1. Announcements
Lab 7 due Wednesday
Assignment 4 due Friday

2. Today in COMP 110 Review from last time Arrays in Classes & Methods
Intro to Sorting
Programming Demo

3. Arrays
Review from last time
Questions?

4. Arrays
A special kind of object in Java used to store a collection of data
What if you wanted to store 80 basketball scores?
Instead of declaring 80 integer variables, declare a single array!

5. Array Details Syntax for creating an array: Example: Alternatively:
Base_Type[] Array_Name = new Base_Type[Length];
Example:
int[] pressure = new int[100]; //create 100 variables of type int that can //be referred to collectively
Alternatively:
int[] pressure; //declare an integer array called pressure
pressure = new int[100]; //allocate memory for the array to hold 100 ints

6. Arrays
The array itself is referred to by a name “scores” or “vector” (in our examples)
Indices 1 2 3 4 68 73 57
102 94
the array scores
scores[3]

7. Array Length An array is a special kind of object
It has one public instance variable: length
length is equal to the length of the array
Pet[] pets = new Pet[20];
int sizeOfArray = pets.length; //sizeOfArray will have the value 20
You cannot change the value of length (it is final)

8. For Loops and Arrays
For loops are often perfectly suited to processing arrays
Why?
Because we know the number of iterations (array.length)
int[] pressure = new int[100];
for(int index = 0; index < pressure.length; index++)
scores[index] = 0;

9. Be Careful with Indices
Indices MUST be in bounds
double[] entries = new double[5];
entries[5] = 3.7;
Your code WILL compile with an out-of-bounds index
But it will result in a run-time error (crash)
//RUN-TIME ERROR! Index out of bounds

10. Arrays in Classes & Methods

11. Arrays as Instance Variables
Arrays can be used as instance variables in classes
public class Course {
private Student[] enrolledStudents; }
public class Pressure {
private double[] pressure;

12. Arrays as Instance Variables
public class Pressure { private double[] pressure; //ask user for the number of pressure values and read them in public void getData() { System.out.println("Hi. How many pressure values would you like to enter?"); Scanner keyboard = new Scanner(System.in); int num = keyboard.nextInt(); pressure = new double[num]; System.out.println("Ok. Please enter " + num + " integers"); for(int i = 0; i < pressure.length; i++) { pressure[i] = keyboard.nextDouble(); } } }
An array instance variable
Create storage for the array
Write values into the array

13. Arrays of Objects
Creating an array of objects does not initialize the individual objects
Student[] students = new Student[35];
students[0].getGPA(); //run-time error, students[0] holds no object
Each object in the array must be instantiated
students[0] = new Student();
students[1] = new Student(); …
students[34] = new Student();
Do this in a loop

14. Arrays of Objects 1045 2584 2836 major class GPA major class GPA major
Student[] students = new Student[3]; for(int i = 0; i < students .length; i++) { students[i] = new Student(); }
1045
2584
2836
students ? ? ?
major
class
GPA
major
class
GPA
major
class
GPA

15. Indexed Variables as Arguments
The same as using a regular variable
public void printNum(int num) {
System.out.println(num); }
public void doStuff() {
int[] scores = { 15, 37, 95 };
for(int index = 0; index < scores.length; index++) {
printNum(scores[index]);

16. Array Assignment & Equality
Arrays are objects
The value of an array is a memory address
Using == to compare arrays two arrays a & b returns whether they point to the same memory address
int[] a = {4, 5, 6};
int[] b = {4, 5, 6};
//a == b is false!

17. Comparing Arrays
To determine whether two arrays hold the same data, compare the two arrays element by element
Lab 7
equals method

18. What is the Output? int a = 7; int b = a; b = 8;
System.out.println("A: " + a);
System.out.println("B: " + b);
//a is not changed by this
Output
A = 7
B = 8

19. This is like giving the array two names (a & b)
What is the Output?
int[] a = {4, 5, 6};
int[] b = a;
b[0] = 3;
System.out.println("a: {" + a[0] + ", " + a[1] + ", " + a[2] + "}");
System.out.println("b: {" + b[0] + ", " + b[1] + ", " + b[2] + "}");
This is like giving the array two names (a & b)
//b holds same memory address as a
//we’re changing both b & a!
Output
a = {3, 5, 6}
b = {3, 5, 6}

20. Array Assignment 4 5 6 int[] a = {4, 5, 6}; int[] b = a; b[0] = 3; a 3

21. Copying Arrays int[] a = {4, 5, 6};
int[] b = new int[a.length]; //create a new array b
//copy a’s entries into b
for(int i = 0; i < b.length; b++) {
b[i] = a[i]; }

22. What is the Output? public void changeNumber(int num) { num = 7; }
public static void main(String[] args) {
int a = 9;
changeNumber(a);
System.out.println("a = " + a);
int num = 9;
changeNumber(num);
System.out.println("num = " + num);
Output
a = 9
num = 9

23. What is the Output? public void changeNumber(int num) { num = 7; }
public static void main(String[] args) {
int[] a = {4, 5, 6};
changeNumber(a[0]);
System.out.println("a: {" + a[0] + ", " + a[1] + ", " + a[2] + "}");
Output
a = {4, 5, 6}

24. What is the Output? public void changeArray(int[] array) { a[0] = 7; }
public static void main(String[] args) {
int[] a = {4, 5, 6};
changeArray(a);
System.out.println("a: {" + a[0] + ", " + a[1] + ", " + a[2] + "}");
Output
a = {7, 5, 6}

25. What is the Output? public void changeArray(int[] a) { a = new int[3];}
public static void main(String[] args) {
int[] a = {4, 5, 6};
changeArray(a);
System.out.println("a: {" + a[0] + ", " + a[1] + ", " + a[2] + "}");
Output
a = {4, 5, 6}

26. Programming Demo
Sorting (Selection Sort)

27. Introduction to Sorting
Given an array of numbers, sort the numbers into ascending order
Input array:
Sorted array: 4 7 3 9 6 2 8 2 3 4 6 7 8 9

28. Selection Sort 4 7 3 9 6 2 8 2 7 3 9 6 4 8 2 3 7 9 6 4 8 …

29. Pseudocode for i = 0 to array.length - 1 2 3 7 9 6 4 8
Find the index s of the smallest element starting at index i
Swap elements i & s in the array 2 3 7 9 6 4 8 i s

30. Programming Demo
Programming

31. Wednesday
Multi-Dimensional Arrays