1. queue

3. Avoid confusion

4. Britain
Italy

6. Applications of Queues
Direct applications
Waiting lists, bureaucracy
Access to shared resources (e.g., printer)
Multiprogramming
Simulations
Indirect applications
Auxiliary data type for algorithms
Component of other data types
ADS Lecture 11

7. The Queue ADT (GoTa §5.2) Auxiliary queue operations: Exceptions
Queues
11/19/2018 7:59 AM
The Queue ADT (GoTa §5.2)
The Queue ADT stores arbitrary objects
Insertions and deletions follow the first-in first-out scheme
Insertions are at the rear of the queue and removals are at the front of the queue
Main queue operations:
enqueue(object): inserts an element at the end of the queue
object dequeue(): removes and returns the element at the front of the queue
Auxiliary queue operations:
object front(): returns the element at the front without removing it
integer size(): returns the number of elements stored
boolean isEmpty(): indicates whether no elements are stored
Exceptions
Attempting the execution of dequeue on front of an empty queue throws an EmptyQueueException
ADS Lecture 11

9. Queue interface in Java contd.
/**
* Inspects the element at the front of the queue
element at the front of the queue
EmptyQueueException if the queue is empty */
public E front() throws EmptyQueueException;
* Inserts an element at the rear of the queue
element new element to be inserted
public void enqueue (E element);
* Removes the element at the front of the queue
element removed
public E dequeue() throws EmptyQueueException; }
ADS Lecture 11

10. Queue Interface in Java
Java interface corresponding to our Queue ADT
Requires the definition of class EmptyQueueException
No corresponding built-in Java class
public interface Queue<E> {
/**
* Returns the number of elements in the queue
number of elements in the queue */
public int size();
* Returns whether the queue is empty
true if queue is empty, false otherwise
public boolean isEmpty();
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11. A naïve implementation
We could have a one dimensional array Q
An integer pointing to the front of the queue and
integer pointing to the end of the queue
Q.enqueue(x) would do this
increment end
Q[end] = x
Need also Q.peek(), Q.size(), Q.isEmpty(), Q.isFull()
Q.dequeue() would do this
discuss!!!

12. A naïve implementation
We could have a one dimensional array Q
Q.dequeue() would do this
discuss!!!
result = Q[front];
for (int i=front+1;i<=end;i++) Q[i-1] = Q[i];
end--;
Obviously with tests for empty queue etc.
What is the complexity of dequeue?

13. Can use a restricted/disciplined linked list
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29. Array-based Queue Use an array of size N in a circular fashion
Three variables keep track of the front, rear, and size
f index of the front element
r index immediately past the rear element, where we add new elements (enqueue)
size is number of entries in the queue

30. wrapped-around configuration
Array-based Queue
Use an array of size N in a circular fashion
Three variables keep track of the front, rear, and size
f index of the front element
r index immediately past the rear element, where we add new elements (enqueue)
size is number of entries in the queue
normal configuration Q 1 2 r f
wrapped-around configuration Q 1 2 f r
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31. Queue Operations We use the modulo operator (remainder of division)
Operation enqueue throws an exception if the array is full
This exception is implementation-dependent Q 1 2 r f Q 1 2 f r
Algorithm enqueue(o)
if size() = N then
throw FullQueueException
else
Q[r]  o
r  (r + 1) mod N
Algorithm size()
return size
Algorithm isEmpty()
return size == 0
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32. Queue Operations (cont.)
Operation dequeue throws an exception if the queue is empty
This exception is specified in the queue ADT
Algorithm dequeue()
if isEmpty() then
throw EmptyQueueException
else
o  Q[f]
f  (f + 1) mod N
return o
Pros and cons of array based implementation:
Again: quick and easy, all methods run in constant time
But again, need good idea of capacity a priori
ADS Lecture 11

33. Application: Round Robin Schedulers
We can implement a round robin scheduler using a queue, Q, by repeatedly performing the following steps:
e = Q.dequeue()
Service element e
Q.enqueue(e)
The Queue
Shared
Service 1 .
Dequeue the
next element 3
Enqueue the
serviced element 2
Service the
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34. Double-Ended queue (deque)
Supports insertion & deletion at the front and rear of the queue.
Pronounced “deck”
Fundamental methods are:
addFirst(e)
addLast(e)
removeFirst()
removeLast()
Since deque requires insertion & removal at both ends of list,
using singly linked list would be inefficient
Could use a doubly linked list
See GoTa p. 213
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35. Your mission
See assessed exercise 2