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Queues

Queues !?

Here's our Second data structure , which is called a queue . The most important property of a queue , which makes it different from a stack is that it's is "FIFO : First input first output " , it's just like real life queues.
You can think of a queue as a special kind of lists , where items are added from one end , and removed from the other end ; while the elements order is preserved.
We've two types of queues :
(1)Linear queues : in case elements are read from the front , we need to shift ... time delay.
(2)Circular queues : This's the one we're going to implement in this course.

Notes.

There are two methods of implementing the queues :
_Approach one :
We here will initialize " Front = 0 " , and " Rear = -1 " ; and the test for empty queue " Front = = NextPosition(Rear) ? " . But `the main problem here is that this condition also holds for a full queue .
Increase rear then put the value.
Front points to the first element.
Rear points to the last element.
_Approach two :
This is a better one , as we'll let " Front = = Rear ?" indicate an empty queue .
Front points to a never used item , and real data starts from the next position to the front .
Front must always points to an empty cell , and this means that we'll lose a place in memory ( never used cell ) but on the other hand it's make it easier to detect an empty queue .

Syntax.

< queue.h >
#include < iostream.h >
#include < stdlib.h >
#include < assert.h >
/* To use assertion in the programme */


const int MaxQueueSize=100 ;
typedef int QueueElemType ;
/* Define a new variable type called QueueElemType */


class Queue {

Public :
Queue( ) ;

void EnQueue(QueueElemType) ;


Applications

(1)Operating systems and computer networks ... programmes or messages are ordered in a queue to be executed one by one.
(2)Simmulations of situations.

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