1 queues and priority queues chapter 8. 2 introduction to queues a queue is a waiting line – seen...

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Queues and Priority Queues

Chapter 8

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Introduction to Queues

• A queue is a waiting line – seen in daily life– Real world examples – toll booths, bank, food– Plenty of CS examples:

• Printer queue, server queues• Event queue for programs (controls program

interaction)• Game programming queue (Networking Game

Programming) • Communication queues – between threads

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Queue Basics

• A queue is a sequence of data elements• In the sequence

– Items can be removed only at the front– Items can be added only at the other end, the back

• 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)

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Designing and Building a Queue ClassArray-Based

• Consider an array in which to store a queue

• Note additional variables needed– myFront, myBack

• Picture a queueobject like this

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• Problems– We quickly "walk off the end" of the array

• Possible solutions– Shift array elements– Use a circular queue

– Note that both emptyand full queuegives myBack == myFront


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• Using a static array–QUEUE_CAPACITY specified

– Enqueue increments myBack using mod operator, checks for full queue

– Dequeue increments myFront using mod operator, checks for empty queue


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Using Dynamic Array to Store Queue Elements

• Similar problems as with list and stack– Fixed size array can be specified too large or too

small

• Dynamic array design allows sizing of array for multiple situations

• Results in structure as shown–myCapacity

determinedat run time

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Linked Queues

• Even with dynamic allocation of queue size– Difficult to adjust during run of program

• Could use linked list to store queue elements– Can grow and shrink to fit the situation

– No need for upper bound (myCapacity)

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Linked Queues

• Constructor initializes myFront, myBack

• Front– return myFront->data

• Dequeue– Delete first node (watch for empty queue)

• Enqueue– Insert node at end of list

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Application of Queues: Buffers and Scheduling

• Important use of queues is I/O scheduling– Use buffers in memory to improve program

execution

– Buffer arrangedin FIFO structure

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• Also times when insertions, deletions must be made from both ends– Consider a scrolling

window on the screen

• This requires a doubleended queue– Called a deque (pronounced "deck")

– Could also be considered a double ended stack (or "dack")

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• Consider a keyboard buffer– Acts as a queue

– But elements may be removed from the back of the queue with backspace key

• A printer spool is a queue of print jobs

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• Queues used toschedule taskswithin an operating system

• Job moves from disk to ready queue

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• Ready queue may actuallybe a priority queue … job may get to "cut the line" based on its priority

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Deque“double-ended queue”

• Push and pop from either side• Deques have the following properties:

– Individual elements can be accessed by their position index.

– Iteration over the elements can be performed in any order.

– Elements can be efficiently added and removed from any of its ends (either the beginning or the end of the sequence).

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Deque• What methods are available?

Iterators:begin Return iterator to beginning end Return iterator to end rbegin Return reverse iterator to reverse beginning rend Return reverse iterator to reverse end

Capacity:size Return size max_size Return maximum size resize Change size (public member functions)empty Test whether container is empty

Element access:operator[] Access element at Access element front Access first element back Access last element

Modifiers:assign Assign container content push_back Add element at the end push_front Insert element at beginning pop_back Delete last element pop_front Delete first element insert Insert elements erase Erase elements swap Swap content clear Clear content

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Deque#include <iostream>#include <deque>using namespace std;int main (){ unsigned int i; deque<int> first; deque<int> second (4,100); deque<int> third (second.begin(),second.end()); deque<int> fourth (third);

for (deque<int>::iterator it = third.begin();it!=third.end();it++) cout << *it << " "; cout << endl; int myints[] = {16,2,77,29}; deque<int> fifth (myints, myints + sizeof(myints) / sizeof(int) ); cout << "The contents of fifth are:"; for (i=0; i < fifth.size(); i++) cout << " " << fifth[i]; cout << endl; return 0;}

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Deque

• How does it work internally?• Divided in several chunks of

storage• Deque class keeps all this

information and provides a uniform access to the elements.

• Deques are a little more complex internally.

1 queues and priority queues chapter 8. 2 introduction to queues a queue is a waiting line – seen...

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