Description
Arrays and ArrayLists
1 Introduction
In this lab you will use two-dimensional arrays and ArrayLists to handle data. In the first section, you will deal with matrices. In the second section, you will perform set operations that can tell you information about different data sets.
2 Exercises
2.1 Two-Dimensional Arrays
In this section, you will write methods that handle 2-dimensional arrays. Exercise 1 Create a class TwoDArrayMethods that implements the following method. 1. public static int[][] transpose(int[][] anArray) c 1 This method receives a 2-dimensional array of integers and returns the transpose of that matrix. Suppose we are given an n by m matrix, A. Then the tranpose of that matrix, denoted AT , is the m by n matrix with entries (A T )i,j = (A)j,i.
In English, the entry in the ith row and jth column of the transpose is the entry in the jth row and ith column of the original. A = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 AT = 1 4 7 10 13 2 5 8 11 14 3 6 9 12 15 Hint: Do not change the original matrix! If you want to get the number of columns in a non-empty matrix, use matrix[0].length. Test your code against TwoDArrayMethodsTest.java. Exercise 1 Complete Run: git add . git commit -m “Completed exercise 1” git push origin master 2
CPSC250L Lab 2
2.2 ArrayList
In this section we will practice using ArrayLists. Before beginning this exercise find the JavaDoc for ArrayList and have it open in your web browser. Reference the JavaDoc for any questions you have about ArrayList. Exercise 2 Create a new Eclipse project named Lab02 with a ArrayListMethods class in it. Implement the following methods in ArrayListMethods.
1. public static void doReverse(ArrayList list) This method reverses the elements of an ArrayList. If your ArrayList is {1,2,3,4}, then the output of this method would be {4,3,2,1}.
2. public static ArrayList getUnion( ArrayList one, ArrayList two) This method returns the union of the lists one and two. The union of two sets, A and B, is defined as A ∪ B := {x such that x ∈ A or x ∈ B}. In English, the union of A and B is the set that contains each element of A and each element of B. Note that the union is a set and thus the order of the elements in it do not matter. Additionally, no element should be listed in the union more than once.
3. public static ArrayList getIntersection( ArrayList one, ArrayList two) This method returns the intersection of the lists one and two. The intersection of two sets, A and B, is defined as A ∩ B := {x such that x ∈ A and x ∈ B}. The intersection of A and B is the set of each element that is in both A and B. Note that the intersection is a set and thus the order of the elements in it do not matter. 3 Additionally, no element should be listed in the intersection more than once.
4. public static ArrayList getDifference( ArrayList one, ArrayList two) This method returns the difference of the lists one and two. The difference of two sets, A and B, is defined as A4B := {x such that (x ∈ A and x /∈ B) or (x ∈ B and x /∈ A)}. The difference of A and B is the set of each element that is in either A or B but not both A and B. Note that the difference is a set and thus the order of the elements in it do not matter.
Additionally, no element should be listed in the difference more than once. Test Your Code Test your code against ArrayListMethodsTest.java. Correct any compilation errors and ensure that your code passes the JUnit tests. Exercise 2 Complete Run: git add . git commit -m “Completed exercise 2” git push origin master 4
3 Common Mistakes
Some solutions to common mistakes for this lab are as follows.
1. Be sure to ensure that any array or ArrayList that you deal with is not null! If you try to do an operation (other than assignment) on a null reference, then your program will throw a NullPointerException and crash.
2. 2D arrays are indexed like array[row][column].
3. You can get the number of columns of a 2D array by looking at array[0].length.
4. When using ArrayList, you still need to be cautious about going out of bounds. You can get the size of ArrayList arr, by calling arr.size();.
5. When creating a list that represents a set, the order of the objects in the list does not matter. However each object should only be in there once! 5
CPSC250L Lab 2
