CMSC 202

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CMSC 202

• Stacks and Queues

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Whats a Queue?

• A queue is a linear collection of homogeneous
  data in which items added to the queue must be
  placed at the end of the queue and items removed
  from the queue are removed from the front.

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An AnalogyBank Teller Line

• When you walk into the bank you go to the end of
  the line.
• When the teller is ready to help the next person,
  the person at the beginning of the line is
  removed from the line.
• Each person then moves one position closer to the
  beginning of the line.
• Eventually, you will be the first person in the
  line and the teller will call you up causing you
  to be removed from the line.

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Whats it used for?

• Items waiting to be printed
• Representing a waiting line in a simulation
  (traffic light, cafeteria, grocery store
  checkout, etc.)
• Passing information from one process to another
  in the order it arrived

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The Queue ADT

• Description of the data
• A linear collection of homogeneous data
• A description of the operations
• Add an element to the end of the queue. This is
  known as enqueueing.
• Remove an element from the front of the queue.
  This is known as dequeueing.
• A queue is a First-In, First-Out (FIFO) data
  structure
• What other operations might be helpful?

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Queue Implementation

• Since the Queue contains homogeneous data, it
  should be implemented as a class template.
• That way, we can create queues that contain any
  data type or object.
• Enqueueing seems to be a special kind of
  inserting.
• Dequeueing seems to be a special kind of
  removing.
• What run-time errors may occur?

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Queue.H

• template lt class Tgt
• class Queue
• public
• // constructor(s), destructor,
  operator enqueue ( ) // what is/are the
  parameter(s) and //return type? dequeue (
  ) // what is/are the parameter(s) and
  //return type?
• private
• // data representation

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Using the Queue object

• main ( )
• Queueltintgt iQ
• iQ.enqueue ( 7 )
• iQ.enqueue ( 42 )
• int x iQ.dequeue ( ) // x 7
• cout ltlt iQ.dequeue ( ) ltlt endl // prints 42
• x iQ.dequeue ( ) // is an error

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Whats the data representation?

• A queue seems to be very much like a list (in the
  abstract sense), except with different (or maybe
  additional) insert and remove functionality.

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Whats a stack

• A stack is a linear collection of homogeneous
  items, in which an item to be added to the stack
  must be placed on top of the stack and and an
  item that removed from the stack must be removed
  from the top.

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Stack AnalogyCafeteria Trays

• When you go to the UC or dining hall for some
  scrumptious food, you probably use a tray. Which
  one do you pick-up? The one on the top of the
  pile.
• When the cafeteria work brings out new trays,
  where does he put them? On top of the pile.
• Trays are both added and and removed from the top
  of the pile.

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Whats it used for??

• Remembering where you were function call return
  addresses
• Reversing the order of stuff

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The Stack ADT

• Description of the data
• A linear collection of homogeneous data
• A description of the operations
• Add an element to the top (front) of the stack.
  This is known as pushing.
• Remove an element from the top (front) of the
  stack. This is known as popping.
• A stack is a Last-In, First-Out (LIFO) data
  structure
• What other operations might be helpful?

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Stack Implementation
Since the Stack contains homogeneous data, it
should be implemented as a class template. That
way, we can create stacks that contain any data
type or object. pushing seems to be a special
kind of inserting. poping seems to be a special
kind of removing. . What run-time errors may
occur?
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Stack.H
template lt class Tgt class Stack public //
constructor(s), destructor, operator push (
) // what is/are the parameter(s) and
//return type? pop ( ) // what is/are the
parameter(s) and //return type? private //
data representation
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Using the Stack object
main ( ) Stackltintgt iS iS.push ( 7
) iS.push ( 42 ) int x iS.pop ( ) // x
42 cout ltlt iS.pop ( ) ltlt endl // prints
7 x iS.pop ( ) // is an error
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Whats the data representation?

• A stack seems to be very much like a list (in the
  abstract sense), except with different (or maybe
  additional) insert and remove functionality.

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Stack Example

• Matching parenthesis, braces and brackets in a
  C program.

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A challenge

• Explain how to simulate a Queue (ie. A FIFO data
  structure) using two stacks.