Queues 15-121 Fall 2020 Margaret Reid-Miller Logistics - - PowerPoint PPT Presentation

Queues

15-121 Fall 2020 Margaret Reid-Miller

Logistics

Midsemester Grades:

• 37% A
• 30% B
• 20% C
• 13% < C (mostly poor homework grades)

Exam 2:

• Thursday Nov 5?
• Tuesday Nov 9?

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Today

Today:

• Stack applications
• Queues
• ListQueue class
• Exercise: Reverse queue values
• Circular ArrayQueue

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Queues

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Queue ADT

“last in last out” Operations:

• ENQUEUE – adds an element to the BACK of the

queue

• DEQUEUE – removes the element from the FRONT
• f the queue
• PEEK – returns the element in the front of the queue

without removing it (optional?)

• Test if the queue is EMPTY

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Queue

Uses:

• A store checkout line
• A playlist of songs on a jukebox, mp3 player, etc.
• Putting a new bottle of milk behind the old one.

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Queue Operations

public interface FIFOQueue<E> { void enqueue(E obj); // at back E dequeue(); // from front E peek(); // at front boolean isEmpty(); } Ideally, each of these operations should be O(1) time.

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Queue Implementations

Want O(1) for all operations, so what underlying data structure can we use?

• ArrayList?

No, one of the enqueue or dequeue must be O(n)

• array?

Yes, surprisingly.

• linked list?

Singly? Singly with reference to the tail?

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Doubly? No Yes Yes

Linked List Queue

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Queues using a singly-linked list

• Store the elements of the queue from front to back in
• rder in the list.
• Why do we need an additional reference to the tail?

null front back

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Linked List Implementation

Fields

public class ListQueue<E> implements FIFOQueue<E> { private Node front; private Node back; // methods (next slides) }

references to nodes with front and back queue elements in list

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Linked List Implementation

Constructor & isEmpty

public ListQueue() { front = null; back = null; } public boolean isEmpty() { return (front == null); }

indicates an empty queue

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Linked List Implementation

enqueue

public void enqueue(E obj) { Node newNode = new Node(obj); if (back != null) { back.next = newNode; back = newNode; } else { _________________ _________________ } } null

newNode

null front

❶ ❷

back back = newNode; front = newNode;

❶ ❷

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Linked List Implementation

dequeue

public E dequeue() { if (front == null) throw new NoSuchElementException(); E element = front.data; front = front.next; ______________________________ return element; }

data

null front

❶ ❷ ❶

element data back

❷

if (front == null) back = null;

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Linked List Implementation

peek

public E peek() { if (front == null) throw new NoSuchElementException(); return front.data; }

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Exercise: Reverse

// reverses the queue objects using a stack public static void reverse(FIFOQueue<String> q){

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Reverse

// reverses the queue objects using a stack public static void reverse(FIFOQueue<String> q){ LIFOStack<string> s = new ArrayStack<String>(); while (!q.isEmpty()) s.push(q.dequeue()); while (!s.isEmpty()) q.enqueue(s.pop()); }

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Reverse (recursively)

// reverses the queue objects, recursively public static void reverse(Queue<String> q) { if (!q.isEmpty()) String front = q.dequeue(); reverse(q); q.enqueue(front); }

Where’s the stack?

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Array Queue

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Queues using an array

1 2 3 4 5 6 7 8 9 10 11

• Store the elements of the queue from front to back in
• rder in the array.
• What happens if we store the front of the queue always

in position 0?

• Are all operations O(1)?

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No, dequeue is O(n)

Queues using an array

1 2 3 4 5 6 7 8 9 10 11

• Store the elements of the queue from front to back in
• rder in the array.
• What happens if we store the back of the queue always

in last position in the array?

• Are all operations O(1)?

FRONT BACK Fall 2020 15-121 (Reid-Miller) 22

No, enqueue is O(n)

Queues using an array

1 2 3 4 5 6 7 8 9 10 11

• Track positions of both front and back of queue.
• To enqueue, increment back and add element at back.
• To dequeue, remove the element at front and increment

front.

• What if the back is last element of the array and want to

enqueue?

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Queues using a “circular” array

1 2 3 4 5 6 7 8 9 10 11

• Allow the queue to “wrap around” from the last cell of the

array back to the first cell so no shifting is necessary.

• Problem?
• If the array is full, we can grow the array and copy the

queue data into the new array starting at position 0 again.

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Array Implementation

Fields

public class ArrayQueue<E> implements FIFOQueue<E> { private E[] dataArray; private int front; private int back; private int numElements; // methods (next slides) }

indices of front and back queue elements in array

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Array Implementation

Constructor & isEmpty

public ArrayQueue() { dataArray = (E[])new Object[1]; front = -1; back = -1; numElements = 0; } public boolean isEmpty() { return (numElements == 0); }

indicates an empty queue

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Array Implementation

enqueue

public void enqueue(E element) { if (______________________________________) grow(); back = ____________________________; dataArray[back] = element; if (front == -1) front = back; numElements++; }

numElements == dataArray.length

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(back+1) % dataArray.length

Array Implementation

grow

1 2 3 4 5

FRONT BACK

1 2 3 4 5 6 7 8 9 10 11

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What's wrong? If you keep dequeuing, it tries to dequeue at index 6.

Array Implementation

grow

1 2 3 4 5

FRONT BACK

1 2 3 4 5 6 7 8 9 10 11

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Array Implementation

grow

private void grow() { E[] newArray = (E[])new Object[numElements*2]; int from = front; for (int to = 0; to < numElements; to++) { newArray[to] = dataArray[from]; from = ___________________________; } front = 0; back = numElements-1; dataArray = newArray; }

(from + 1) % numElements

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Array Implementation

peek

public E peek() { if (__________________________) throw new NoSuchElementException(); return dataArray[front]; }

numElements == 0

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Array Implementation

dequeue

public E dequeue() { E obj = peek(); dataArray[front] = null; if (front == back) { // was one element ______________________________________ } else ______________________________________ numElements--; return obj; }

front = -1; back = -1; front = (front+1)%dataArray.length;

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Other uses for queues

• Printer queues
• Packet router
• Simulating a queuing system
• Supermarket checkout lanes
• Highway traffic congestion models
• Internet traffic

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Java Queue<E> interface

• The java.util package includes a Queue<E>

interface.

• Queue<E> includes specifications for these methods:
• boolean offer(E item)
• E element() *
• E peek()

**

• E remove() *
• E poll()

**

* Throws an exception if queue is empty. ** Returns null if the queue is empty. enqueue peek dequeue

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Using Java Queue<E>

• LinkedList<E> implements Queue<E>.
• Example: A queue of customers

Queue<Customer> customerQ = new LinkedList<Customer>(); Customer c = new Customer("Andrew"); customerQ.offer(c); // OK customerQ.addFirst(c); // BAD (Why?)

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