6.queue

Data Structure and
Algorithm (CS 102)
Ashok K Turuk

Queue
• A queue is a linear list of elements in
which
– deletion can take place only at one end
called Front, and
– Insertion takes place at one end called
Rear
• Queues are also known as First-In-
First-Out (FIFO) list

Queue
• Queue are represented in two-ways
– Linear Array
– One-way Linked List

Array representation of Queue
• A queue is maintained by a
–linear array QUEUE
– Two pointer variable
• FRONT : Containing the location of the front
element of the queue
• REAR : Containing
• FRONT == NULL indicates that the
queue is empty

Queue
AA BB CC DD …
1 2 3 4 5 6 7 N
FRONT: 2
REAR: 4
BB CC DD …
1 2 3 4 5 6 7 N
Whenever an element is deleted from the
queue, the value of FRONT is increased by 1
FRONT = FRONT + 1
FRONT: 1
REAR: 4
Delete an element

Queue
BB CC DD …
1 2 3 4 5 6 7 N
FRONT: 2
REAR: 4
Insert an element
BB CC DD EE …
1 2 3 4 5 6 7 N
FRONT: 2
REAR: 5
Whenever an element is inserted into the
queue, the value of REAR is increased by 1
REAR = REAR + 1

Queue
• REAR = N and Insert an element into
queue
XX … ZZ
1 2 3 4 5 6 7 N
FRONT: 7
REAR: N
Move the entire queue to the beginning of the
array
Change the FRONT and REAR accordingly
Insert the element
This procedure is too expensive

Queue
• Queue is assumed to be circular
• QUEUE[1] comes after QUEUE[N]
• Instead of increasing REAR to N +1, we
reset REAR = 1 and then assign
QUEUE[REAR] = ITEM

Queue
• FRONT = N and an element of QUEUE
is Deleted
XX … ZZ
1 2 3 4 5 6 7 N
FRONT: N
REAR:
We reset FRONT = 1, instead of increasing
FRONT to N + 1

Queue
• QUEUE contain one element
FRONT = REAR  NULL
XX …
1 2 3 4 5 6 7 N
FRONT: 7
REAR: 7
FRONT = NULL and REAR = NULL

Algorithm to Insert in Q
[1] If FRONT = 1 and REAR = N or if FRONT =
REAR + 1 then Print: Overflow and Exit
[2] If FRONT = NULL then
Set FRONT = 1 and REAR = 1
Else If REAR = N then
Set REAR = 1
Else
Set REAR = REAR + 1
[3] Set QUEUE[REAR] = ITEM
[4] Exit

Queue
AA BB CC DD EE FF XX … ZZ
1 2 3 4 5 6 7 N
FRONT: 7
REAR: 6
FRONT = REAR + 1 [FULL QUEUE]

Algorithm to Delete from Q
[1] If FRONT = NULL then Print: Underflow and
Exit
[2] Set ITEM = QUEUE[FRONT]
[3] If FRONT = REAR then
Set FRONT = NULL and REAR = NULL
Else If FRONT = N then
Set FRONT = 1
Else
Set FRONT = FRONT + 1
[4] Exit

Linked List Representation of Queue
• A linked queue is a queue implemented
as linked list with two pointer variable
FRONT and REAR pointing to the nodes
which is in the FRONT and REAR of the
queue

Linked List Representation of
Queue
CC XAABB
FRONT
15
REAR

Insertion in a Queue
AA XCCBB
FRONT
16
REAR
DD
NEW

Insertion in a Queue
AA XCCBB
FRONT
17
REAR
DD

Delete from a Queue
CC XAABB
FRONT
18
REAR

Delete from a Queue
XAABB
FRONT
19
REAR

Linked Queue
• No need to check for overflow condition
during insertion
• No need to view it as circular for
efficient management of space

Insertion
[1] NEW -> INFO = ITEM
NEW -> LINK = NULL
[2] If (FRONT = NULL) then
FRONT = REAR = NULL
else
Set REAR -> LINK = NEW
REAR = NEW
[3] Exit

Deletion
[1] If (FRONT = NULL) then
Print: Underflow, and Exit
[2] FRONT = FRONT -> LINK
[3] Exit

Deque
• A deque is a linear list in which
elements can be added or removed at
either end but not in the middle
• Deque is implemented by a circular
array DEQUE with pointers LEFT and
RIGHT which points to the two end of
the deque

Deque
• LEFT = NULL indicate deque is empty
AA BB CC DD
1 2 3 4 5 6 7 8
LEFT: 4
RIGHT: 7
YY ZZ WW XX
1 2 3 4 5 6 7 8
LEFT: 4
RIGHT: 7

Variation of deque
• There are two variation of deque
[1] Input-restricted queue: Deque which
allows insertions at only one end of the list
but allows deletion at both ends of the list
[2] Output-restricted queue: Deque which
allows deletion at only one end of the list
but allows insertion at both ends of the
list

Deque
A C D
1 2 3 4 5 6
LEFT: 2
RIGHT: 4
F is added to the right
A C D F
1 2 3 4 5 6
LEFT: 2
RIGHT: 5

Deque
A C D F
1 2 3 4 5 6
LEFT: 2
RIGHT: 5
Two Letters on right is deleted
A C
1 2 3 4 5 6
LEFT: 2
RIGHT: 3

Deque
A C
1 2 3 4 5 6
LEFT: 2
RIGHT: 3
K, L and M are added to the Left
K A C M L
1 2 3 4 5 6
LEFT: 5
RIGHT: 3

Priority Queue
• A priority queue is a collection of elements
such that each elements has been assigned
a priority and such that the order in which
elements are deleted and processed comes
from the following rules:
[1] Elements of higher priority is processed
before any elements of lower priority
[2] Two elements with the same priority are
processed according to the order in which
they were added to the queue

Priority Queue
• There are different ways a priority
queue can be represented such as
[1] One-way List
[2] Multiple queue

One-Way List Representation of
a Priority Queue
[1] Each node in the list will contain three
items of information: an information
field INFO, a priority number PRN,
and a link number LINK
[2] A node X precedes a node Y in the list
(a) when X has higher priority than Y or
(b) when both have same priority but X
was added to the list before Y

Insertion and Deletion
• Deletion : Delete the first node in the
list.
• Insertion: Find the location of Insertion
Add an ITEM with priority number N
[a] Traverse the list until finding a node X
whose priority exceeds N. Insert ITEM
in front of node X
[b] If no such node is found, insert ITEM
as the last element of the list

Array representation of Priority
Queue
• Separate queue for each level of
priority
• Each queue will appear in its own
circular array and must have its own
pair of pointers, FRONT and REAR
• If each queue is given the same amount
space then a 2D queue can be used

1 2 3 4 5
1 AA
2 BB CC DD
3
4 FF DD EE
5 GG
1 2
2 1
3 0
4 5
5 4
2
3
0
1
4
FRONT REAR QUEUE

Deletion Algorithm [outline]
[1] Find the smallest K such that
FRONT[K]  NULL
[2] Delete and process the front element
in row K of QUEUE
[3] Exit

Insertion Algorithm [outline]
[1] Insert ITEM as the rear element in
row M of QUEUE
[2] Exit

6.queue

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6.queue