Queues

Chapter 5

Queue Definition• A queue is an ordered collection of data

items such that:– Items can be removed only at one end

(the front of the queue)– Items can be added only at the other end

(the back of the queue)

Basic Operations

• construct a queue (usually empty)• empty Check if queue is empty• addQ Add an item to back of

queue• front Retrieve element at front of

queue• RemoveQ Remove element at front of

queue

Queue FAQs

• A queue is a FIFO structureFirst-In First-Out

• Examples of queues– I/O buffers (stream of bytes from keyboard)– Scheduling queues in a multiuser OS – Printer queue for

(;;){while (

printerQueue.empty()) sleep

1;printFile = printerQueue.removeQ();Print

(printFile);}

Scheduling Queues

• Resident queue– on disk waiting for

memory• Ready queue

– in memory, onlyneeds CPU to run

• Suspended queue– waiting for I/O transfer or to be reassigned to

the CPU

Contrasting Stacks and Queues

Stacks Queues•LIFO•Elements stored in reverse of order received•Used in applications where reversal or "unwinding" needed

•FIFO•Elements stored in same order as received

•Used where service rendered is relative to order received

Interesting Application Evaluate If A String Is A Palindrome

#include “Stack.h”;#include “Queue.h”;#include <iostream.h>using namespace std;int main(){

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

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

}

bool pal = true;while (!Q.empty())

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

cout << “Palindrome!!” << endl;return 0;

}

This code will not do what it claims. Do a white box

test of the code.

Implementation with An Array

Requirements• Array space for storage for the data • A pointer to indicate the front of the queue• A pointer to indicate the back of the queue

• Structures would includeitemType myQueue [queueQuantity];int myFront, myBack;

Implementation with An Array• Additions to the queue

– place new element where back points, increment myBack

• Deletions from the queue– increment myFront

• What problems can you see with this?myFront myBack

myQueue

Implementation with An Array

• Obviously we would run out of space

• Possible solutions– Shifting the array elements downward with

each deletion– Viewing array as a circular buffer, i.e.

wrapping the end to the front

"Circular" Array-Implementation

• Wraparound keeps the addition/deletion cycle from walking off the edge of the array

• Given itemType myQueue [queueQuantity];int myFront, myBack;

• When myBack (or myFront) hits the end of myQueue, it should wrap around to the beginningmyBback = (myBack + 1) % queueQuantity;

"Circular" Array-Implementation

• Initially queue is emptymyFront = myBack = 0;

• How do we know queue is empty at any time?? myFront == myBack

• What would happen if array fills up … what would be value of myFront and myBack?– they would be the same … why?

• So … how to tell empty from full??

"Circular" Array-Implementation

Common solutions• Keep an empty slot between myFront and myBack– declare array asitemType myQueue [queueQuantity +1];

• Keep an auxiliary counter to track actual number of elements in queue

Queue Class SpecificationNote again, capacity and

element type declared outside class.

#ifndef QUEUE#define QUEUEconst int QUEUE_CAPACITY = 128;typedef int QueueElement;class Queue{ /***** Function Members *****/ public:

Queue();bool empty() const;bool full() const;void addQ(const QueueElement & value);QueueElement front const(); //nondestructive “peek”QueueElement removeQ(); // retrieve and remove

/***** Data Members *****/ private:

QueueElement myArray[QUEUE_CAPACITY];int myFront,

myBack;}; // end of class declaration#endif

Queue Class Implementation

• Use the class declaration to write the implementation

• Also write a driver to test the implementation• It should

– instantiate multiple queues– test for empty, full– add, remove, and look at the front element– enter values from keyboard, from file– print output of queue to screen, to file

Linked List Implementation

• Interface would remain the same– empty(), full(), front(), addQ(), removeQ()

• The storage structure would be a linked list.

• The markers would be pointers• instead of indices into an array• myFront contains pointer to first node• myBack contains pointer to last node

Linked List Implementation• empty

– returns true if myFront points to NULL• addQ

– allocates new node– link it off myBack– update myBack

• removeQ– remove and return first element– update myFront

Linked List Implementation• front

– returns value of node where myFront points to

• full???– returns true if myFront = = myBack

• Write specification and implementation as an exercise

Dequeue

• Double-ended-queue• Similar to a queue• But … additions and deletions can occur

at either end• Methods must have

– either a parameter specifying at which end the action should occur

– or multiple methods (functions) provided

addQ( ) Example for Dequeue

Version with parameter for where addition takes place

Multiple Method Versionvoid Deque::push_front( int item){ if (( myBack +1)% QUEUE_CAPACITY == myFront)

cout << "FULL, cannot add to queue." << endl; else // enqueue at front { if (! myFront)

myFront = QUEUE_CAPACITY;else

myFront--;myArray[ myFront] = item;

} return;}

void Deque::push_back( int item){ if (( myBack +1)% QUEUE_CAPACITY == myFront)

cout << "FULL, cannot add to queue." << endl; else // regular enqueuing {

myArray[ myBack] = item;myBack = ( myBack+ 1) % QUEUE_CAPACITY;

} return;}

Multiple Methods for Removing

Comments on deque

• A standard container provided by the STL• Easily implemented by the vector type• More coming in Chapter 6

Priority Queues

• Definition : A queue where the order of the elements …– is by some priority– rather than by the order in which they are

received• Any queue implementation can be used

– Only the addQ() method must be altered– Element is not added at back– Queue must be scanned for correct location

for insertion

Priority Queues

• What would be the drawback of using an implementation that used arrays?

– shifting elements

• Hence a linked list implementation is preferred

Priority Queues

What changes would be made to this to use a linked list version?