Lists3 Study Guide
Author: Kartik Kapur and Wayne Li

## Lecture Code

Code from this lecture available at

## Overview

SLList Drawbacks addLast() is slow! We can’t add to the middle of our list. In addition, if our list is really large, we have to start at the front, and loop all the way to the back of our list before adding our element.

A Naive Solution Recall that we cached the size of our list as an instance variable of SLList. What if we cached the last element in our list as well? All of a sudden, addLast() is fast again; we access the last element immediately, then add our element in. But removeLast() is still slow. In removeLast(), we have to know what our second-to-last element is, so we can point our cached last variable to it. We could then cache a second-to-last variable, but now if I ever want to remove the second-to-last element, I need to know where our third-to-last element is. How to solve this problem?

DLList The solution is to give each IntNode a prev pointer, pointing to the previous item. This creates a doubly-linked list, or DLList. With this modification, adding and removing from the front and back of our list becomes fast (although adding/removing from the middle remains slow).

Incorporating the Sentinel Recall that we added a sentinel node to our SLList. For DLList, we can either have two sentinels (one for the front, and one for the back), or we can use a circular sentinel. A DLList using a circular sentinel has one sentinel. The sentinel points to the first element of the list with next and the last element of the list with prev. In addition, the last element of the list’s next points to the sentinel and the first element of the list’s prev points to the sentinel. For an empty list, the sentinel points to itself in both directions.

Generic DLList How can we modify our DLList so that it can be a list of whatever objects we choose? Recall that our class definition looks like this:

public class DLList { ... }


We will change this to

public class DLList<T> { ... }


where T is a placeholder object type. Notice the angle bracket syntax. Also note that we don’t have to use T; any variable name is fine. In our DLList, our item is now of type T, and our methods now take T instances as parameters. We can also rename our IntNode class to TNode for accuracy.

Using Generic DLList Recall that to create a DLList, we typed:

DLList list = new DLList(10);


If we now want to create a DLList holding String objects, then we must say:

DLList<String> list = new DLList<>("bone");


On list creation, the compiler replaces all instances of T with String! We will cover generic typing in more detail in later lectures.

Arrays Recall that variables are just boxes of bits. For example, int x; gives us a memory box of 32 bits. Arrays are a special object which consists of a numbered sequence of memory boxes! To get the ith item of array A, use A[i]. The length of an array cannot change, and all the elements of the array must be of the same type (this is different from a Python list). The boxes are zero-indexed, meaning that for a list with N elements, the first element is at A[0] and the last element is at A[N - 1]. Unlike regular classes, arrays do not have methods! Arrays do have a length variable though.

Instantiating Arrays There are three valid notations for creating arrays. The first way specifies the size of the array, and fills the array with default values:

int[] y = new int[3];


The second and third ways fill up the array with specific values.

int[] x = new int[]{1, 2, 3, 4, 5};
int[] w = {1, 2, 3, 4, 5};


We can set a value in an array by using array indexing. For example, we can say A[3] = 4;. This will access the fourth element of array A and sets the value at that box to 4.

Arraycopy In order to make a copy of an array, we can use System.arraycopy. It takes 5 parameters; the syntax is hard to memorize, so we suggest using various references online such as this.

2D Arrays We can declare multidimensional arrays. For 2D integer arrays, we use the syntax:

int[][] array = new int[4][];


This creates an array that holds integer arrays. Note that we have to manually create the inner arrays like follows:

array[0] = new int[]{0, 1, 2, 3};


Java can also create multidemensional arrays with the inner arrays created automatically. To do this, use the syntax:

int[][] array = new int[4][4];


We can also use the notation:

int[][] array = new int[][]{{1}, {1, 2}, {1, 2, 3}}


to get arrays with specific values.

Arrays vs. Classes

• Both are used to organize a bunch of memory.
• Both have a fixed number of “boxes”.
• Arrays are accessed via square bracket notation. Classes are accessed via dot notation.
• Elements in the array must be all be the same type. Elements in a class may be of different types.
• Array indices are computed at runtime. We cannot compute class member variable names.

## Exercises

### C Level

1. Complete the exercises from the online textbook here and here.

2. Can you create a 2 dimensional array with different types? For example, one sub array would be composed of all Strings and another sub array would be made of only ints.

### B Level

1. At each step follow the instructions

 public class Deck{
public static  int[] cards;
Deck(){
cards = [1, 3, 4, 10];
}
}


Write down the contents of dingie’s array cards.

   Deck dingie = new Deck();
dingie.cards[3] = 3;


Write the contents of pilates’s array and dingie’s array.

   Deck pilates = new Deck();
pilates.cards[1] = 2;


Write the contents of pilates’s array and dingie’s array.

   int[] newArrWhoDis = {2, 2, 4, 1, 3};
dingie.cards = pilates.cards;
pilates.cards = newArrWhoDis;
newArrWhoDis = null;

2. Say we have a 2 dimensional DList. We want this 2-D DList to be as even as possible. To do this we will try to fill up rows as uniformly as possible- meaning that not row will have a greater size than any other row by more than 1 element. Write a method that will take in a sub DList and add the given element if it fulfills the constraints. If the constraints are not fulfilled, the item will be attempted to be put in the DList below the one you attempted to insert in originally and so forth until the bottom most DList is reached in which case move to the top DList.

### A level

1. Complete problem 10 from practice midterm 1 in Kartik’s textbook

2. Complete problem 7 from midterm 1 from Spring 2015 here

### A+ level

1. You are a lonesome tortoise trying to find their way in the galaxy. Suddenly, a portal opens up in front of you and holy mackerel, it leads to a new galaxy! You almost leap into the new galaxy, but you are scared- after all, you want to be able to come home so, at some point, you can have your porridge. After some asking around, you realized that at certain points in a given galaxy you can transport to a different galaxy (though you may not be able to take the same point back); however, if you are in a galaxy, you can reach any point in that galaxy for sure.

You want to try to maximize the amount of galaxies that you can visit while still ensuring that you can make it back home. Provide an idea of how you will do this (no need to code; however, you can if you want).