CS221Week 4 - Monday

Last time

What did we talk about last time? Queues Implementing queues with circular

arrays

Questions?

Bitmap Manipulator

Project 1

Assignment 1 issues

Most people messed up the add() method that takes an index

Most people didn't throw exceptions at quite the right times

There was a lot of inconsistent formatting I don't specify what your formatting should be

too closely, but it should be consistent Check out the coding standards for the course:▪ http://users.etown.edu/w/wittmanb/cs221/standards/

Specific Assignment 1 don'ts Don't create a new Node unless you have to:

Never look at the next thing unless you have to:

Node temp = new Node(); //wastes memorytemp = head;

public boolean contains(int element) {Node temp = head;while(temp.next != null){ //two different bugsif( temp.value == element )return true;temp = temp.next;}return false;

}

Specific Assignment 1 do's Use other methods to simplify code:

Except when it doesn't make sense Using get() in addAll() is inefficient

Keep it simple:

Test, test, test!

public boolean contains(int element) {return indexOf(element) != -1;

}

public boolean isEmpty() {return size == 0;

}

Linked Lists

Purpose and design of linked listsImpromptu student lecture

Linked lists

What is a linked list? Why not just use (dynamic) arrays

for everything?

X

head

23 47 58

Pros

Insert at front (or back) O(1)

Delete at front (or back) O(1)

Arbitrary amounts of storage with low overhead

Cons

Search O(n)

Go to index O(n)

Potentially significant memory overhead if data is small

Much easier to make pointer and memory errors (especially in C/C++)

Implementations

Levels of flexibility

Class protecting nodes implementation

Generic class providing nodes with arbitrary type

Generic class with the addition of iterators

Wait, what's an iterator?

I'm glad you asked They allow a collection to be used in a foreach loop So, what's a foreach loop?

It allows you to read (but not change) each value in a list

public static int sum( int[] array ) {int total = 0;for( int value: array )

total += value;return total;

}

So what?

Foreach loops work for any iterable list of any type

public static double weigh(LinkedList<Wombat> list) {double total = 0.0;for( Wombat wombat: list )

total += wombat.getWeight();return total;

}

public static double weigh(ArrayList<Wombat> list) {double total = 0.0;for( Wombat wombat: list )

total += wombat.getWeight();return total;

}

public static double weigh(Wombat[] list) {double total = 0.0;for( Wombat wombat: list )

total += wombat.getWeight();return total;

}

Singly linked list

Node consists of data and a single next pointer

Advantages: fast and easy to implement

Disadvantages: forward movement only

X

head

23 47 58

Doubly linked list

Node consists of data, a next pointer, and a previous pointer

Advantages: bi-directional movement Disadvantages: slower, 4 pointers must

change for every insert/delete Xhead

23 47 58

X tail

Interview question

You are given a singly linked list It may have a loop in it, that is, a

node that points back to an earlier node in the list

If you try to visit every node in the list, you’ll be in an infinite loop

How can you see if there is a loop in a linked list?

Code

Definition

Let’s try a simple definition for a linked list:public class LinkedList {

private static class Node {public int data;public Node next;public Node previous;

}

private Node head = null;private Node tail = null;…

}

Insert at head

Delete from head

Find

Insert in order

Upcoming

Next time…

Continued implementation of a linked list

Circular linked lists and skip lists Implementing a stack with a linked

list Keep reading section 1.3

Reminders

Keep reading section 1.3 Keep working on Project 1

Due this Friday, September 18 by 11:59pm