Queues

04/15/23 CS 201

Queue ADT

• Queues implement the FIFO (first-in first-out) policy– An example is the printer/job queue!

enqueue(o)dequeue()

isEmpty()getFront() createQueue()

04/15/23 CS 201

Sample OperationQueue *Q;

enqueue(Q, “a”);

enqueue(Q, “b”);

enqueue(Q, “c”);

d=getFront(Q);

dequeue(Q);

enqueue(Q, “e”);

dequeue(Q);

q

front back

a b c e

d

04/15/23 CS 201

Queue ADT interface

• The main functions in the Queue ADT are (Q is the queue)

void enqueue(o, Q) // insert o to back of Q void dequeue(Q); // remove oldest item Item getFront(Q); // retrieve oldest item boolean isEmpty(Q); // checks if Q is empty boolean isFull(Q); // checks if Q is full

void clear(Q); // make Q empty }

04/15/23 CS 201

Implementation of Queue (Linked List)

• Can use LinkedListItr as underlying implementation of Queues

a1 a2 a3 a4

head tail

Queue

lst

LinkedListaddTail

04/15/23 CS 201

Codestruct Node { int element; Node * next;};

struct QUEUE { Node * front; Node * rear;};

04/15/23 CS 201

More code

More code

04/15/23 CS 201

CELL is a list node

04/15/23 CS 201

Implementation of Queue (Array)

• use Array with front and back pointers as implementation of queue Queue

arr 0 1 7 8 92 3 4 5 6

A B C D E F G

frontback

04/15/23 CS 201

Circular Array• To implement queue, it is best to view arrays as circular structure

0 1 7 8 92 3 4 5 6

A B C D E F G

frontback

front

back

AB

C

DEF

G

0

1

7

8

9

2

3

456

Circular view of arrays.

04/15/23 CS 201

How to Advance

• Both front & back pointers should make advancement until they reach end of the array. Then, they should re-point to beginning of the array

front = adv(front);back = adv(back);

int adv(int p) { return ((p+1) % maxsize); }

Alternatively, use modular arithmetic:

mod operator

int adv(int p) { int r = p+1; if (r<maxsize) return r; else return 0; }

upper bound of the array

04/15/23 CS 201

Sample

Queue *Q;

enqueue(Q, “a”);

enqueue(Q, “b”);

enqueue(Q, “c”);

dequeue(Q);

dequeue(Q);

enqueue(Q, “d”);

enqueue(Q, “e”);

dequeue(Q);

a

Q

F=front B=back

F

B

b c d

F

B B B

F F

B B

e

04/15/23 CS 201

Checking for Full/Empty State

What does (F==B) denote?

FB

QueueEmptyState

c de

B

F

f QueueFullState

size 0 size 4

c de

B F

Alternative - Leave a Deliberate Gap!

No need for size field.

Full Case : (adv(B)==F)

04/15/23 CS 201

Summary• The definition of the queue operations

gives the ADT queue first-in, first-out (FIFO) behavior

• The queue can be implemented by linked lists or by arrays

• There are many applications– Printer queues,– Telecommunication queues,– Simulations,– Etc.