1. A Wide Array of Possibilities
Java’s Central Casting CS
Lecture 3-2 1

2. What’s an Array
An array is a group of values of the same type (usually stored in consecutive memory locations)
Arrays are objects
Can be assigned to variables of type Object
Dynamically created

3. An Array of Examples From Figure 5.3
int n[]; // declare an array of integers
// initialize instance variables
public void init() {
n = new int[ 10 ]; //dynamic allocation }

4. The Declaration of Array
Array declarations
int[] ai; // array of int
short[][] as; // array of array of short
Object[] ao, // array of Object
otherAo; // array of Object

5. Declaring Arrays Can’t specify the length in a declaration
int test [] = new int [100]; // Okay
int test2 [100]; // No good
int test3 [100]=new int [100]; // No good

6. Some Array Definitions
These create array objects:
Exception ae[] = new Exception[3];
Object aao[][] = new Exception[2][3];
int[] factorial = { 1, 1, 2, 6, 24 };
String[] aas = {"array", "of", "String"};

7. An Array Example int face; int frequency[];
// initialize instance variables
public void init() {
frequency = new int[ 7 ];
for (int roll=1; roll <= 6000; roll++ ) {
face = 1 + (int) (Math.random() * 6 );
++frequency[ face ]; }

8. An Array of Observations
Arrays know their own length
test.length == 100
What kind of loops do we usually use with arrays?
Arrays start at index 0
Last index == ??
Arrays must be indexed by int values (short, byte, or char are okay, but long is no good)

9. Operating on an Array Square brackets are an operator
Brackets have very high precedence
Same as ()’s
Example
-17*studentGrades[12] * 2

10. "Can he do that?" What’s wrong with the following code?
int scores[] = {98, 76, 84, 97, 101, 78};
int counter=0;
for (; counter <= scores.length; counter++) {
System.out.println("Test #" + counter +
": " + scores[counter]); }

11. Array Initializers Use {}’s to set initial values
int grades[] = {87, 92, 76, 94}

12. Finally, a Constant Variable
What’s wrong with the following code?
// Figure 5.6
// This Applet won’t compile
import java.applet.Applet;
public class FinalTest extends Applet {
final int x; }

13. Pass the Salt, and an Array of int
From last time: primitive types and reference types
Parameter passing: offering data to a method in the method call
public void paint(Graphics graphObject)
When you call paint, give it a Graphics object
How does Java pass data around?

14. Call Me Ishmael, But Don't Call Me Late for Dinner
Call-by-value
The called method gets a copy of the actual data (i.e., its value)
Called method can't corrupt the original data, because it's only got a copy
Call-by-reference
The called method gets a reference to the data

15. Arrays & Call-by-Reference
Reference types are always passed call-by-reference
Pro: Don’t need to copy lots of data
Con: Since you can access the original object, you can change it
Arrays are a reference type
Whole arrays are passed call-by-reference

16. What About Array Elements?
If an array is an array of a reference type, then individual elements are passed as call-by-____________
Graphics[] graphicsArray = new Graphics[10];
int[] scores = new int[10];

17. Searching & Sorting Data
The Second Law of Thermodynamics
Sorting restores order to the world
Sorting in everyday life
Bank checks by account
Searching is easier when sorted
Reverse directories of phone numbers

18. Sorting in Computers Usually start with an array of items to be sorted
Integers
Social Security numbers
Last names and first names
Lots of data
All the SSN’s in the U.S.
All of GM’s employees

19. Different Ways to Sort Bubble sort
Compare two neighbors, swap if out of order

20. The Code for Bubble Sort
public void sort() {
int hold; // temporary holding area for swap
// passes
for ( int pass = 1; pass < a.length; pass++ )
// one pass
for ( int i = 0; i < a.length - 1; i++ )
// one comparison
if ( a[ i ] > a[ i + 1 ] ) {
// one swap
hold = a[i]; a[ i ] = a[ i + 1 ]; a[ i + 1 ] = hold; }
Nested for loops

21. There’s More Than One Way to Sort a Cat
Other sorts
Quicksort
Heapsort
Bucket sort
Different sorts have different characteristics
Memory usage
Speed

22. Searching Here, There & Everywhere
Need to find a value? Search for it!
Linear search
Is it the next one?
No, then look again
Yes, stick a fork in me, ‘cuz I’m done
Works on unsorted items
Binary search
Cut in half every time

23. You’re a Detective in a Small Town
The killer called the victim
Get the LUDs
Look up the number in a reverse directory

24. Arrays of Arrays Arrays in multiple dimensions Not just a list
A table (an array of arrays)
A cube (an array of arrays of arrays)

25. An Example An array of arrays int b[][] = { {1,2}, {3, 4, 5}}
Row 0: 1 2
Row 1: 3 4 5
A 3x3 array
int b[][];
b = new int[3][3];