Data structures & Algorithms ssss-04.ppt

Data structures & Algorithms ssss-04.ppt

• 1.
  Queue
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  What is aqueue?  It is an ordered group of homogeneous items of elements.  Queues have two ends:  Elements are added at one end.  Elements are removed from the other end.  The element added first is also removed first (FIFO: First In, First Out).
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  Note: In queue deletioncan take place only at one end called FRONT end and insertion can take place only at the other end called REAR end. The order in which the element enter a queue is the order in which they will leave 3
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  Purpose of queue 4 The purpose of queue is to provide some form of buffering. In a computer system, queues are used  Process Management: For example, in a timesharing system in computer, programs are added to queue and are executed one after the other.  Buffer between the fast computer and a slow printer: Documents sent to the printer for printing is added to a queue. The document sent first is printed first and document sent last is printed last.
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  Example of queue An important example of queue in computer science occurs in a timesharing system in which programs with different priorities form a queue while waiting to be executed (priority queue).  Queues used in everyday life. The automobiles are waiting to pass through an intersection from a queue in which the first car is line is the first can through,  The people is waiting in line at a bank from a queue, where the first person in line is the first person to be waited on and so on. 5
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  Operation on Queue 7 1.Insertion or enqueue ( ): The insertion operation [insert (QUEUE, ITEM)] inserts an ITEM at the REAR of a QUEUE. 2. Deletion or dequeue ( ): The deletion operation [remove (QUEUE, ITEM)] removes the ITEM from the FRONT of a QUEUE. 3. Empty: The third operation [empty (QUEUE)] return FALSE when the QUEUE is not empty otherwise it returns TRUE if the QUEUE is empty. 4. Full: There is another operation performed on a QUEUE. The full operation [full (QUEUE)]) return TRUE value if the QUEUE is full otherwise it returns FALSE.
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  Representation of Queues Queue can be represented in the computer memory in various ways usually in one way list or linear array.  Queue which we will mention the algorithm will be a linear array 'Queue' and two pointer variables.  FRONT will contain the location number of the front element of the queue and REAR will contain the location number of the rear element of the queue.  The condition FRONT = NULL will indicate that the queue is empty. 8
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  Implementation of Queueusing Array 11
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  Note: 12
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  1) Algorithm toinsert in Q  ALGORITHM: INSERTION (FRONT,REAR,ITEM, MAX , QUEUE)  Step #01: [Check for overflow] If (Rear = = max-1) then Write(“Overflow or Queue is full”) Return null or exit;  Step # 02: [Set REAR Pointer] If ( REAR = = -1 and FRONT = = -1) then REAR= FRONT = 0 Else REAR= REAR+1 [ End of if structure]  Step # 03: [Insert the item] QUEUE [REAR]=ITEM  Step # 04: [Finish] Exit 13
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  2) Algorithm todelete from Q  ALGORITHM: Deletion (FRONT,REAR,ITEM, QUEUE)  Step #01: [Check for under flow] If ( FRONT = = -1 and REAR = = -1) then Write(“Under flow or Queue is empty”) Return null or exit;  Step # 02: [Delete the item] ITEM = QUEUE [FRONT]  Step # 03: [Set FRONT Pointer] If ( FRONT = = REAR) then FRONT = REAR = -1 Else FRONT= FRONT+1 [ End of if structure]  Step # 04: [Finish] Exit 14
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   Deque  Deque,pronounced “deck”, which stands for double- ended queue.  A deque is a linear list in which elements can be added or removed at either end but not at the middle.  A deque, also known as a double-ended queue, is an ordered collection of items similar to the queue.  It has two ends, a front and a rear, and the items remain positioned in the collection. What makes a deque different is the unrestrictive nature of adding and removing items.  New items can be added at either the front or the rear. Likewise, existing items can be removed from either end. 15
• 16.
  Deque  In asense, this hybrid linear structure provides all the capabilities of stacks and queues in a single data structure.  It is important to note that even though the deque can assume many of the characteristics of stacks and queues, it does not require the LIFO and FIFO orderings that are enforced by those data structures.  It is up to you to make consistent use of the addition and removal operations. 16
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  Types of Deque a)Input restricted Deque: It is the type of Deque which allows insertion from one end and deletion from both ends. b) Output restricted Deque: It is the type of Deque which allows deletion from one end and insertion from both ends. 17