Data Structures
Queue
Namiq Sultan
1
Queue
• A queue is an ordered collection of items into which items may be
added at one end (rear) and from which items may be deleted form the
end (front).
• A queue is logically a first in first out (FIFO) linear data structure.
front
rear
2
Queue
 Representing queues
const MAX = 8;
struct queue{
int items[MAX];
int front, rear;
} q;
q.items[7]
 Queue Basic Operations
 add(q, x)
Adds item x at the rear of the
queue q
q
q.items[0]
 x = remove(q)
Removes the front element from
the queue q.
q.front
q.rear
3
Queue
 Demonstration
q.front =0;
q.rear = -1;
Initial
conditions
7
add(q,4);
add(q, 7);
add(q, 3);
6
x=remove(q);
y=remove(q);
3
9
2
3
add(q, 9);
1
7
0
4
5
4
q.front
q.rear
201
1
0
-1
32
4
Important !!!!!
If q.rear==MAX-1 we cannot add elements even though
there are free cells in the front of the queue.
What will we do?
5
Queue
This problem can be solved by one of the following:
1. Shifting the elements whenever q.rear equals MAX-1 and there are
empty cells in the front of the queue.
2. Converting this linear queue to a circular queue.
6
Circular Queue
• To add an element, the element is
inserted in first location of the queue.
q.rear
• It behaves like a circular list.
Add(q, 5)
q.front
7
2
6
4
5
7
4
3
3
9
2
1
q.rear
0
5
7
Circular Queue
• The previous initial condition:
q.fron = 0
q.rear
7
q.rear = -1
6
is not suitable here.
5
• It is difficult to differentiate between an
empty queue and full queue.
4
3
2
1
q.front
0
8
Circular Queue
• Solution: Make front points to the element immediately preceding the
first item.
• Initial condition: front = rear = 0;
• The following conditions are satisfied:
 Queue is empty if:
front == rear
 Queue is full if
front == rear + 1
OR
front == 0 && rear == MAX-1
9
Program 5.1
//PROGRAM TO IMPLEMENT CIRCULAR QUEUE USING ARRAY
#include <process.h>
#include <iostream>
#using namespace std;
const MAX = 10;
class c_queue{
int cq_arr[MAX];
int front,rear;
public:
//a constructor is created to initialize the variables
c_queue()
{
front= 0;
rear = 0;
}
//public function declarations
void insert();
void del();
void display();
};
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Program 5.1
void c_queue::insert()
{
int added_item;
//Checking for overflow condition
if ((front == 0 && rear == MAX-1) || (front == rear +1))
{
cout<<"\nQueue Overflow \n";
return;
}
else
if (rear == MAX-1) /*rear is at last position of queue */
rear = 0;
else
rear = rear + 1;
cout<<"\nInput the element for insertion in queue:";
cin>>added_item;
cq_arr[rear] = added_item;
}/*End of insert()*/
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Program 5.1
//This function will delete an element from the queue
void c_queue::del()
{
//Checking for queue underflow
if (front == rear)
{
cout<<"\nQueue Underflow\n";
return;
}
else
if (front == MAX-1)
front = 0;
else
front = front + 1;
cout<<"\nElement deleted from queue is:"<<cq_arr[front]<<"\n";
}/*End of del()*/
12
Program 5.1
// Display the elements in the queue
void c_queue::display()
{
int front_pos = front, rear_pos = rear;
if (front == rear) {
// is empty or not
cout<<"\nQueue is empty\n";
return;
}
cout<<"\nQueue elements:\n";
//Displaying the queue elements
if (front_pos < rear_pos )
while(front_pos < rear_pos){
cout<<cq_arr[++front_pos]<<", ";
}
else{
while(front_pos < MAX-1){
cout<<cq_arr[++front_pos]<<", ";
}
front_pos = -1;
while(front_pos < rear_pos) {
cout<<cq_arr[++front_pos]<<", ";
}
} /*End of else*/
cout<<"\n";
} /*End of display() */
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Program 5.1
void main()
{
int choice;
c_queue co; //Creating the object
while(1){
//Menu options
cout <<"\n1. Insert\n";
cout <<"2. Delete\n";
cout <<"3. Display\n";
cout <<"4. Quit\n";
cout <<"\n Enter your choice: ";
cin>>choice;
switch(choice){
case 1: co.insert();
break;
case 2 : co.del();
break;
case 3: co.display();
break;
case 4:
exit(1);
default:
cout<<"\nWrong choice\n";
} /*End of switch*/
} /*End of while*/
} /*End of main()*/
14
APPLICATIONS OF QUEUE
1. Round robin techniques for processor scheduling is implemented using
queue.
2. Printer server routines (in drivers) are designed using queues.
3. All types of customer service software (like Railway/Air ticket
reservation) are designed using queue to give proper service to the
customers.
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