QUEUE OPERATIONS in DATASTRUCTURE AND ALGORITHMS

QUEUE OPERATIONS in DATASTRUCTURE AND ALGORITHMS

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  QUEUE By Mrs. P. Usha.,MCA., M.Phil., SET.,NET.,(Ph.D) Assistant Professor, Department of Information Technology, Bishop Heber College (Autonomous), Trichy-17.
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  QUEUE  Queue isan abstract (LINEAR) data structure  A queue is open at both its ends. One end is always used to insert data (enqueue) and the other is used to remove data (dequeue).  Element is inserted from one end called the REAR(also called tail), and the removal of existing element takes place from the other end called as FRONT(also called head).  Queue follows First-In-First-Out methodology, i.e., the data item stored first will be accessed first.
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  Basic features ofQueue 1.Like stack, queue is also an ordered list of elements of similar data types. 2.Queue is a FIFO( First in First Out ) structure. 3.Once a new element is inserted into the Queue, all the elements inserted before the new element in the queue must be removed, to remove the new element. 4.peek( ) function is oftenly used to return the value of first element without dequeuing it.
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  Basic Operations ofQueue • Enqueue: Add an element to the end of the queue • Dequeue: Remove an element from the front of the queue • IsEmpty: Check if the queue is empty • IsFull: Check if the queue is full • Peek: Get the value of the front of the queue without removing it
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  Applications of QueueData Structure • CPU scheduling, Disk Scheduling • When data is transferred asynchronously between two processes.The queue is used for synchronization. eg: IO Buffers, pipes, file IO, etc • Handling of interrupts in real-time systems. • Call Center phone systems use Queues to hold people calling them in an order
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  Algorithm for ENQUEUEoperation • Step 1 − Check if the queue is full. • Step 2 − If the queue is full, produce overflow error and exit. • Step 3 − If the queue is not full, increment rear pointer to point the next empty space. • Step 4 − Add data element to the queue location, where the rear is pointing. • Step 5 − return success.
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  Case 1: A Bc D e F RN F=1 R=5 N=5 Step 1: FRONT=1 and REAR=N F=1 1=1 true & REAR = N 5=5true OR FRONT=REAR+1FRONT=6 Print Overflow Case 2: Let us the queue Front F=1 R=3 N=5 ITEM=D A B c F R N Step 1: FRONT=1 & REAR=N FRONT=1  1=1 true & REAR=N  3=5false OR FRONT=REAR+11=4False Step 2: FRONT=NULL FRONT=01=0false Else if REAR=N3=5false Else Set REAR=REAR+1REAR=3+1=4REAR=4 Step 3: QUEUE[REAR]= ITEM QUEUE[4]=D A B c D F R N Step 4: Exit
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  Dequeue Operation • Step1 − Check if the queue is empty. • Step 2 − If the queue is empty, produce underflow error and exit. • Step 3 − If the queue is not empty, access the data where front is pointing. • Step 4 − Increment front pointer to point to the next available data element. • Step 5 − Return success.
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  A Case 1: FR N FRONT=1REAR=1 N=5 Step 1: FRONT=-11=-1False Step 2: Element=Queue[FRONT] Element=Queue[1] Element=A ADeleted Step 3: FRONT=REAR1=1true FRONT=REAR=-1 Step 4: Return Case 2: FRONT=-1 REAR=-1 N=5 Step 1: FRONT=-1-1=- 1true Print “UNDERFLOW” & Return. F=1 N=5 R=4 Case 3: A B c d F R N Step 1: FRONT=-1 1=-1false Step 2: Element=Queue[FRONT] Element= Queue[1]Element=A A deleted B c d F R N Step 3: FRONT=REAR 1=4false FRONT=FRONT+1FRONT=1+1=2 FRONT=2 B c d F R N