Queues

CSCI 3333 Data Structures

Acknowledgement

Dr. Bun Yue Mr. Charles Moen Dr. Wei Ding Ms. Krishani Abeysekera Dr. Michael Goodrich Dr. Richard F. Gilberg

Queues

First In First Out (FIFO) data structure.

Queue Concept

Queues

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

Queue Operations

Some 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 or front on an empty queue throws an EmptyQueueException

Queue ExampleOperation Output Q enqueue(5) – (5)enqueue(3) – (5, 3)dequeue() 5 (3)enqueue(7) – (3, 7)dequeue() 3 (7)front() 7 (7)dequeue() 7 ()dequeue() “error” ()isEmpty() true ()enqueue(9) – (9)enqueue(7) – (9, 7)size() 2 (9, 7)enqueue(3) – (9, 7, 3)enqueue(5) – (9, 7, 3, 5)dequeue() 9 (7, 3, 5)

Some Implementation Criteria

As queues are general data structures for widespread use, there are variations to fit different applications. Some considerations: Implementation: array vs. linked list. Thread safe or not. Blocking or not. Synchronous or not. Allow null elements or not.

Applications of Queues

Some direct applications Waiting lists, bureaucracy Access to shared resources (e.g., printer) Simulation Breadth-first traversals Multiprogramming Messaging

Indirect applications Auxiliary data structure for algorithms Component of other data structures

Array-based Queue Use an array of size N in a circular fashion Two variables keep track of the front and rear

f index of the front elementr index immediately past the rear element

Array location r is kept empty

Q0 1 2 rf

normal configuration

Q0 1 2 fr

wrapped-around configuration

Queue Operations

We use the modulo operator (remainder of division)

Algorithm size()return (N f + r) mod N

Algorithm isEmpty()return (f r)

Q0 1 2 rf

Q0 1 2 fr

Queue Operations (cont.)

Algorithm enqueue(o)if size() = N 1 then

throw FullQueueException else

Q[r] or (r + 1) mod N

Operation enqueue throws an exception if the array is full

This exception is implementation-dependent

Q0 1 2 rf

Q0 1 2 fr

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 Nreturn o

Q0 1 2 rf

Q0 1 2 fr

Linked List Implementation

We have already implemented methods necessary for the queue ADT (under different method names) using a singly linked list.

Application: Round Robin Schedulers

We can implement a round robin scheduler using a queue, Q, by repeatedly performing the following steps:

1. e = Q.dequeue()2. Service element e3. Q.enqueue(e)

The Queue

Shared Service

1. Deque the next element

3. Enqueue the serviced element

2 . Service the next element

Palindrome in C++: Spot the error #include “Stack.h”;#include “Queue.h”;#include <iostream.h>using namespace std;int main() { // Incorrect solution. Can you see it?

Stack S;Queue Q;char ch;cout << “Enter string to be tested as palindrome: “;while (cin >> ch) {

S.push(ch);Q.enqueue(ch);

}bool pal = true;while (!Q.isEmpty())

pal = Q.dequeue() = = S.pop();if (pal)

cout << “Palindrome!!” << endl;return EXIT_SUCCESS; }

Example: Queue in Ruby

Ruby is a newer object oriented scripting language using some software engineering principles.

We use a queue example in Ruby to illustrative the use of queues for concurrent processes.

There is no need to understand all details of the example.

Queue.rbrequire 'thread'queue = Queue.new

producer = Thread.new do # concurrent thread 1 5.times do |i| sleep rand(i) # simulate expense queue << i puts "#{i} produced" endend

consumer = Thread.new do # concurrent thread 2 5.times do |i| value = queue.pop sleep rand(i/2) # simulate expense puts "consumed #{value}" endend

consumer.join

Executing Queue.rb

0 produced1 producedconsumed 02 producedconsumed 1consumed 23 producedconsumed 34 producedconsumed 4

Questions and Comments?