Arrays and Detailed explanation of Array

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Arrays in Java
Jasmine Beulah Gnanadurai, PhD

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Opening Problem
Read one hundred numbers, compute their average, and
find out how many numbers are above the average.

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Introducing Arrays
Array is a data structure that represents a collection of the same types
of data.

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

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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.

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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];

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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

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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, 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];

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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];

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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.

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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;

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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};

Liang, Introduction to Java Programming, Tenth Edition, (c) 2015 Pearson Education, Inc. All rights reserved.
13
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
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After i++, i becomes 2
0
1
2
3
4
0
1
0
0
0
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public class Test {
public static void main(String[]
args) {
for (int i = 1; i < 5; i++) {
}
values[0] = values[1] +
values[4];
}
}
i (= 2) is less than 5
0
1
2
3
4
0
1
0
0
0
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
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
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this, i becomes 3.
0
1
2
3
4
0
1
3
0
0
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
i (=3) is still less than 5.
0
1
2
3
4
0
1
3
0
0
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this line, values[3] becomes 6 (3 + 3)
After the third iteration
0
1
2
3
4
0
1
3
6
0
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this, i becomes 4
0
1
2
3
4
0
1
3
6
0
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
i (=4) is still less than 5
0
1
2
3
4
0
1
3
6
0
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this, values[4] becomes 10 (4 + 6)
After the fourth iteration
0
1
2
3
4
0
1
3
6
10
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After i++, i becomes 5
0
1
2
3
4
0
1
3
6
10
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
i ( =5) < 5 is false. Exit the loop
0
1
2
3
4
0
1
3
6
10
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public class Test {
for (int i = 1; i < 5; i++) {
}
}
}
After this line, values[0] is 11 (1 + 10)
0
1
2
3
4
11
1
3
6
10
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Initializing arrays with input values
java.util.Scanner input = new java.util.Scanner(System.in);
System.out.print("Enter " + myList.length + " values: ");
for (int i = 0; i < myList.length; i++)
myList[i] = input.nextDouble();

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Initializing arrays with random values
for (int i = 0; i < myList.length; i++) {
myList[i] = Math.random() * 100;
}

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Printing arrays
System.out.print(myList[i] + " ");
}

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Summing all the elements
double total = 0;
total += myList[i];
}

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Finding the largest element
double max = myList[0];
for (int i = 1; i < myList.length; i++)
{
if (myList[i] > max)
max = myList[i];
}

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Shifting Elements

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Enhanced for Loop (for-each loop)
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.

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Self-check
Write an enhanced for loop that prints all elements in the array
values.
Answer:
for (double x:values)
{
System.out.println(x);
}

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Read one hundred numbers, compute their average, and
find out how many numbers are above the average.
int n1, n2,…, n100;
Scanner input = new Scanner(System.in);
n1 = input.nextInput();
.....

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Solution through Arrays
int [] n = new int[100];
Scanner input = new Scanner(System.in);
for (int i=0; i < 100; i++)
{
n[i] = input.nextInt();
}

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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;

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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];

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Passing Arrays to Methods
public static void printArray(int[] array) {
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
}
Invoke the method
int[] list = {3, 1, 2, 6, 4, 2};
printArray(list);
Invoke the method
printArray(new int[]{3, 1, 2, 6, 4, 2});
Anonymous array

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Searching Arrays
Searching is the process of looking for a specific element in an array;
for example, discovering whether a certain score is included in a list of scores.
Searching is a common task in computer programming.
There are many algorithms and data structures devoted to searching.
In this section, two commonly used approaches are discussed, linear search and
binary search.
public class LinearSearch {
/** 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;
}
}
list
key Compare key with list[i] for i = 0, 1, …
[0] [1] [2] …

41
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
Key List
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Binary Search
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For binary search to work, the elements in the array must already be ordered.
Without loss of generality, assume that the array is in ascending order.
e.g., 2 4 7 10 11 45 50 59 60 66 69 70 79
The binary search first compares the key with the element in the middle of
the array.

