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Web Based Services, Lecture Notes - Computer Science - 6, Study notes of Computers and Information technologies

University of SussexComputers and Information technologies

Data structures, Queue, FIFO, Implementation using link list

Typology: Study notes

2010/2011

Uploaded on 09/09/2011

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Data Structures G5029
Kingsley Sage
Room 5C16, Pevensey III
[email protected]
© University of Sussex 2006
Lecture 3
Queues and Linked Lists
Lecture 3
Queues
–A typical analogy is the “queue of people”.
–A way of buffering a stream of objects, in which the the First In
is the First Out (FIFO).
–What might we use a queue for?
Functional requirements of a queue.
–A queue is similar to a stack, except that you join the queue at
one end and leave it at the other.
–We shall again use an array to storing the items in the queue.
–We shall need two pointers, one to indicate where the next item
to be placed in the queue should be stored (top), and another
to indicate the location of the next item to be retrieved (base).
–We shall treat the underlying array as a circular buffer.
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Data Structures G

Kingsley Sage

Room 5C16, Pevensey III

[email protected]

© University of Sussex 2006

Lecture 3

Queues and Linked Lists

Lecture 3

 Queues

– A typical analogy is the “queue of people”.

– A way of buffering a stream of objects, in which the the First In

is the First Out (FIFO).

– What might we use a queue for?

 Functional requirements of a queue.

– A queue is similar to a stack, except that you join the queue at

one end and leave it at the other.

– We shall again use an array to storing the items in the queue.

– We shall need two pointers, one to indicate where the next item

to be placed in the queue should be stored (top), and another

to indicate the location of the next item to be retrieved (base).

– We shall treat the underlying array as a circular buffer.

Queues – Conceptual diagram

top

base

Queues – Conceptual diagram

Adding an item

base

top

Simple implementation

 Items to be enqueued will be written in the array at

queue[top].

 Items to be dequeued will be read from queue[base].

 After each write top is incremented by one.

 After each read base is incremented by one.

 The amount by which top is “in the lead” is given by a

variable size which will always lie between 0 when the

queue is empty and queue.length when the queue

is full.

 Incrementing of top and base is always modulo

queue.length.

public class SimpleQueue { private Object[] queue; private int size; // size of the queue private int top; // for next insertion private int base; // for next removal public SimpleQueue(int capacity) { queue = new Object[capacity]; size = top = base = 0; } public boolean isEmpty() { return (size == 0); } public boolean isFull() { return (size==queue.length); } public void enqueue(Object item) { if (isFull()) { throw new RuntimeException(“queue overflow”); } else { queue[top++] = item; if (top == queue.length)top = 0; ++size; } } public Object dequeue() { if (isEmpty()) { throw new RuntimeException(“queue underflow”); } else { Object item = queue[base++]; if (base == queue.length)base = 0; --size; return item; } } }

Simple implementation

 We can use modulo arithmetic as an

alternative bounds length checking ..

if (top == queue.length) {

top = 0;

// Can use modulo arithmetic instead ..

top = top % queue.length;

// which can also be written as ..

top %= queue.length

Simple implementation

 We can use techniques similar to the ones we

saw in the previous lecture for stacks to define

an interface class for queues, and to increase

the queue size by copying it to a larger one if

required. See on-line course notes for details.

Linked lists implementation

 We define a Java class to represent a list

member (or node) …

class ListNode {

Object data;

ListNode next;

ListNode(Object data, ListNode next) {

this.data = data;

this.next = next;

// this operator is self referential …

Stacks using linked lists

import java.util.NoSuchElementException; public class StackList implements Stack { private ListNode top; public StackList() { top = null; } public boolean isEmpty() { return (top == null); } public void push(Object item) { top = new ListNode(item, top); } public Object pop() { if (top == null) { throw new NoSuchElementException(); } else { Object item = top.data; top = top.next; return item; } } }

Linked lists

 We shall see more about linked lists in the

next lecture.

 For details of the linked list implementation of

the queue interface, see the on-line course

notes.

Next time …

 Enumerations and iterators.

 More on linked lists.