Arrays

1. Object – Oriented Programming
Week 5 –Arrays
Ferdin Joe John Joseph, PhD
Faculty of Information Technology
Thai-Nichi Institute of Technology

2. Arrays
Faculty of Information Technology,
Thai-Nichi Institute of Technology
2

3. 3
Introducing Arrays
Array is a data structure that represents a collection
of the same types of data.
5.6
4.5
3.3
13.2
4
34.33
34
45.45
99.993
11123
double[] myList = new double[10];
myList reference
myList[0]
myList[1]
myList[2]
myList[3]
myList[4]
myList[5]
myList[6]
myList[7]
myList[8]
myList[9]
Element value
Array reference
variable
Array element at
index 5

4. 4
Declaring Array Variables
• datatype[] arrayRefVar;
Example:
double[] myList;
• datatype arrayRefVar[]; // This style is
allowed, but not preferred
Example:
double myList[];

5. 5
Creating Arrays
arrayRefVar = new datatype[arraySize];
Example:
myList = new double[10];
myList[0] references the first element in the
array.
myList[9] references the last element in the
array.

6. 6
Declaring and Creating
in One Step
• datatype[] arrayRefVar = new
datatype[arraySize];
double[] myList = new double[10];
• datatype arrayRefVar[] = new
datatype[arraySize];
double myList[] = new double[10];

7. 7
The Length of an Array
Once an array is created, its size is fixed. It
cannot be changed. You can find its size using
arrayRefVar.length
For example,
myList.length returns 10

8. 8
Default Values
When an array is created, its elements are
assigned the default value of
0 for the numeric primitive data types,
'u0000' for char types, and
false for boolean types.

9. 9
Indexed Variables
The array elements are accessed through the
index. The array indices are 0-based, i.e., it starts
from 0 to arrayRefVar.length-1. In the example in
Figure 6.1, myList holds ten double values and the
indices are from 0 to 9.
Each element in the array is represented using the
following syntax, known as an indexed variable:
arrayRefVar[index];

10. 10
Using Indexed Variables
After an array is created, an indexed
variable can be used in the same way as a
regular variable. For example, the following
code adds the value in myList[0] and
myList[1] to myList[2].
myList[2] = myList[0] + myList[1];

11. 11
Array Initializers
• Declaring, creating, initializing in one step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand syntax must be in one
statement.

12. 12
Declaring, creating, initializing
Using the Shorthand Notation
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent to the
following statements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;

13. 13
CAUTION
Using the shorthand notation, you
have to declare, create, and
initialize the array all in one
statement. Splitting it would cause
a syntax error. For example, the
following is wrong:
double[] myList;
myList = {1.9, 2.9, 3.4, 3.5};

14. 14
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = values[i] + values[i-1];
}
values[0] = values[1] + values[4];
}
}
Declare array variable values, create
an array, and assign its reference to
values
After the array is created
0
1
2
3
4
0
0
0
0
0
animation

15. 15
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = values[i] + values[i-1];
}
values[0] = values[1] + values[4];
}
}
i becomes 1
After the array is created
0
1
2
3
4
0
0
0
0
0
animation

16. 16
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = values[i] + values[i-1];
}
values[0] = values[1] + values[4];
}
}
i (=1) is less than 5
After the array is created
0
1
2
3
4
0
0
0
0
0
animation

17. 17
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After this line is executed, value[1] is 1
After the first iteration
0
1
2
3
4
0
1
0
0
0
animation

18. 18
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = values[i] + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After i++, i becomes 2
animation
After the first iteration
0
1
2
3
4
0
1
0
0
0

19. 19
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = values[i] + values[i-1];
}
values[0] = values[1] + values[4];
}
}
i (= 2) is less than 5
animation
After the first iteration
0
1
2
3
4
0
1
0
0
0

20. 20
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After this line is executed,
values[2] is 3 (2 + 1)
After the second iteration
0
1
2
3
4
0
1
3
0
0
animation

21. 21
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After this, i becomes 3.
After the second iteration
0
1
2
3
4
0
1
3
0
0
animation

22. 22
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
i (=3) is still less than 5.
After the second iteration
0
1
2
3
4
0
1
3
0
0
animation

23. 23
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After this line, values[3] becomes 6 (3 + 3)
After the third iteration
0
1
2
3
4
0
1
3
6
0
animation

24. 24
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After this, i becomes 4
After the third iteration
0
1
2
3
4
0
1
3
6
0
animation

25. 25
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
i (=4) is still less than 5
After the third iteration
0
1
2
3
4
0
1
3
6
0
animation

