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![Array Implementation
• A queue can be implemented with an array, as
shown here. For example, this queue contains the
integers 4 (at the front), 8 and 6 (at the rear).
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We
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part
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the
array.](https://image.slidesharecdn.com/queues-adt-240128054242-f1a76523/75/Queue-ADT-for-data-structure-for-computer-20-2048.jpg)
![Array Implementation
• The easiest implementation also keeps
track of the number of items in the
queue and the index of the first
element (at the front of the queue), the
last element (at the rear).
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![A Dequeue Operation
• When an element leaves the queue,
size is decremented, and first changes,
too.
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![An Enqueue Operation
• When an element enters the queue,
size is incremented, and last changes,
too.
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![At the End of the Array
• There is special behavior at the end of
the array. For example, suppose we
want to add a new element to this
queue, where the last index is [5]:
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![At the End of the Array
• The new element goes at the front of
the array (if that spot isn’t already
used):
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![Array Implementation
• Easy to implement
• But it has a limited capacity with a fixed array
• Or you must use a dynamic array for an
unbounded capacity
• Special behavior is needed when the rear reaches
the end of the array.
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![Data Structures - Lecture 6 [queues]](https://cdn.slidesharecdn.com/ss_thumbnails/lecture-6queues-150114114904-conversion-gate01-thumbnail.jpg?width=640&height=640&fit=bounds)
Queue ADT for data structure for computer
- 1. Data Structures: Queues & ADT CS 1112 Mona Diab
- 2. Queues • Defini9on of a Queue • Examples of Queues • Design of a Queue Class • Different Implementa9ons of the Queue Class
- 3. Introduction to Queues •A queue is a waiting line • It’s in daily life: – A line of persons waiting to check out at a supermarket – A line of persons waiting to purchase a ticket for a film – A line of planes waiting to take off at an airport – A line of vehicles at a toll booth
- 4. Defini*on of a Queue • A queue is a data structure that models/enforces the first-‐come first-‐serve order, or equivalently the first-‐ in first-‐out (FIFO) order. • That is, the element that is inserted first into the queue will be the element that will deleted first, and the element that is inserted last is deleted last. • A wai9ng line is a good real-‐life example of a queue. (In fact, the Bri9sh word for “line” is “queue”.)
- 5. Queues • Unlike stacksin which elements are popped and pushed only at the ends of the list Collection of data elements: • items are removed from a queue at one end, called the FRONT of the queue; • and elements are added at the other end, called the BACK
- 6. Introduction to Queues •Difference between Stack and Queues: – Stack exhibits last-in-first-out (LIFO) – Queue exhibits first-in-first-out (FIFO)
- 7. The Queue Operations ❐A queue is like a line of people wai9ng for a bank teller. The queue has a front and a rear. $ $ Front Rear
- 8. The Queue Operations ❐New people must enter the queue at the rear. This is called an enqueue opera9on. $ $ Front Rear
- 9. The Queue Operations ❐When an item is taken from the queue, it always comes from the front. This is called a dequeue opera9on. $ $ Front Rear
- 10. Queue Abstract DataType • Basic operations – Construct a queue – Check if empty – Enqueue (add element to back) – Front (retrieve value of element from front) – Dequeue (remove element from front)
- 11. A Graphic Model of a Queue Tail: All new items are added on this end Head: All items are deleted from this end
- 12. Opera*ons on Queues • Insert(item): (also called enqueue) – It adds a new item to the tail of the queue • Remove( ): (also called delete or dequeue) – It deletes the head item of the queue, and returns to the caller. If the queue is already empty, this opera9on returns NULL • getHead( ): – Returns the value in the head element of the queue • getTail( ): – Returns the value in the tail element of the queue • isEmpty( ) – Returns true if the queue has no items • size( ) – Returns the number of items in the queue
- 13. Queue as a Class • Much like stacks and linked lists were designed and implemented as classes, a queue can be conveniently packaged as a class • It seems natural to think of a queue as similar to a linked list, but with more basic opera9ons, to enforce the FIFO order of inser9on and dele9on
- 14. Designing and Buildinga Queue Class Array-Based • Consider an array in which to store a queue • Note additional variables needed – myFront, myBack • Picture a queue object like this
- 15. Queue Operation • EmptyQueue Enqueue(70)
- 16. Queue Operation • Enqueue(80) •Enqueue(50)
- 17. Queue Operation • Dequeue() •Dequeue()
- 18. Queue Operation • Enqueue(90) •Enqueue(60)
- 19. Circular Queue • Problems –We quickly "walk off the end" of the array • Possible solutions – Shift array elements – Use a circular queue – Note that both empty and full queue gives myBack == myFront
- 20. Array Implementation • Aqueue can be implemented with an array, as shown here. For example, this queue contains the integers 4 (at the front), 8 and 6 (at the rear). [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . An array of integers to implement a queue of integers 4 8 6 We don't care what's in this part of the array.
- 21. Array Implementation • Theeasiest implementation also keeps track of the number of items in the queue and the index of the first element (at the front of the queue), the last element (at the rear). [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . 4 8 6 size 3 first 0 last 2
- 22. A Dequeue Operation •When an element leaves the queue, size is decremented, and first changes, too. [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . 4 8 6 size 2 first 1 last 2
- 23. An Enqueue Operation •When an element enters the queue, size is incremented, and last changes, too. [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . 2 8 6 size 3 first 1 last 3
- 24. At the Endof the Array • There is special behavior at the end of the array. For example, suppose we want to add a new element to this queue, where the last index is [5]: [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] 2 1 6 size 3 first 3 last 5
- 25. At the Endof the Array • The new element goes at the front of the array (if that spot isn’t already used): [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] 2 1 6 size 4 first 3 last 0 4
- 26. Array Implementation • Easyto implement • But it has a limited capacity with a fixed array • Or you must use a dynamic array for an unbounded capacity • Special behavior is needed when the rear reaches the end of the array. [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . 4 8 6 size 3 first 0 last 2
- 27. Linked List Implementation 10 15 7 null 13 • A queue can also be implemented with a linked list with both a head and a tail pointer. head_ptr tail_ptr
- 28. Linked List Implementation 10 15 7 null 13 • Which end do you think is the front of the queue? Why? head_ptr tail_ptr
- 29. Linked List Implementation 10 15 7 null head_ptr 13 • The head_ptr points to the front of the list. • Because it is harder to remove items from the tail of the list. tail_ptr Front Rear
- 30. Abstract Data Type • Abstract Data Type as a design tool • Concerns only on the important concept or model • No concern on implementa9on details. • Stack & Queue is an example of ADT • An array is not ADT.
- 31. What is the difference? • Stack & Queue vs. Array – Arrays are data storage structures while stacks and queues are specialized DS and used as programmer’s tools. • Stack – a container that allows push and pop • Queue -‐ a container that allows enqueue and dequeue • No concern on implementa9on details. • In an array any item can be accessed, while in these data structures access is restricted. • They are more abstract than arrays.
- 32. Ques9ons? • Array is not ADT • Is Linked list ADT?