The document discusses data structures like stacks and queues. It provides examples of implementing a queue using both a linked list and an array. It describes how a queue is a FIFO structure where elements are inserted at the rear and removed from the front. It also discusses some uses of queues, providing a bank simulation as an example where a queue models customers waiting to be served by tellers.

1. Class No.09 Data Structures http://ecomputernotes.com

2. Memory Organization Code Static data Stack Heap Process 1 (browser) Process 3 (word) Process 4 (excel) Windows OS Process 2 (dev-c++) http://ecomputernotes.com

3. Stack Layout during a call Here is stack layout when function F calls function G: Parameters(F) Local variables(F) Return address(F) Parameters(G) Parameters(F) Local variables(F) Return address(F) Parameters(F) Local variables(F) Return address(F) Parameters(G) Local variables(G) Return address(G) During execution of G After call At point of call sp sp sp http://ecomputernotes.com

4. Queues 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. http://ecomputernotes.com

5. Queue Operations Enqueue(X) – place X at the rear of the queue. Dequeue() -- remove the front element and return it. Front() -- return front element without removing it. IsEmpty() -- return TRUE if queue is empty, FALSE otherwise http://ecomputernotes.com

6. Implementing Queue 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. http://ecomputernotes.com

7. Implementing Queue Using linked List: front 2 5 7 1 1 7 5 2 front rear rear http://ecomputernotes.com

8. 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 dequeue() http://ecomputernotes.com

9. Implementing Queue Using linked List: front 2 5 7 1 1 7 5 2 front rear rear front 2 5 7 9 7 5 2 front rear rear enqueue(9) 9 http://ecomputernotes.com

10. Implementing Queue int dequeue() { int x = front->get(); Node* p = front; front = front->getNext(); delete p; return x; } void enqueue(int x) { Node* newNode = new Node(); newNode->set(x); newNode->setNext(NULL); rear->setNext(newNode); rear = newNode; } http://ecomputernotes.com

11. Implementing Queue int front() { return front->get(); } int isEmpty() { return ( front == NULL ); } http://ecomputernotes.com

12. Queue using Array If we use an array to hold queue elements, both insertions and removal at the front (start) of the array are expensive. This is because we may have to shift up to “n” elements. For the stack, we needed only one end; for queue we need both. To get around this, we will not shift upon removal of an element. http://ecomputernotes.com

13. Queue using Array front 2 5 7 1 rear 6 5 7 0 0 1 3 2 4 front 1 7 5 2 3 rear http://ecomputernotes.com

14. Queue using Array front 2 5 7 1 rear 6 5 7 0 0 1 3 2 4 front 1 7 5 2 4 rear enqueue(6) 6 6 http://ecomputernotes.com

15. Queue using Array front 2 5 7 1 rear 6 5 7 0 0 1 3 2 4 front 1 7 5 2 5 rear enqueue(8) 6 6 8 8 http://ecomputernotes.com

16. Queue using Array front 2 5 7 rear 6 5 7 1 0 1 3 2 4 front 7 5 2 5 rear dequeue() 6 6 8 8 http://ecomputernotes.com

17. Queue using Array front 2 5 rear 6 5 7 2 0 1 3 2 4 front 5 2 5 rear dequeue() 6 6 8 8 http://ecomputernotes.com

18. Queue using Array front 2 5 rear 6 5 7 2 0 1 3 2 4 front 5 2 7 rear enqueue(9) enqueue(12) 6 6 8 8 9 9 12 12 enqueue(21) ?? http://ecomputernotes.com

19. Queue using Array We have inserts and removal running in constant time but we created a new problem. Cannot insert new elements even though there are two places available at the start of the array. Solution: allow the queue to “wrap around”. http://ecomputernotes.com

20. Queue using Array Basic idea is to picture the array as a circular array . front 2 5 rear 2 front 7 rear 6 8 9 12 6 5 7 0 1 3 2 4 5 2 6 8 9 12 http://ecomputernotes.com

21. Queue using Array void enqueue(int x) { rear = (rear+1)%size; array[rear] = x; noElements = noElements+1; } front 2 5 rear 2 front 0 rear 6 8 9 12 6 5 7 0 1 3 2 4 5 2 6 8 9 12 enqueue(21) 21 21 8 size 7 noElements http://ecomputernotes.com

22. Queue using Array int isFull() { return noElements == size; } int isEmpty() { return noElements == 0; } front 2 5 rear 2 front 1 rear 6 8 9 12 6 5 7 0 1 3 2 4 5 2 6 8 9 12 enqueue(7) 21 21 8 size 8 noElements 7 7 http://ecomputernotes.com

23. Queue using Array int dequeue() { int x = array[front]; front = (front+1)%size; noElements = noElements-1; return x; } front rear 4 front 1 rear 6 8 9 12 6 5 7 0 1 3 2 4 6 8 9 12 dequeue() 21 21 8 size 6 noElements 7 7 http://ecomputernotes.com

24. Use of Queues Out of the numerous uses of the queues, one of the most useful is simulation . A simulation program attempts to model a real-world phenomenon. Many popular video games are simulations, e.g., SimCity, FlightSimulator Each object and action in the simulation has a counterpart in real world. http://ecomputernotes.com

25. Uses of Queues If the simulation is accurate, the result of the program should mirror the results of the real-world event. Thus it is possible to understand what occurs in the real-world without actually observing its occurrence. Let us look at an example. Suppose there is a bank with four tellers. http://ecomputernotes.com

26. Simulation of a Bank A customer enters the bank at a specific time (t 1 ) desiring to conduct a transaction. Any one of the four tellers can attend to the customer. The transaction (withdraw, deposit) will take a certain period of time (t 2 ). If a teller is free, the teller can process the customer’s transaction immediately and the customer leaves the bank at t 1 +t 2 . http://ecomputernotes.com