Introduction to C Programming CE00312-1

1. Introduction to C ProgrammingCE00312-1 Lecture 12 Circular Queue and Priority Queue Data Structures

2. Addition to a queue (enqueue) addq (item , queue)begin if rear = size then queuefull else begin          q[rear] = item increment rear endend Deletion from a queue (dequeue) deleteq (item , queue) beginif front = rear then queueempty else begin item = q[front]         front = front+1 endend Recap on Queues

3. 4 4 4 4 3 3 3 3 2 2 2 2 1 1 1 1 0 0 0 0 Queues e d c c c b a Front = 2 Rear = 5 Array size = 5 Front = 0 Rear = 0 Front = 0 Rear = 3 Front = 2 Rear = 3

4. Circular Array • Allows array to wrap round to the front • Array bounds no longer dictate empty or full • How do I define empty /Full • Underflow/Overflow • If pointer to front catches up with rear on dequeuing then underflow • If result of enqueing means rear pointer = front then overflow

5. 4 4 4 4 3 3 3 3 2 2 2 2 1 1 1 1 0 0 0 0 C Queue e e e d d c c g f f f Front = 2 Rear = 0 Front = 2 Rear = 1 Front = 4 Rear = 1 Front = 0 Rear = 2

6. Circular Queue • Front = rear is used to define both empty and full • Sacrifice one element in the array by initialising size to size –1 • If rear = front can’t add element • Test for remove happened before front is updated

7. Circular Queues 4 • If rear++ == front • Insertion would cause overflow • If rear = front • Removal would cause underflow c 3 d 2 1 b a 0 Front = 3 Rear = 2

8. Addition to a queue (enqueue) addq (item , queue)begin if rear + 1 = front then queueoverflow else begin q[rear] = item increment rear rear = rear mod (size –1)endend Deletion from a queue (dequeue) deleteq (item , queue) beginif front = rear then queueunderflow else begin item = q[front]        front = front+1 front = front mod (size –1) endend Circular Queue Functions

9. Priority Queue • Stacks and queues are linear structures • Very efficient in terms of insertion and deletion • Not so efficient for locating specific data • We have to do several operations of load and unload to access specific data • Priority is a means of storing data such that unloading produces most relevant data to an operation • E.g. most important process running in job scheduler • Uses ‘heap sort’ which always puts highest priority at head of queue • Not the same as a conventional ordinal sort

10. Priority • Priority is defined as the largest or highest ranking • Stack deletes newest • Queue deletes oldest • Priority queue deletes highest priority • Newest item inserted to retain integrity of priority • Employs heap sort

11. Heap sort

12. Heap sort Add 44 to heap

13. Heap sort

14. Heap sort

15. Heap sort Now add 47

16. Heap sort

17. Heap sort End result

18. Heap • Attempts to maintain complete tree • Balanced • Fills from left to right on each level • No more than one level between leaves • Root always contains highest priority value • Deletion always is from root • Heap reorganised on deletion • How?

19. Heap sort Root removed

20. Heap sort

21. Heap sort

22. Heap sort

23. 45 44 33 22 29 12 24 1 2 3 4 5 6 7 8 9 Array Implementation Where leaf nodes are 2n and 2n+1 Or root is n div 2 using integer division

24. 45 44 33 22 29 12 24 21 24 1 2 3 4 5 6 7 8 9 21 is in position 8 – 8/2 = 4 22 is in pos 4 – no swap 24 is in position 9 – 9/2 = 4 22 is in pos 4 – swap Array Implementation

25. Recap • Circular queues more efficient than standard queue • Linked list implementation of queue obviates need for circular queue. Dynamic. • Priority Queue always yields highest priority for deletion • Implements heap sort • Maintains complete tree structure