Queues and its application, Study notes of Data Structures and Algorithms

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Data Structures and Algorithms

LECTURE 09 – QUEUES

Queue

• A queue is a “waiting line”, that grows by adding elements to its end and shrinks by

taking elements from its front

• Unlike stack, a queue is a structure in which both ends are used:
  • One for adding new elements
  • And one for removing elements
• Last element has to wait until all the elements preceding it are removed
• Queue is a FIFO : First In, First Out, structure

Queue Operations

• Enqueue
  • Adding items at the back of queue
• Dequeue
  • Removing items from the front

Queue Operations clear() // clear the queue isEmpty() // check to see if the queue is empty isFull() // check to see if the queue is full enqueue(el) // Add element el at the back of queue dequeue() // remove the first element from the //front firstEl() // return the value of the first element in the // queue without removing it

Queue Implementation

• The queue is empty when
  • First == −
• The queue is full when
  • First == 0 && Last == size- 1
  • OR
  • First == Last +1 (we will see this condition next)

Queue Operations

• Enqueue(66)
• Enqueue(8)
• Enqueue(12)
• Queue is now full

Queue Operations

• Dequeue()
• Two options
  2. Remove the first element and then increment the First index – Circular Queue

Circular Queue

Circular Queue

• Dequeue()

Array Queue Implementation

• Please see the hand out - Queue