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Introduction to C Programming CE Lecture 12 Circular Queue and Priority Queue Data Structures

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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

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Queues Front = 0 Rear = Front = 0 Rear = 3 Front = 2 Rear = 3 Front = 2 Rear = 5 Array size = 5 d c e a b cc

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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

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C Queue Front = 2 Rear = Front = 2 Rear = 1 Front = 4 Rear = 1 Front = 0 Rear = 2 d c e e c d e f ff g

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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

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Circular Queues If rear++ == front Insertion would cause overflow If rear = front Removal would cause underflow Front = 3 Rear = 2 c d b a

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Circular Queue Functions 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) 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

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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

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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

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Heap sort

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Add 44 to heap

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Heap sort

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Now add 47

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Heap sort

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End result

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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?

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Heap sort Root removed

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Heap sort

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Array Implementation Where leaf nodes are 2n and 2n+1 Or root is n div 2 using integer division

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Array Implementation 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

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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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