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AL-HUSEEN BIN TALAL UNIVERSITY
College of Engineering
Department of Computer Engineering
Algorithms and Data Structures
Course No.: 0511363
Fall 2014
Queues
Queues
• Queue: is a linear data structure in which the elements
are added at one end, called the back or rear, and
deleted from the other end, called the front
• Example:
 line of customers in a bank
 A queue of documents is often waiting to be printed at a
printer.
• The only difference between stack and queue is that in
case of stack insertion and removable (PUSH and POP)
operations are at one end (TOP) only, but in case of a
queue Insertion and Deletion take place at two ends
called REAR and FRONT of the queue respectively.
Queues
• A queue is a container that implements that the first-infirst-out (FIFO) protocol. This means that the data in
the queue is processed in the same order as it was
entered. The following figure represents a queue
structure:
• Queue operations:


Insert (Item)
Remove ()
Representation of queues
• Queue may be represented in the memory in
various ways. Mainly there are two ways:
 Using one dimensional array
 Using single linked list
• To represent queues as array we need:
 An array to store the queue elements
 Two variables to keep track of the first and last
elements of the queue
 Variable to specify the maximum size of the
queue.
Array representation of queues
# include<iostream.h>
#include<conio.h>
#include<assert.h>
int front = 0 , rear = -1;
const int size = 7;
char q[size];
char item;
insert(char item)
{
assert(rear != size);
q[++rear] = item;
}
char remove(char item)
{
assert(rear >= front );
item = q[front++];
return item;
}
Array representation of queues…cont.
main( )
{
for(int i = 0 ; i < size ; i++)
{
cout<<"Insert an item : ";
cin>>item;
insert(item);
}
clrscr( );
cout<<" items removed are: " ;
for(int j = 0 ; j < size ; j ++)
cout << remove(item);
}
Example: Queue to store Array of
characters
# include<iostream.h>
#include<string.h>
#include<conio.h>
#include<assert.h>
int front = 0 , rear = -1;
const int size = 7, n = 7;
char q[size][n];
char item[n];
insert(char item[])
{
assert(rear != size);
strcpy(q[++rear],item);
}
remove(char item[])
{
assert(rear >= front );
strcpy(item,q[front++]);
cout<<"Deleted item is : "<<item<<endl;
}
Example ..cont.
int frnt( )
{ return front ;}
int rer( )
{ return rear ;}
main( )
{
for(int i = 0 ; i
{
cout<<"Insert the
cin>>item;
insert(item);
}
clrscr( );
for(int j = 0 ; j
{
remove(item);
}
cout<<" The index
cout<<" The index
}
< size ; i++)
value of the item : ";
< size ; j ++)
at the rear is: "<<rer( )<<endl;
at the front is: "<<frnt( )<<endl;
Abstract Data Type (Queue)
# include<iostream.h>
#include<conio.h>
#include<assert.h>
class queue{
int front;
int rear;
int count;
unsigned size ;
char* q;
public:
queue (unsigned s) ; // constructor
unsigned Get_s( ) ; // returns size of the queue
void insert(char item) ; // inserts given item at the rear
char remove() ; // remove an item from the front
int Get_C( ) ; // returns the number of elements in the queue
char Get_frnt( ) ; // returns the element in the front
char Get_rer( ) ; // returns the element of the rear
void initialize( ) ; // initialize front, rear and count
int isEmpty( ) ; // check if queue is empty
int isFull( ) ; // check if queue is full
void Destroy( ) ; // Destroy all elements in the queue
};
General program..Cont.
void queue:: initialize( ){
front = 0 ;
rear = -1 ;
count = 0 ;
}
int queue::isEmpty( ){
return ( count == 0 ) ;
}
int queue::isFull( ){
return ( count == size ) ;
}
void queue::Destroy( ){
front = 0 ;
rear = -1 ;
count = 0 ;
}
General program..Cont.
queue::queue (unsigned s){
if(s <= 0){
cout<< " The size of the array should be positive "<<endl ;
cout<< " By default it can be S. "<<endl ;
size =10 ;}
else
size = s ;
q = new char [size] ;
assert (q ) ;
}
unsigned queue::Get_s( ){
return size;
}
void queue::insert(char
assert(rear != size);
q[++rear] = item;
count++;
}
item){
General program..Cont.
char queue::remove(){
assert(rear >= front );
--count;
return q[front++];
}
int queue::Get_C( ) { //return number of item in queu
return count;
}
char queue::Get_frnt( ){
assert( !isEmpty( ) ) ;
return q[front] ;
}
char queue::Get_rer( ){
assert( !isEmpty( ) ) ;
return q[rear] ;
}
Main function
main(){
int i, s;
char it;
cout<<"Enter the size of queue ";
cin>>s;
queue f(s);
f. initialize( );
for (i=0;i<s;i++){
cout<<"Enter the element"<<(i+1)<<"\t";
cin>>it;
f.insert(it);
}
cout<<"get front item "<<f.Get_frnt( )<<endl;
cout<< "the number of item in queue is "<<f.Get_C( )<<endl;
for (i=0;i<s;i++){
cout<<"remove item number "<< (i+1)<<"\t" <<f.remove( )<<endl;
}
}
Linked list representation of queues
• Queues can be implemented using single
linked-list (SLL):
 The operation of adding an item to queue is similar
to add a node to SLL
 The operation of deleting an item from queue is
similar to delete a node from SLL.
Linked list representation of queues
Cont.
Cont.
Cont.
Various queue structures
• Deque (Double Ended Queue)
In this type both the insertion and deletion will
be from both ends
• This structure is between Queue and Stack
To implement this type there are two ways:
1- Using double linked list
2- Circular array
Priority Queue
• Priority Queue : is a data structure where
element removed according to there priority
and there current queue position
• An element with high priority is served before an
element with low priority. If two elements have the
same priority, they are served according to their order in
the queue
• Priority Queue Implementation:
 Array
 Double Linked List
Application of Queues
• Simulation: design computer models to study
the behavior of real systems.
• Multiprogramming
• Schedule: Examples include CPU scheduling
(Operating systems often maintain a queue of
processes that are ready to execute or that are
waiting for a particular event to occur), Disk
Scheduling.