Queues.ppt

Queues
CS 308 – Data Structures

What is a queue?
• It is an ordered group of homogeneous items of
elements.
• Queues have two ends:
– Elements are added at one end.
– Elements are removed from the other end.
• The element added first is also removed first
(FIFO: First In, First Out).

Queue Specification
• Definitions: (provided by the user)
– MAX_ITEMS: Max number of items that might be on
the queue
– ItemType: Data type of the items on the queue
• Operations
– MakeEmpty
– Boolean IsEmpty
– Boolean IsFull
– Enqueue (ItemType newItem)
– Dequeue (ItemType& item)

Enqueue (ItemType newItem)
• Function: Adds newItem to the rear of the
queue.
• Preconditions: Queue has been initialized
and is not full.
• Postconditions: newItem is at rear of queue.

Dequeue (ItemType& item)
• Function: Removes front item from queue
and returns it in item.
• Preconditions: Queue has been initialized
and is not empty.
• Postconditions: Front element has been
removed from queue and item is a copy of
removed element.

Implementation issues
• Implement the queue as a circular structure.
• How do we know if a queue is full or
empty?
• Initialization of front and rear.
• Testing for a full or empty queue.

Make front point to the element preceding the front
element in the queue (one memory location will be
wasted).

Queue is empty now!!
rear == front

Queue Implementation
template<class ItemType>
class QueueType {
public:
QueueType(int);
QueueType();
~QueueType();
void MakeEmpty();
bool IsEmpty() const;
bool IsFull() const;
void Enqueue(ItemType);
void Dequeue(ItemType&);
private:
int front;
int rear;
ItemType* items;
int maxQue;
};

Queue Implementation (cont.)
QueueType<ItemType>::QueueType(int
max)
{
maxQue = max + 1;
front = maxQue - 1;
rear = maxQue - 1;
items = new ItemType[maxQue];
}

QueueType<ItemType>::~QueueType()
{
delete [] items;
}

void QueueType<ItemType>::
MakeEmpty()
{
front = maxQue - 1;
rear = maxQue - 1;
}

bool QueueType<ItemType>::IsEmpty() const
{
return (rear == front);
}
bool QueueType<ItemType>::IsFull() const
{
return ( (rear + 1) % maxQue == front);
}

void QueueType<ItemType>::Enqueue
(ItemType newItem)
{
rear = (rear + 1) % maxQue;
items[rear] = newItem;
}

void QueueType<ItemType>::Dequeue
(ItemType& item)
{
front = (front + 1) % maxQue;
item = items[front];
}

Queue overflow
• The condition resulting from trying to add
an element onto a full queue.
if(!q.IsFull())
q.Enqueue(item);

Queue underflow
• The condition resulting from trying to
remove an element from an empty queue.
if(!q.IsEmpty())
q.Dequeue(item);

Example: recognizing palindromes
• A palindrome is a string that reads the same
forward and backward.
Able was I ere I saw Elba
• We will read the line of text into both a
stack and a queue.
• Compare the contents of the stack and the
queue character-by-character to see if they
would produce the same string of
characters.

#include <iostream.h>
#include <ctype.h>
#include "stack.h"
#include "queue.h“
int main()
{
StackType<char> s;
QueType<char> q;
char ch;
char sItem, qItem;
int mismatches = 0;
cout << "Enter string: " << endl;
while(cin.peek() != 'n') {
cin >> ch;
if(isalpha(ch)) {
if(!s.IsFull())
s.Push(toupper(ch));
if(!q.IsFull())
q.Enqueue(toupper(ch));
}
}

while( (!q.IsEmpty()) && (!s.IsEmpty()) ) {
s.Pop(sItem);
q.Dequeue(qItem);
if(sItem != qItem)
++mismatches;
}
if (mismatches == 0)
cout << "That is a palindrome" << endl;
else
cout << That is not a palindrome" << endl;
return 0;
}

Case Study: Simulation
• Queuing System: consists of servers and
queues of objects to be served.
• Simulation: a program that determines how
long items must wait in line before being
served.

Case Study: Simulation (cont.)
• Inputs to the simulation:
(1) the length of the simulation
(2) the average transaction time
(3) the number of servers
(4) the average time between job arrivals

Case Study: Simulation (cont.)
• Parameters the simulation must vary:
(1) number of servers
(2) time between arrivals of items
• Output of simulation: average wait time.

Exercises (Chapt 4)
• 26, 29-34, 39-41, 46, 47.

Queues.ppt

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