QUEUES WAITING LINE

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QUEUES - WAITING LINE

• A Queue is an ordered collection of data items.
  Items are added to one end of the queue called
  the back, and removed from the other end called
  the front.
• Example Check-out lane in a grocery.
• Queue is First-In-First-Out (FIFO) whereas the
  Stack is Last-In-First-Out (LIFO)

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Examples of queue
a. I/O buffers queues, scrolls, deques ?? From
a file (queue)
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?? Interactively (scroll queue on one end,
stack on the other
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?? Screen handling (deque double-ended
queue)
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• b. Scheduling queues in a multi-user computer
  system
• ?? Printer queue When files are submitted to a
  printer, they are placed in the printer queue.
  The printer software executes an algorithm
  something like
• for ()
• while (printerQueue.empty())
• sleep 1
• printFile printerQueue.removeQ()
• Print(printFile)

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?? Other Queues Resident queue On disk,
waiting for memory Ready queue In memory has
everything it needs to run, except the
CPU Suspended queue Waiting for I/O transfer or
to be reassigned the CPU
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c. CPU Scheduling Probably uses a priority
queue Items with lower priority are behind
all those with higher priority. (Usually a new
item is inserted behind those with the same
priority.)
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Queue ADT

• Data ordered collection of data items
• Basic Operations
• empty()
• enqueue(DataElt) adds DataElt to back of queue
• front() returns item at front of queue
• dequeue() removes item from front of queue
• Display() output the values stored in queue

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Implementation of Queue ADT

• Simplest implementation is to put items into an
  array with 0 being the front. This implementation
  is not used because it is too inefficient. Every
  time an item leaves the queue, every item has to
  be moved just like a real waiting line.
• A circular array is used instead with a moving
  front and back.

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Circular Array Implementation

• Keep a gap (empty element) between front and back
  elements let the index of the empty element be
  the back.
• As you add elements, advance myBack as you
  remove elements, advance myFront. (Use
  operator to keep indexes within the range 0
  through the capacity-1.)

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QUEUE ClassConstants and Data Fields

• Global constants
• typedef type QueueElement
• const int QUEUE_CAPACITY ...
• Private data fields
• QueueElement myArrayQUEUE_CAPACITY
• int myFront, myBack

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QUEUE ClassFunction Members

• //default constructor
• Queue()
• myFront 0 myBack 0
• // empty?
• bool empty() const
• // to add a new element
• void enqueue (const QueueElement value)

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functions continued

• // to retrieve front element
• QueueElement front () const
• // to remove front element
• void dequeue()

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Example of Software Development(A Queue
Application)

• Recall from Chapter 1 --
• Phases of Software Development
• Requirements (Problem analysis and specification)
• Design
• Coding
• Testing

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Check-out Lane SimulationRequirements

• Functional Requirements The program will
• Simulate servicing customers in checkout lanes by
  
• randomly generating customer arrivals and
  expected service time for each
• opening and closing lanes as needed
• Summarize results of the simulation
• total customers serviced
• maximum minutes spent in line
• average minutes spent in line
• Design Constraint
• Each checkout lane will be represented as a queue.

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Requirements Analysis

• Many details about the requirements need to be
  filled in.
• Clarification of terminology
• time
• elapsed time in simulation
• service time for a customer

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Requirements Analysis cont.

• Steps in simulation
• Count off minutes of simulation and for each
  minute
• randomly determine if a customer arrives
• if so
• if all lanes are full, display error
• else
• randomly generate service time for this customer
• place customer in first lane which is not full,
  opening a new lane if necessary

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Requirements Analysis cont.

• Steps in simulation cont.
• Provide 1 minute of service to customer at the
  front of each lane.
• Determine for each lane whether the customer has
  finished checking out, and if so
• Remove the customer from the lane.
• If lane is now empty, close it.
• When simulation time is up, finish servicing
  remaining customers in all lanes.

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Requirements Analysis cont.

• Outputs
• for each minute
• current time in simulation
• lanes full error message if needed
• message when a lane opens/closes
• status and contents of each lane at end of minute
• at end of simulation, summary report
• total customers serviced
• maximum minutes spent in line by a customer
• average minutes spent in line by customers

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Requirements Analysis cont.

• Simulation parameters supplied by client
• Max num lanes
• Customers per lane
• Minutes for simulation
• Minimum service minutes
• Maximum service minutes
• Number of customers expected per hour

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Design

• What are the objects and how to represent them.
• Customer Struct
• CustId, serviceMinutes, arrivalTime
• Output function
• Lane Queue
• Collection of lanes Array of lanes
• Simulation parameters
• Statistics
• Total Customers
• Total time in line
• Max time in line

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Design cont.

• Types and classes
• RandomInt
• Queue modified as follows
• SetFront
• Count
• Top-down design of functions (begin with main and
  work down)

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main function

• GetUserParameters
• Loop to count off minutes
• Process customer arrival
• Service customers at front of lanes
• Display lanes
• Service remaining customers
• Report statistics

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Input User Parameters
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Process Customer Arrivals
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Service Customer at Front of Each Lane
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Display Lanes
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Service Remaining Customers
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Display Report of Statistics