Queue Operations
advertisement
Queues Chapter 5 Ceng-112 Data Structures I 2007 1 Linear Lists Operations are; 1. Insertion 2. Deletion 3. Retrieval 4. Traversal (exception for restristed lists). Ceng-112 Data Structures I 2007 Figure 3-2 2 Queue A queue is a linear list. Data can be inserted at one end (rear) and deleted from the other end (front). First In First Out - FIFO Figure 5-1 Ceng-112 Data Structures I 2007 3 Queue Operations There are four basic queue operations. Data can be inserted at the rear and processed from the front. 1. Enqueue ; inserts an element at the rear of the queue. 2. Dequeue ; deletes an element at the front of the queue. 3. Queue Front; examines the element at the front of the queue. 4. Queue Rear; examines the element at the rear of the queue. Ceng-112 Data Structures I 2007 4 Queue Operations Figure 5-2 Ceng-112 Data Structures I 2007 5 Queue Operations Figure 5-3 Ceng-112 Data Structures I 2007 6 Queue Operations If there are no data in the queue, then the queue is in an underflow state. Figure 5-4 Ceng-112 Data Structures I 2007 7 Queue Operations If there are no data in the queue, then the queue is in an underflow state. Figure 5-5 Ceng-112 Data Structures I 2007 8 Figure 5-6 Ceng-112 Data Structures I 2007 9 Queue Linked List Design Figure 5-7 Ceng-112 Data Structures I 2007 10 Queue Data Structure Figure 5-8 Ceng-112 Data Structures I 2007 11 Figure 5-9, Part I Ceng-112 Data Structures I 2007 12 Queue Algorithms - Create Queue algorithm createQueue Allocates memory for a queue head node from dynamic memory and returns its address to the caller. Pre Nothing Post head has been allocated and initialized Return head’s address if successful, null if memory owerflow. 1. if (memory available) 1. 2. 3. 4. 5. 2. allocate (newPtr) newPtrfront = null pointer newPtrrear = null pointer newPtrcount = 0 return newPtr else 1. return null pointer end createQueue Ceng-112 Data Structures I 2007 13 Queue Algorithms - Enqueue Figure 5-10 Ceng-112 Data Structures I 2007 14 Queue Algorithms - Enqueue algorithm enqueue(val queue <head pointer>, val item <dataType>) This algorithm inserts data into queue. Pre queue has been create Post item data have been inserted Return boolean, true if successful, false if overflow. 1. if (queue full) 1. 2. 3. 4. 5. return false allocate (newPtr) newPtrdata = item newPtrnext = null pointer if (queuecount zero) //inserting into null queue 1. 6. queuefront = newPtr else // insert data and adjust meta data 1. queuerearnext = newPtr 7. queuerear = newPtr 8. queuecount = queuecount +1 9. return true end enqueue Ceng-112 Data Structures I 2007 15 Figure 5-9, Part II Ceng-112 Data Structures I 2007 16 Figure 5-11 Ceng-112 Data Structures I 2007 17 Queue Algorithms - Dequeue algorithm dequeue(val queue <head pointer>, ref item <dataType>) This algorithm deletes a node from a queue. Pre queue has been create Post data at front of the queue returned to user through item and front element deleted and recycled. Return boolean, true if successful, false if overflow. 1. if (queuecount is 0) 1. 2. 3. 4. return false item = queuefrontdata deleteLoc = queuefront if (queuecount is 1) // deleting only item in queue 1. queuerear = null pointer 5. queuefront = queuefrontnext 6. queuecount = queuecount – 1 7. recycle (deleteLoc) 8. return true end dequeue Ceng-112 Data Structures I 2007 18 Queuing Theory • A single server queue can provide service to only one customer at a time. • Multiserver queues, can provide service to many customers at a time. • A customer is any person or thing needing service. • The service is any activity needed to accomplish the required result. • The rate at which customers arrive in the queue for service is known as the arrival rate. • Service time is the avarage time required to complete the processing of a customer request. Figure 5-12 Ceng-112 Data Structures I 2007 19 Queuing Theory • Queuing theory attempts to predict some patterns, such as; – Queuing time, which is defined the avarage length of time customers wait in the queue, • Avarage and maximum size of queue, – Response time is an important statistical tool for online computer systems • There are two factors that affect the performance of queues; – Arrival rate – Service time Ceng-112 Data Structures I 2007 20 Queue Data Structures Figure 5-13 Ceng-112 Data Structures I 2007 21 Queues Array Implementation Figure 5-15 Ceng-112 Data Structures I 2007 22 Queues Array Implementation Figure 5-16 Ceng-112 Data Structures I 2007 23 • Store the address of the queue array. • Store the max. number of elements in array. Figure 5-17 Ceng-112 Data Structures I 2007 24 Exercise Imagine the contents of queue Q1 and Q2 are as shown. What would be the content of Q3 after the following code is executed? The queue contents are shown front (left) to rear (right). Q1: 42 30 41 31 19 20 25 14 10 11 12 15 Q2: 1 4 5 4 10 13 1 Q3 = createQueue 2 count = 0 3 loop (not empty Q1 and not empty Q2) 1 count = count + 1 2 dequeue (Q1, x) 3 dequeue (Q2, y) 4 if (y equal count) 1 enqueue (Q3, x) Ceng-112 Data Structures I 2007 25 Exercise Imagine the contents of queue Q1 and Q2 are as shown. What would be the content of Q3 after the following code is executed? The queue contents are shown front (left) to rear (right). Q1: 42 30 41 31 19 20 25 14 10 11 12 15 Q2: 1 4 5 4 10 13 1 Q3 = createQueue 2 count = 0 3 loop (not empty Q1 and not empty Q2) 1 count = count + 1 2 dequeue (Q1, x) 3 dequeue (Q2, y) 4 if (y equal count) 1 enqueue (Q3, x) Step count Q1 Q2 Q3 1 0,1 42,30,.. 1,4,5,4,.. 2 2 30,41,.. 3 3 4 y 42 1 4,5,4,.. 30 4 42,31,19,.. 5,4,10,13 42 5 4 31,19,20,.. 4,10,13 31 4 5 5 19,20,25,.. 10,13 19 10 6 6 20,25,14,.. 13 20 13 25,14,10,.. NULL 7 42 x 42,31 Q3: 42, 31 Ceng-112 Data Structures I 2007 26
