) 7

Front Rear

InsertQatBeg(dq,10) 7

Front Rear

InsertQatBeg(dq, 7
10)

Front Rear
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InsertQatBeg(dq,10) 10 7

Front Rear

InsertQatEnd(dq,4)
10 7 4

Front Rear

InsertQatBeg(dq,15) 15 10 7 4

Front Rear

InsertQatEnd(dq,55)
15 10 7 4 55

Front www.eshikshak.co.in Rear

Removing Element from Deque
delQBeg() 10 7 4 55

Front Rear

delQBeg() 7 4 55

Front Rear

InsertQatEnd(dq,23) 7 4 55 23

Front Rear

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Deque : Insert
● There are two variations of a deque.
These are
○ Input Restricted Deque
■ An input restricted deque restricts the insertion of
elements at one end only, but the deletion of
elements can perform at both the ends.

○ Output Restricted Deque
■ An output restricted queue, restricts the deletion
of elements at one end only, and allows insertion
to be done at both the ends of deque

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Input Restricted queue : Insert Element
Step 1: start
Step 2:[check condition for overflow]
If(rear==N-1 && front==0 or front=rear+1)
Print “over flow”
else
goto step 3
If(front==null)
front = 0 and rear=-1
else
goto step 4
Step 4: [check condition and value]
If (rear== N -1)
rear=0
Else
rear=rear+1
Step 5: [Add value]
dq[rear]=value
Step 6:end

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Input Restricted queue : Delete Beginning
Step 1: start
Step 2: [check condition for underflow]
If(rear==null & front==null)
Print”underflow”
else
goto step 3
Step 3: [delete]
dq[rear]=null
If(rear==front)
front=rear=null
else
rear--;
Step 5: end

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Input Restricted queue : Delete End
Step 1: start
Step2 : [check condition for under flow)
If(rear==null & front==null)
Print “under flow”
else
goto step 3
Step 3: [delete]
dq[front]=null
If(rear==front)
rear=-1 and front=null
else
front=front+1
Step 5: end

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Queue as data_structure

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