26. 26
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After this, values[4] becomes 10 (4 + 6)
After the fourth iteration
0
1
2
3
4
0
1
3
6
10
animation

27. 27
Trace Program with Arrays
public class Test {
public static void main(String[]
args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After i++, i becomes 5
animation
After the fourth iteration
0
1
2
3
4
0
1
3
6
10

28. 28
Trace Program with Arrays
public class Test {
public static void main(String[] args) {
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
i ( =5) < 5 is false. Exit the loop
animation
After the fourth iteration
0
1
2
3
4
0
1
3
6
10

29. 29
Trace Program with Arrays
public class Test {
public static void main(String[] args)
{
int[] values = new int[5];
for (int i = 1; i < 5; i++) {
values[i] = i + values[i-1];
}
values[0] = values[1] + values[4];
}
}
After this line, values[0] is 11 (1 + 10)
0
1
2
3
4
11
1
3
6
10
animation

30. 30
Processing Arrays
See the examples in the text.
1. (Initializing arrays)
2. (Printing arrays)
3. (Summing all elements)
4. (Finding the largest element)
5. (Finding the smallest index of the
largest element)

31. 31
Enhanced for Loop
JDK 1.5 introduced a new for loop that enables you to traverse the
complete array sequentially without using an index variable. For example,
the following code displays all elements in the array myList:
for (double value: myList)
System.out.println(value);
In general, the syntax is
for (elementType value: arrayRefVar) {
// Process the value
}
You still have to use an index variable if you wish to traverse the array in a
different order or change the elements in the array.
JDK 1.5
Feature

32. 32
Example: Assigning Grades
• Objective: read student scores (int), get the
best score, and then assign grades based on
the following scheme:
– Grade is A if score is >= best–10;
– Grade is B if score is >= best–20;
– Grade is C if score is >= best–30;
– Grade is D if score is >= best–40;
– Grade is F otherwise.

33. 33
Copying Arrays
Often, in a program, you need to duplicate an array or a part of
an array. In such cases you could attempt to use the assignment
statement (=), as follows:
list2 = list1;
Contents
of list1
list1
Contents
of list2
list2
Before the assignment
list2 = list1;
Contents
of list1
list1
Contents
of list2
list2
After the assignment
list2 = list1;
Garbage

34. 34
Copying Arrays
Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new
int[sourceArray.length];
for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];

35. 35
The arraycopy Utility
arraycopy(sourceArray, src_pos,
targetArray, tar_pos, length);
Example:
System.arraycopy(sourceArray, 0,
targetArray, 0, sourceArray.length);

36. 36
Linear Search
The linear search approach compares the
key element, key, sequentially with each
element in the array list. The method
continues to do so until the key matches
an element in the list or the list is
exhausted without a match being found. If
a match is made, the linear search returns
the index of the element in the array that
matches the key. If no match is found, the
search returns -1.

37. 37
Linear Search Animation
6 4 1 9 7 3 2 8
6 4 1 9 7 3 2 8
6 4 1 9 7 3 2 8
6 4 1 9 7 3 2 8
6 4 1 9 7 3 2 8
6 4 1 9 7 3 2 8
3
3
3
3
3
3
animation
Key List

38. 38
From Idea to Solution
/** The method for finding a key in the list */
public static int linearSearch(int[] list, int key) {
for (int i = 0; i < list.length; i++)
if (key == list[i])
return i;
return -1;
}
int[] list = {1, 4, 4, 2, 5, -3, 6, 2};
int i = linearSearch(list, 4); // returns 1
int j = linearSearch(list, -4); // returns -1
int k = linearSearch(list, -3); // returns 5
Trace the method

39. Two-Dimensional Arrays
• A one-dimensional array stores a list of
elements
• A two-dimensional array can be thought
of as a table of elements, with rows and
columns
one
dimension
two
dimensions

40. Two-Dimensional Arrays
• To be precise, in Java a two-dimensional array is an
array of arrays
• A two-dimensional array is declared by specifying the
size of each dimension separately:
int[][] scores = new int[12][50];
• A array element is referenced using two index values:
value = scores[3][6]
• The array stored in one row can be specified using
one index

41. Arrays of Arrays
• Two-Dimensional arrays
– float[][] temperature=new float[10][365];
– 10 arrays each having 365 elements
– First index: specifies array (row)
– Second Index: specifies element in that array
(column)
– In JAVA float is 4 bytes, total
Size=4*10*365=14,600 bytes

42. Graphical Representation
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
Sample[0]
Sample[1]
Sample[2]

43. Initializing Array of Arrays
int[][] array2D = { {99, 42, 74, 83, 100}, {90,
91, 72, 88, 95}, {88, 61, 74, 89, 96}, {61,
89, 82, 98, 93}, {93, 73, 75, 78, 99}, {50,
65, 92, 87, 94}, {43, 98, 78, 56, 99} };
//5 rows with 5 elements each