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Binary Search, cont.
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• If the key is less than the middle element, you only need to search the
key in the first half of the array.
• If the key is equal to the middle element, the search ends with a match.
• If the key is greater than the middle element, you only need to search the
key in the second half of the array.
Consider the following three cases:

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Binary Search
1 2 3 4 6 7 8 9
1 2 3 4 6 7 8 9
1 2 3 4 6 7 8 9
8
8
8
Key List

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Binary Search
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
2 4 7 10 11 45 50 59 60 66 69 70 79
key is 54
key > 50
list
mid
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
key < 66
key < 59
high
low
mid high
low
list
[7] [8]
mid high
low
list
59 60 66 69 70 79
59 60
[6] [7] [8]
high
low
59 60

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Binary Search
The binarySearch method returns the index of the element in the list that
matches the search key if it is contained in the list. Otherwise, it returns
-insertion point - 1.
The insertion point is the point at which the key would be inserted into the
list.

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2D Array Declaration and Creation
 2D arrays are used to store a matrix or a table
Declaring Array Variables
elementType[][] arrayRefVar;
e.g. int[][] matrix;
 Declaration of an array variable does not allocate any space in
memory for the array
Creating Arrays
arrayRefVar = new elementType[rowSize][columnsize];
e.g. int[][] matrix = new int[5][5];

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2D Array Declaration and Creation
Assigning Value to elements
arrayRefVar[index2][index2] = value;
e.g. matrix[2][1] = 7;

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Array Size
 A two-dimensional array is actually an array in which each element
is a one-dimensional array
 If x = new int[3][4] then x[0], x[1], and x[2] are one-dimensional
arrays and each contains four elements

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Initializing and Processing 2D Arrays

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Some Examples of Processing 2D Arrays
 Initializing arrays with input values
java.util.Scanner input = new Scanner(System.in);
System.out.println("Enter " + matrix.length + " rows and " + matrix[0].length + " columns: ");
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
matrix[row][column] = input.nextInt();
}
}
 Initializing arrays with random values
matrix[row][column] = (int)(Math.random() * 100);
}
}
 Displaying arrays
System.out.print(matrix[row][column] + " ");
}

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Multi Dimensional Arrays
 n-dimensional Array means collection of (n-1)-dimensional Arrays
 The way to declare two-dimensional array variables and create two-dimensional
arrays can be generalized to declare n-dimensional array variables
 For example, the following syntax declares a three-dimensional array variable
scores, creates an array, and assigns its reference to scores.
double[][][] scores = new double[10][5][2];

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ArrayList
 Useful class for storing objects
 Arrays can be used to store objects but once the array is created, its size is fixed.
 Java provides the ArrayList class that can be used to store an unlimited number
of objects
 ArrayList supports dynamic arrays
 Defined in java.util package

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ArrayList
 Constructors of ArrayList class are
ArrayList() : creates an empty array list with an initial capacity sufficient to
hold 10 elements.
e.g. ArrayList list =new ArrayList();
ArrayList(int capacity) : creates an array list that has the specified initial
capacity.
e.g. ArrayList list =new ArrayList(20);
ArrayList(Collection c) : creates an array list that is initialized with the
elements of the collection c.
e.g. ArrayList l = new ArrayList(list);
Array lists are created with an initial size. When this size is exceeded, the
collection is automatically enlarged

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Methods of ArrayList
1. void add(int index, Object element)
 Inserts the specified element at the specified position index in this list.
 Throws IndexOutOfBoundsException if the specified index is is out of range.
2. boolean add(Object o)
 Appends the specified element to the end of this list.
3. boolean addAll(Collection c )
 Appends all of the elements in the specified collection to the end of this list.
 Throws NullPointerException if the specified collection is null.
4. boolean addAll(int index, Collection c )
 Inserts all of the elements of the specified collection into this list, starting at the
specified position.
 Throws NullPointerException if the specified collection is null.

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5. void clear() : Removes all of the elements from this list.
6. Object remove(int index) : Removes the element at the specified position in
this list.
7. Object clone() : Returns a shallow copy of this ArrayList.
8. int size(): Returns the number of elements in the list.
9. boolean contains(Object o): Returns true if this list contains the specified
element.

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10. Object get(int index): Returns the element at the specified position in this list.
11. Object set(int index, o Object): Sets the element at the specified index.
12. int indexOf(Object o): Returns the index in this list of the first occurrence of the
specified element, or -1 if the List does not contain the element.
13. int lastIndexOf(Object o):Returns the index in this list of the last occurrence of the
specified element, or -1.
14. void ensureCapacity(int minCapacity) : Increases the capacity of this ArrayList
instance, if necessary, to ensure that it can hold at least the number of elements specified by the
minimum capacity argument.

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Arrays vs ArrayList

Arrays and Detailed explanation of Array

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