queues, deques, and priority queues chapter 14. 2 chapter contents specifications for the adt queue...
Post on 22-Dec-2015
222 views
TRANSCRIPT
Queues, Deques, and Priority Queues
Chapter 14
2
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 Track Your Assignments
3
Specifications for the ADT Queue
Queue organizes entries according to order of entry,
Front of queue has items added first, end of queue has items added last. It organizes items chronologically• Exhibits first-in, first-out behavior• Contrast to stack which is last-in, first-out
All additions are at the back of the queue, removals are at the front of the queue.
4
Specifications for the ADT Queue
Some everyday queues.
5
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
6
Specifications for the ADT Queue
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.
7
Using a Queue to Simulate a Waiting Line
A line, or queue of people.
Short waiting time increase satisfaction.
8
Waiting Line
Time-driven simulation, a counter enumerates simulated units of time - minutes etc.
Customers arrive at random times and are assigned a random transaction time. • Math.random();• Maximal_transaction_time * Math.random()
At the conclusion, summary statistics are created including total number of customers served and the average waiting time.
9
Classes WaitLine and Customer
A diagram of the classes WaitLine and Customer to generate a new customer
10
Classes WaitLine and Customer
A simulated waiting line … continued →
11
Classes WaitLine and Customer
(ctd) A simulated waiting line.
12
Pseudocode of Method SimulateAlgorithm simulate (duration, arrivalProbability, maxTransactionTime)for( clock = 0; clock < duration; clock++){
if( a new customer arrives){ numberOfArrivals++ transactionTime = a random time does not exceed maxTransactionTime nextArrival = a new customer containing clock, transactionTime, and a customer number that is numberOfArrival
line.enqueue(nextArrival)}if( transactionTimeLeft > 0 ) // if present customer is still being served transactionTimeLeft—else if(!line.isEmpty)
{ nextCustomer = line.dequeue() transactionTimeLeft = nextCustomer.getTransactionTime() -1 timeWaited = clock() – nextCustomer.getArrivalTime() totalTimeWaited = totalTimeWaited + timeWaited numberServed++}
}
13
Using a Queue to Compute Capital Gain in a Sale of Stock
Buy n shares of a stock for d dollars each. Later you sell some of these shares.
If the sale price exceeds the purchase price, you have made a profit - a capital gain.
Otherwise, you experience a loss.
Let’s assume you sell shares in the order in which you purchase them. ( first-in, first-out)
14
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
15
Classes StockLedger and StockPurchase
A diagram of the class StockLedger and StockPurchase.
16
30 shares at $45 eachtwo queues
(a) Adds each of the 30 individual shares to a queue
(b) Encapsulate 30 shares into one object
If we only wan to sell 20 shares with 10 shares left. (a) is easy to implement. (b). Need to place 10 left shares
to the front of the queue.
17
Specifications of the ADT Deque
A Double ended queue• At a post office, when it is your turn, you
need to fill up a form, step aside and get served when finished.
Has operations that • Add, remove, or retrieve entries • At both its front and back
Combines and expands the operations of queue and stack
18
Specifications of the ADT Deque
An instance of a deque.
19
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
20
Specifications of the ADT Deque
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.
21
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: assign a priority number to each patient to override the arrival time.
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
22
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
23
Using Priority Queue to Track Your Assignment
Class TrackAssignment• Include data field DueDate • Method compareTo is made available• This field is the priority – earliest due date is
highest priority• Organize tasks in the order in which we should
complete them.