introduction to c programming ce00312-1 lecture 12 circular queue and priority queue data structures
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Introduction to C Programming
CE00312-1
Lecture 12
Circular Queue and Priority Queue Data Structures
Recap on Queues
Addition to a queue (enqueue)
addq (item , queue)begin if rear = size then queuefull
else begin
q[rear] = item increment rear
end end
Deletion from a queue (dequeue)
deleteq (item , queue)
begin if front = rear then queueempty else begin
item = q[front] front = front+1 end end
Queues
Front = 0
Rear = 0
4
3
1
2
0
4
3
1
2
0
4
3
1
2
0
4
3
1
2
0
Front = 0
Rear = 3
Front = 2
Rear = 3
Front = 2
Rear = 5
Array size = 5
d
c
e
a
b
c c
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
C Queue
Front = 2
Rear = 0
4
3
1
2
0
4
3
1
2
0
4
3
1
2
0
4
3
1
2
0
Front = 2
Rear = 1
Front = 4
Rear = 1
Front = 0
Rear = 2
d
c
e e
c
d
e
f f f
g
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
Circular Queues
If rear++ == front Insertion would
cause overflow If rear = front
Removal would cause underflow
4
3
1
2
0
Front = 3
Rear = 2
c
d
b
a
Circular Queue FunctionsAddition to a queue (enqueue) addq (item , queue)
begin if rear + 1 = front then queueoverflow else begin
q[rear] = itemincrement rear
rear = rear mod (size –1) end end
Deletion from a queue (dequeue) deleteq (item , queue)
begin if front = rear then queueunderflow else
begin item = q[front]
front = front+1 front = front mod (size –1)
end end
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
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
Heap sort
1 0 7
2 2
1 3
2 6
2 9
1 2 2 4
3 3
4 5
Heap sort
1 0 7
2 2
1 3 4 4
2 6
2 9
1 2 2 4
3 3
4 5
Add 44 to heap
Heap sort
1 0 7
2 2
1 3 2 6
4 4
2 9
1 2 2 4
3 3
4 5
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
1 2 2 4
3 3
4 5
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
4 7
1 2 2 4
3 3
4 5
Now add 47
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
1 2
4 7 2 4
3 3
4 5
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
1 2
2 4
4 7
End result
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?
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
1 2
3 3 2 4
4 5
E m p ty
Root removed
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
3 3 2 4
4 5
1 2
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
3 3 2 4
1 2
4 5
Heap sort
1 0 7
2 2
1 3 2 6
2 9
4 4
1 2 2 4
3 3
4 5
Array Implementation
45 44 33 22 29
1 2 3 4 5 6 7 8 9
12 24
Where leaf nodes are 2n and 2n+1
Or
root is n div 2 using integer division
Array Implementation
45 44 33 22 29
1 2 3 4 5 6 7 8 9
12 24 21 24
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
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
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