44. Arrays of Arrays of Varying
Length
• All arrays do not have to be of the same
length
float[][] samples;
samples=new float[6][];//defines # of arrays
samples[2]=new float[6];
samples[5]=new float[101];
• Not required to define all arrays

45. Initializing Varying Size Arrays
int[][] uneven = { { 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 } };
//Three arrays
//First array has 3 elements
//Second array has 2 elements
//Third array has 5 elements

46. Array of Arrays Length
long[][] primes = new long[20][];
primes[2] = new long[30];
System.out.println(primes.length); //Number of arrays
System.out.println(primes[2].length);//Number of elements in the second array
OUTPUT:
20
30

47. Sample Program
class unevenExample3
{
public static void main( String[] arg )
{ // declare and construct a 2D array
int[][] uneven = { { 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 } };
// print out the array
for ( int row=0; row < uneven.length; row++ ) //changes row
{
System.out.print("Row " + row + ": ");
for ( int col=0; col < uneven[row].length; col++ ) //changes column
System.out.print( uneven[row][col] + " ");
System.out.println();
}
}
}

48. Output
Row 0: 1 9 4
Row 1: 0 2
Row 2: 0 1 2 3 4

49. Faculty of Information Technology,
Thai-Nichi Institute of Technology
49
An Ordered Collection: ArrayList
• ArrayList is a Java class that specializes in
representing an ordered collection of things
• The ArrayList class is defined in the Java
libraries
– part of the java.util package
• We can store any kind of object in an ArrayList
– myList.add(theDog);
• We can retrieve an object from the ArrayList by
specifying its index number
– myList.get(0)

50. Faculty of Information Technology,
Thai-Nichi Institute of Technology
50
ArrayList
• ArrayList()
– This constructor builds an empty list with an initial
capacity of 10
• int size()
– This method returns the number of elements in this
list
• boolean add(Object o)
– This method appends the specified element to the
end of this list and increases the size of the array if
needed
• Object get(int index)
– This method returns the element at the specified
position

51. Faculty of Information Technology,
Thai-Nichi Institute of Technology
51
Using ArrayLists
• ArrayList is part of the java.util package
– import java.util.*; to use ArrayList
• Creating a list
• ArrayList names = new ArrayList ( );
• Getting the size
• int numberOfNames = names.size( );
• Adding things
• names.add("Billy");
• names.add("Susan");
• names.add("Frodo");
NameList.java

52. Faculty of Information Technology,
Thai-Nichi Institute of Technology
52
Using ArrayLists : import
• ArrayList is part of the java.util package
– import java.util.ArrayList; to use ArrayList
• The import statement tells the Java
compiler where to look when it can’t find a
class definition in the local directory
– We tell the compiler to look in package
java.util for the definition of ArrayList by
putting an import statement at the top of the
source code file
– Java always looks in package java.lang on its
own

53. Faculty of Information Technology,
Thai-Nichi Institute of Technology
53
Using ArrayLists : constructor
• Creating a new ArrayList object
• ArrayList names = new ArrayList ( );
• There are several constructors available
– ArrayList()
• Construct an empty list with an initial capacity of 10
– ArrayList(int initialCapacity)
• Construct an empty list with the specified initial capacity
– ArrayList(Collection c)
• Construct a list containing elements from another collection

54. Faculty of Information Technology,
Thai-Nichi Institute of Technology
54
Using ArrayLists : size
• Getting the size
• int numberOfNames = names.size( );
• size() method returns integer value that
caller can use to control looping, check for
limits, etc
– Design pattern: The object keeps track of
relevant information, and can tell the caller when
there is a need to know

55. Faculty of Information Technology,
Thai-Nichi Institute of Technology
55
Using ArrayLists : add
• Adding things
• names.add("Billy");
• add(Object o) method adds an object to the
list at the end of the list
• The object can be of any class type
– String, File, InputStream, …
– can’t add “primitive” types like int or double
directly
• Can use the wrapper classes like Integer to store
primitives

56. Faculty of Information Technology,
Thai-Nichi Institute of Technology
56
Using ArrayLists: get
• ArrayLists provide indexed access
– We can ask for the ith item of the list, where
the first item is at index 0, the second at index
1, and the last item is at index n-1 (where n is
the size of the collection).
ArrayList names = new ArrayList ( );
names.add("Billy");
names.add("Susan");
Object x = names.get(0);
Object y = names.get(1);