Data Structures and Agorithm: DS 09 Queue.pptx

1. Data Structure
Lecture No. 9
Queue
Engr. Rashid Farid Chishti
http://youtube.com/rfchishti
http://sites.google.com/site/chishti
International Islamic University H-10, Islamabad, Pakistan
Video Lecture

2.  A stack is LIFO (Last-In First Out) structure.
 In contrast, a queue is a FIFO (First-In First-Out ) structure.
 A queue is a linear structure for which items can be only inserted at one end
and removed at another end.
Queue Operations
 Put(X) place X at the rear of the queue.
 Get() remove the front element and return it.
 Front() return front element without removing it.
 Is_Empty() return TRUE if queue is empty, FALSE otherwise
 Is_Full() return TRUE if queue is full, FALSE otherwise
Queues

3. Implementing Queue using Array
Index No. 0 1 2 3 Head Tail Count
A Queue -1 -1 0 An empty queue
Put(‘A’) A -1 0 1 Puts ‘A’ in Queue
Put(‘B’) A B -1 1 2 Puts ‘B’ in Queue
Put(‘C’) A B C -1 2 3 Puts ‘C’ in Queue
Put(‘D’) A B C D -1 (3) → -1 4 Puts ‘D’ in Queue
Put(‘E’) A B C D -1 -1 4 Error: Queue is full
Get() B C D 0 -1 3 Gets ‘A’ from Queue
Put(‘F’) F B C D 0 0 4 Puts ‘F’ in Queue
Get() F C D 1 0 3 Gets ‘B’ from Queue
Get() F D 2 0 2 Gets ‘C’ from Queue
Get() F (3) → -1 0 1 Gets ‘D’ from Queue
Get() 0 0 0 Gets ‘F’ from Queue
Get() 0 0 0 Error: Queue is empty

4. #include <iostream>
using namespace std;
typedef char Type;
#define MAX 4
class Queue{
private:
Type qu[MAX];//array of any type
int head; //index of start of queue
int tail; // index of end of queue
int count; // number of items
public:
Queue::Queue();
bool Is_Full();
bool Is_Empty();
void Put(Type var);
Type Get();
Type Front();
void Show();
};
4
Example 1: Implementing Queue using Array
Queue::Queue(){ //constructor
head = -1; tail = -1;
count = 0;
}
bool Queue::Is_Full(){
return count >= MAX-1;
}
bool Queue::Is_Empty(){
return count <= 0;
}
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5. void Queue::Put(Type var){
// insert at tail of Queue
if(count >= MAX)
cout <<"Error: Queue is full n";
else{
qu[++tail] = var;
count++;
}
if(tail >=MAX-1)
// wrap around if past array end
tail = -1;
}
5
Example 1: Implementing Queue using Array
Type Queue::Get(){
// remove item from queue head
if(count <= 0){ // if queue empty
cout <<"Error: Queue is Emptyn";
return -1;
}
Type temp = qu[++head]; //store item
count--;
if(head >= MAX-1){
// wrap around if past array end
head = -1;
}
return temp; //return item
}
3 4

6. Type Queue::Front(){
if(count <= 0){ // if queue empty
cout <<"Error: Queue is Emptyn";
return -1;
}
Type temp = qu[++head];
--head;
return temp; //return item
}
void Queue::Show(){
for(int i = 0 ; i<MAX ; i++){
cout << qu[i] <<" ";
}
cout << "Head = " << head
<< " Tail = " << tail << endl;
}
6
Example 1: Implementing Queue using Array
int main( ){
Queue q;
cout<<"Put An"; q.Put('A');
cout<<"Put Bn"; q.Put('B');
cout<<"Put Cn"; q.Put('C');
cout<<"Put Dn"; q.Put('D');
cout<<"Put E"; q.Put('E');
cout<<"Got "; cout << q.Get() << endl;
cout<<"Put Fn"; q.Put('F');
cout<<"Got "; cout << q.Get() << endl;
cout<<"Got "; cout << q.Get() << endl;
cout<<"Got "; cout << q.Get() << endl;
cout<<"Got "; cout << q.Get() << endl;
cout<<"Got "; cout << q.Get() << endl;
system("PAUSE");
return 0;
}
5 6

7.  Using linked List: Recall
 Insert works in constant time for either end of a linked list.
 Remove works in constant time only.
 Seems best that head of the linked list be the front of the queue so that all
removes will be from the front.
 Inserts will be at the end of the list.
Implementing Queue using Linked List
front
2
5
7
1 1 7 5 2
front
rear rear
front
2
5
7 1 7 5 2
front
rear rear
get()
front
2
5
7
9
7 5 2
front
rear
rear
put(9)
9

8. #include <iostream>
using namespace std;
typedef int Type;
struct Node{
Type data ;
Node *next ;
};
class Queue{
private :
Node *front, *rear ;
public :
Queue( ) ;
void Put ( Type Data ) ;
Type Get( ) ;
~Queue( ) ;
};
8
Example 2: Implementing Queue using Linked List
Queue :: Queue( ){
front = rear = NULL ;
}
void Queue :: Put ( Type Data ){
Node *newNode ;
newNode = new Node ;
if ( newNode == NULL )
cout << "nQueue is full" ;
newNode -> data = Data ;
newNode -> next = NULL ;
if ( front == NULL ){
rear = front = newNode ;
return ;
}
rear -> next = newNode ;
rear = rear -> next ;
}
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9. Type Queue :: Get( ){
if ( front == NULL ){
cout << "Queue is empty" ;
return -1;
}
Node *current ;
Type Data ;
Data = front -> data ;
current = front ;
front = front -> next ;
delete current ;
if (front == NULL) rear = NULL;
return Data ;
}
Queue :: ~Queue( ){
if ( front == NULL )
return ;
Node *current ;
while ( front != NULL ){
9
Example 2: Implementing Queue using Linked List
current = front ;
front = front -> next ;
delete current ;
}
}
int main( ){
Queue a ;
a.Put ( 11 ) ;
a.Put ( -8 ) ;
a.Put ( 23 ) ;
a.Put ( 19 ) ;
cout << a.Get( ) << endl;
cout << a.Get( ) << endl;
cout << a.Get( ) << endl;
cout << a.Get( ) << endl;
cout << a.Get( ) << endl;
system("PAUSE");
return 0;
}
3 4

10.  The queue data structure is used in various CPU and disk scheduling. Here we
have multiple tasks requiring CPU or disk at the same time. The CPU or disk
time is scheduled for each task using a queue.
 The queue can also be used for print spooling wherein the number of print
jobs is placed in a queue.
 Handling of interrupts in real-time systems. The interrupts are handled in the
same order as they arrive i.e First come first served.
 Breadth-first Search in which the neighboring nodes of a tree are traversed
before moving on to next level uses a queue for implementation.
 In real life scenario, Call Center phone systems uses Queues to hold people
calling them in an order, until a service representative is free.
Applications of Queue

11.  Double ended queue is a more generalized form of queue data structure
which allows insertion and removal of elements from both the ends, i.e , front
and back.
Double Ended Queue
0 1 2 3 4 5 6
Front Rear
Pop Back
Push Back
Push Front
Pop Front

12.  Circular Queue is a linear data structure in which the
 operations are performed based on FIFO (First In First Out)
 principle and the last position is connected back to the first
 position to make a circle. It is also called ‘Ring Buffer’.
Circular Queue

13.  In priority queue, data when inserted, is stored based on its priority.
 When we remove a data from a priority queue, the data with least priority, will
get removed.
Priority Queue
9 0
Index
Number
7 1
7 2
6 3
5 4
3 5
1 6
Data with the highest priority
Data with the least priority