Stacks and Queues

1
Stacks and Queues

• 3/5/99
• B. Ramamurthy

2
Introduction

• Stacks and queues are very commonly used ADTs in
  a computer system.
• For example every process in computer system has
  a user stack to maintain its context.
• The subroutine call structure is maintained in a
  LIFO basis.
• The requests for the processes for various
  operation/ resources are maintained in a queue
  and served in a FIFO basis.
• We will study the stack ADT and queue ADT in this
  discussion.

3
ADT Stack

• Methods
• Constructor
• Push, Pop
• Peek
• Empty
• Java 1.2 Collection Classs stack

4
Application

• Evaluation of postfix expression
• Evaluate ABCD-/ using a stack. A6, B3,C10,D2

5
ADT Queue

• Queue is collection of items where items are
  added to one and removed from the other end in a
  FIFO fashion.
• Constructor()
• Empty()
• Add(NewItem) (to the end)
• Remove(Item) (from the front)
• Front(FrontItem) (similar to peek of stack)

6
Implementation

• Using Arrays, Vectors, or Linked List
• First write the interface
• Then implement it using any of already existing
  ADTs.
• Lets define an interface for Queue class and
  implement it using LinkedList of Java Collection
  class.

7
Application Simulation

• Simulation is a major area for computer
  applications.
• It is a technique for modeling the behavior of a
  system.
• Goal of simulation is to generate statistics that
  summarize the performance of an existing system
  and/or to predict the performance of a proposed
  system.
• Central to simulation is the concept of simulated
  time.
• Time-driven simulation, Event-driven simulation,
  and process-driven simulation.

8
Simulate Bank Line

• Objective To determine the average wait time
  given a arrival time and processing time for a
  set of customers.
• Input
• File containing many pairs of numbers
  representing arrival time, processing time
  Ordered by time of arrival.
• Output requires
• (1) Trace of events executed
• (2) Statistics total number of arrivals,
  average time spent in line.

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Simulate Bank Line ADTs

• Line A queue of arrival events
• Event list
• A list of arrival events and departure events
  that are sorted by the time of the event.
• Possible states empty, Arrival (A) event,
  Departure (D) event, A D and DA.
• Insertion into event list is according to time.

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Simulate Event class

• class BankEvent
• EventKind whichevent
• int Time // arrival time
• int TransTime // processing time
• Event() // constructors
• Event (EventKind E, int Atime, int Ttime)

11
Simulate Statistics

• class Stats
• int TotalNum 0
• int TotalWait 0
• Stats() // construtor

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Simulate methods

• GetArrival ( Instream Afile, Elist EL)
• Reads arrival event from input file and inserts
  it into event list.
• ProcessArrival (BankEvent AE, Instream Afile,
  Elist EL, Queue Line, Stats S)
• Executes arrival event.
• ProcessDeparture (BankEvent DE, Elist EL, Queue
  Line, Stats S)
• Executes a departure event.
• Simulate(String Filename, Stats S)
• Performs the simulation by calling the above
  methods.

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Simulate main method

• main()
• Stats Statistics
• Simulate(bank.dat, Statistics)
• //Print out Statistics

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Simulate method

• 1.Get Arrival from input file Update EventList
• 2. While (EventList is not empty)
• Get Next Event
• If (Event is Arrival)
• Process Arrival
• Else
• Process Departure

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ProcessArrival method

• 1. Update number of arrival statistics.
• 2. Output a message with current time and event.
• 3. AtHead True is Line is empty.
• 4. Add an Arrival event to Line.
• 5. Delete an event from Event list since it is
  being processed now.
• 6. If (AtHead) // start the processing first
  person
• Insert in the Event List a Departure Event
  for CurrentTimeTransTime of current event
• 7. Get Next Arrival from Input File Update Event
  List.

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ProcessDeparture method

• 1. Current Time Arrival time of Departure
  Event
• 2. Output a message with time and event
• 3. Remove Item from Line since a person is
  departing
• 4. Delete an event from the list
• 5. If Line is not empty
• PersonInfo Line front persons info
• Insert in Event List a Departure Event for
  Current Time PersonInfos TranTime
• Update Totalwait Statistics by (Current Time
  - PersonInfo.Time)
• 6. Exit.

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Overall Picture
Line (queue)
A
D
Simulator
Event List
Trace Stats
18
Miscellaneous

• Insert into EventList is according to time of
  occurrence.
• Delete the first element of the Event list.
• Event List contains at most one arrival and one
  departure event.
• Lets try hand simulate the bank line for the
  data

19
Sample data

• Arrival time Service Time
• 20 5
• 22 8
• 26 3
• 30 2
• Then change first service time to 10 and check
  what happens.