Queues and Linked List-Data Structures-Lecture Slides, Slides of Data Structures and Algorithms

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Chap 4. Queues and Linked List

MCCS

A

B

C

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4. Queues

Overview

• Queue. Usage of Queue• Implementation of Queue:

– Array– Linked List

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4. Queues

First-in First-out (FIFO)

When we enqueue entriesin the queue and thendequeue them one by one,we will get the entries inthe same order.

A

B

C

A A

B

B

C

C

The first oneenqueued is thefirst onedequeued. (FIFO)

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4. Queues

Usage of Queue

• Queues in the real world

– Queue of persons in front of counter– Queue of jobs to be done

• Message queue in Windows• Print queue• I/O buffer

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4. Queues

Example

#include <stdio.h>#include "queue.h"int main () {

Queue Q; QueueEntry x;CreateQueue(&Q);Enqueue('a', &Q);Enqueue('b', &Q);Enqueue('c', &Q);PrintQueue(&Q);Dequeue(&x, &Q);PrintQueue(&Q);ClearQueue(&Q);return 0; }

The type Queue and thefunctions CreateQueue,Enqueue and Dequeue aredefined in the queue.h andqueue.cpp.

Remember, call CreateQueuebefore using the queue, andcall ClearQueue when thequeue is no longer used.

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4. Queues

Exercise

#include <stdio.h>#include "stack.h" /* stack of char /#include "queue.h" / queue of char */int main ( ) {

char x;Queue Q; Stack S;CreateQueue(&Q); CreateStack(&S);Enqueue('a', &Q); Enqueue('b', &Q); Enqueue('c', &Q);Enqueue('d', &Q); Enqueue('e', &Q); Enqueue('f', &Q);for (int i=0; i<3; i++) { Dequeue(&x, &Q); Push(x, &S); }while (!StackEmpty(&S)) {

Pop(&x, &S); Enqueue(x, &Q); }

/* draw the content of Q and S at this point of time */return 0; }

Q1. Draw the queueand the stack.

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4. Queues

Circular Array

?

E

F

D

?

?

F E D??^?

rear = 5

entry

front = 3

G

entry

If we view the array as acircle, the cell after the5th cell is the 0th cell.

G

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4. Queues

Array implementation

#define MAXQUEUE 6typedef char QueueEntry;struct Queue {

int count;int front, rear;QueueEntry entry[MAXQUEUE]; };

F E D??^?

entry

D

E

F front = 3^ rear = 5 count = 3

Number of entriesin the Q

Change the definition ofQueueEntry to match yourapplication. E.g. if you needa queue of struct Student,writetypedef Student QueueEntry;

The capacity of thequeue. In arrayimplementation, wehave to fix the sizeof array in compiletime.

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4. Queues

Create Queue

void CreateQueue (Queue *Q) {

Q->count = 0;Q->front = 0;Q->rear = -1;

}

We can also use these values:front : 1

rear :

… 5 = MAXQUEUE - 1

We have to make sure that when we enqueue the firstentry, front and rear point to that entry.

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4. Queues

Enqueue

void Enqueue (QueueEntry x, Queue *Q) {

if (QueueFull(Q))

printf("Cannot … "); else {

Q->count ++;Q->rear = (Q->rear+1) % MAXQUEUE;Q->entry[Q->rear] = x; }

}

This formula is the same asif (Q->rear==MAXQUEUE-1)

Q->rear = 0; else

Q->rear ++;

Remember to update countwhenever the number ofentries in the Q changes!

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4. Queues

Traversing a Queue

void PrintQueue (Queue *Q) {

int c;int i = Q->front;int size=QueueSize(Q);printf("<< ");for (c=1; c<=size; c++) {

printf("%c ", Q->entry[i]);i = (i+1) % MAXQUEUE;

} printf("<<"); }

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4. Queues

Exercise

Q2. int QueueEmpty (Queue *Q);int QueueFull (Queue *Q);void ClearQueue (Queue *Q);int QueueSize (Queue *Q);void QueueFront (QueueEntry *x, Queue *Q);

Write the following functions on

array implementation of Queue.

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4. Queues

Code Listing, 2

#include <stdio.h>#include "queue.h"void CreateQueue (Queue *Q) {

Q->count = 0;Q->front = 0;Q->rear = -1; } void Enqueue (QueueEntry x, Queue *Q) {

if (QueueFull(Q))

printf("Cannot enqueue item into full queue."); else {

Q->count ++;Q->rear = (Q->rear+1) % MAXQUEUE;Q->entry[Q->rear] = x; } }

queue.cpp, p.

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4. Queues

Code Listing, 3

void Dequeue (QueueEntry *x, Queue *Q) {

if (QueueEmpty(Q))

printf("Cannot dequeue from empty queue."); else {

Q->count --;*x = Q->entry[Q->front];Q->front = (Q->front+1) % MAXQUEUE; } }; int QueueEmpty (Queue *Q) {

return Q->count <=0; } int QueueFull (Queue *Q) {

return Q->count >= MAXQUEUE; }

queue.cpp, p.

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