Queues, Deques, and Priority Queues
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Transcript of Queues, Deques, and Priority Queues
Queues, Deques, and Priority Queues
Chapter 22
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Chapter Contents
Specifications for the ADT QueueUsing a Queue to Simulate a Waiting Line• The Classes WaitLine and Customer
Using a Queue to Compute the capital Gain in a Sale of Stock• The Classes StockLedger and StockPurchase
Specifications of the ADT DequeUsing a Deque to Compute the Capital Gain in a Sale of StockSpecifications of the ADT Priority QueueUsing a Priority Queue to Compute the Capital Gain in a Sale of Stock
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Specifications for the ADT Queue
Queue organizes entries according to order of entry• Exhibits first-in, first-out behavior• Contrast to stack which is last-in, first-out
All additions are at the back of the queue
Front of queue has items added first
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Specifications for the ADT Queue
Fig. 22-1 Some everyday queues.
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Specifications for the ADT Queue
Interface for a queue of objectspublic interface queueInterface{ /** Task: Adds a new entry to the back of the queue.
* @param newEntry an object to be added */public void enqueue(Object newEntry);
/** Task: Removes and returns the front of the queue.* @return either the object at the front of the queue or null if the queue was empty */public Object dequeue();
/** Task: Retrieves the front of the queue.* @return either the object at the front of the queue or null if the queue is empty */public Object getFront();
/** Task: Determines whether the queue is empty.* @return true if the queue is empty */public boolean isEmpty();
/** Task: Removes all entries from the queue. */public void clear();
} // end queueInterface
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Specifications for the ADT Queue
Fig. 22-2 Queue of strings after (a) enqueue adds Jim; (b) Jess; (c) Jill; (d) Jane; (e) Joe; (f) dequeue retrieves,
removes Jim; (g) enqueue adds Jerry; (h) dequeue retrieves, removes Jess.
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Using a Queue to Simulate a Waiting Line
Fig. 22-3 A line, or queue of people.
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Classes WaitLine and Customer
Fig. 22-4 A CRC card for the class WaitLine.
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Classes WaitLine and Customer
Fig. 22-5 A diagram of the classes WaitLine and Customer.
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Classes WaitLine and Customer
Fig. 22-6 A simulated waiting line … continued →
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Classes WaitLine and Customer
Fig. 22-6 (ctd) A simulated waiting line.
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Using a Queue to Compute Capital Gain in a Sale of Stock
Must sell stocks in same order they were purchased• Must use the difference in selling price and
purchase price to calculate capital gain
We use a queue to• Record investment transactions
chronologically• Compute capital gain of the stock
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Classes StockLedger and StockPurchase
Fig. 22-7 A CRC card for the class StockLedger
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Classes StockLedger and StockPurchase
Fig. 22-8 A diagram of the class StockLedger and StockPurchase.
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Classes StockLedger and StockPurchase
Fig. 22-9 A queue of (a) individual shares of stock; (b) grouped shares.
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Specifications of the ADT Deque
A Double ended queue
Has operations that • Add, remove, or retrieve entries • At both its front and back
Combines and expands the operations of queue and stack
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Specifications of the ADT Deque
Fig. 22-10 An instance of a deque.
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Specifications of the ADT Deque
A Java interface
public interface DequeInterface{ public void addToFront(Object newEntry);
public void addToBack(Object newEntry);public Object removeFront();public Object removeBack();public Object getFront();public Object getBack();public boolean isEmpty();public void clear();
} // end DequeInterface
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Specifications of the ADT Deque
Fig. 22-11 A comparison of the operations that add, remove, and retrieve the entries of a stack s, queue q,
and a deque d; (a) add; (b) remove; (c) retrieve.
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Using a Deque to Compute Capital Gain in Sale of Stock
Revise the class StockPurchase• Represents purchase of n shares at d dollars
per share• Constructor is changed• Accessor methods for new fields added
Revise class StockLedger• Method ledger now an instance of a deque• Method buy now creates instance of StockPurchase, places it at back of deque
• Method sell also altered
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Specifications of the ADT Priority Queue
Organizes objects according to priorities• Contrast to regular queue in order of arrival
Priority queue example – a hospital ER
Priority can be specified by an integer• Must define whether high number is high
priority or …• Low number (say 0) is high priority
Other objects can specify priority• Object must have a compareTo method
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Specifications of the ADT Priority Queue
public interface PriorityQueueInterface{ public void add(Comparable newEntry);
public Comparable remove();public Comparable get();public boolean isEmpty();public int getSize();public void clear();
} // end PriorityQueueInterface
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Using Priority Queue to Compute Capital Gain in Sale of Stock
Revise class StockPurchase• Include data field date • Method compareTo is made available• This field is the priority – earliest date is
highest priority