The document discusses different implementations of queues using arrays and linked lists, as well as circular queues which avoid issues with arrays filling up by connecting the last element to the first. It covers common queue operations and describes how queues are widely used in applications like operating systems, databases, and simulations for tasks like job scheduling where elements need to be processed in a first-in, first-out order. Circular queues are presented as an efficient approach to implement queues using arrays without capacity issues.

1. Pune Vidyarthi Griha’s
COLLEGE OF ENGINEERING, NASHIK – 3.
“QUEUE”
By
Prof. Anand N. Gharu
(Assistant Professor)
PVGCOE Computer Dept.
01 August 2019
.

2. Introduction of Queue
➢ “A queue is an ordered list in which all insertions are
done at one end, called rear and deletions at another end
called front“
➢ Queue when implemented using arrays has some
drawbacks which can be avoided by circular queue
➢ Queue is used in many applications such as as simulation,
priority queue, job queueetc

3. Introduction of Queue
➢ One of the most common dataprocessing structures
➢ Frequently used in most of the system software's like
operating systems, Network and Database
implementations and in many more other areas
➢ Very useful in time-sharing and distributed computer
systems where many widely distributed users share the
system simultaneously

4. Array representation and
implementation of queue
➢ An array representation of queue require three entities :
1. An array to hold queue element
2. A variable to hold index of the front element
3. A variable to hold index of the rear element

5. Array representation and
implementation of queue

6. Array representation and
implementation of queue

7. Array representation and
implementation of queue

8. Operation on queue
implemented using array

9. Operation on queue
implemented using array

10. Operation on queue
implemented using array

11. Operation on queue
implemented using array

12. Operation on queue
implemented using array

13. Queue as an ADT
➢ Create : Create creates an empty queue, Q
➢ Add (i,Q) : Add adds the element i to the rear end of queue,
Q and returns the newqueue
➢ Delete (Q) : Delete takes out an element i from the
➢ front end of queue and returns the resulting queue
➢ Front(Q) : Front returns the element i which is at the front
position of queue
➢ Is_Empty_Q(Q) : Is_Empty_Q returns true if queue is
➢ empty otherwise returns false

14. Queue as an ADT

15. Queue as an ADT

16. Queue as an ADT

17. STACK VS QUEUE

18. Queue Example

19. Queue Example

20. Queue Example

21. Queue Example

22. Operation on queue implemented using
Linked list

23. Operation on queue implemented using
Linked list

24. Operation on queue implemented using
Linked list

25. Operation on queue implemented using
Linked list

26. Operation on queue implemented using
Linked list

27. Operation on queue implemented using
Linked list

28. Circular Queue

29. 6-29
Second Approach: Queue as a
Circular Array
• If we don't fix one end of the queue at
index 0, we won't have to shift elements
• Circular array is an array that conceptually
loops around on itself
o The last index is thought to “precede” index 0
o In an array whose last index is n, the location “before” index 0 is index n;
the location “after” index n is index 0
• Need to keep track of where the front as
well as the rear of the queue are at any
given time

30. 6-30
Conceptual Example of a Circular Queue
1
0
12
11
10
1
0
12
11
10
1
0
12
11
10
After 7 enqueues
front
rear
After 5
dequeues
front
rear
After 8 more enqueues
front
rear

31. 6-31
Implementation of a
Queue
rear
front
5
queue
count
8
3
0
1
2 3 4
5
6
7
8
9
10
n-1
n-2
n-3
. ..
cq

32. 6-32
A Queue Straddling the
End of a Circular Array
rear
front
4
queue
count
2
98
0
1
2 3 4
5
6
7
8
9
10
99
98
97
. ..
cq

33. 6-33
Circular Queue Drawn
Linearly
rear
front
4
queue
count
2
98
0 4321 96 97 98 99
…
Queue from previous slide
cq

34. 6-34
Circular Array
Implementation
• When an element is enqueued, the value of rear is
incremented
• But it must take into account the need to loop back
to index 0:
rear = (rear+1) % queue.length;
• Can this array implementation also reach capacity?

35. 6-35
Example: array of length 4
What happens?
rear
front
3
queue
count
1
2
0 321
rear
front
4
queue
count
2
2
0 321
Suppose we try to add one
more item to a queue
implemented by an array of
length 4
cq
cq
The queue is now full. How
can you tell?

36. 6-36
Need to expand
capacity…
rear
front
4
queue
count
2
2
0 321
rear
front
4
queue
count
2
2
0 321 4 765
We can’t just double the
size of the array and
copy values to the same
positions as before:
circular properties of the
queue will be lost
These locations
should be in use
cq
cq

37. 6-37
rear
front
4
queue
count
6
2
0 321 4 765
We could build the new array, and copy the queue elements into
contiguous locations beginning at location front:
cq

38. 6-38
rear
front
4
queue
count
4
0
0 321 4 765
Better: copy the queue elements in order to the beginning of the
new array
cq

39. 6-39
rear
front
5
queue
count
5
0
0 321 4 765
New element is added at rear = (rear+1) % queue.length
See expandCapacity() in CircularArrayQueue.java
cq

40. Circular Queue

41. Circular Queue

42. Circular Queue

43. Circular Queue

44. Circular Queue

45. DeQueue

46. DeQueue

47. DeQueue as an ADT

48. Implementation of DeQueue using linked list

49. C function for deque using circular array

50. C function for deque using circular array

51. Priority Queue

52. Priority Queue as an ADT

53. Priority Queue as an ADT

54. Multiple Queue using array

55. Application of Queue