Arrays

(1)

Arrays

Multidimensional Arrays

(2)

Array Declaration

•

int[] someArray;

–

Declares an integer array

–

Brackets after the type = preferred

•

int someArray[];

–

Declares an integer array

–

Brackets after the variable name = not preferred

•

int someArray = new int[10];

–

Declares an array of size 10

–

Primitive types are initialized to their default values

•

int[] someArray = {1,2,3,4,5};

–

Declares and initializes an array for five integers

(3)

Default Values

•

int = 0

•

Boolean = false

•

Char = \u0000

•

Objects = null

(4)

Length of Array

•

int someArray[] = new int[10];

•

someArray.length returns 10

(5)

Array Example

0 0 0 0 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

(6)

Array Example

0 0 0 0 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

6

(7)

Array Example

0 0 0 0 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

7

(8)

Array Example

0 0 0 0 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

8

(9)

Indices

0 0 0 0 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

9

(10)

Indices

0 5 0 0 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

10

(11)

Indices

0 5 5 0 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

11

(12)

Indices

0 5 5 10 0 0 0 0 0 0

Index 0 1 2 3 4 5 6 7 8 9

Length 10

12

(13)

Simple Array Example

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]; }

(14)

Simple Array Example

public class Test {

int[] values = new int[5];

for (int i = 1; i < 5; i++) {

values[0] = values[1] + values[4]; } } 14 Index Value 0 0 1 0 2 0 3 0 4 0 Array

(15)

public class Test {

(16)

Simple Array Example

public class Test {

(17)

public class Test {

values[i] = i + values[i-1];

}

(18)

Simple Array Example

public class Test {

(19)

public class Test {

(20)

Simple Array Example

public class Test {

}

(21)

public class Test {

(22)

Simple Array Example

public class Test {

(23)

public class Test {

}

(24)

Simple Array Example

public class Test {

(25)

public class Test {

(26)

Simple Array Example

public class Test {

}

(27)

public class Test {

(28)

Simple Array Example

public class Test {

(29)

public class Test {

values[0] = values[1] + values[4];

} } 29 Index Value 0 11 1 1 2 3 3 6 4 10 Array

(30)

Useful Things to do with Arrays

•

Print all elements

•

Sum all elements

•

Find the largest element

•

Shift elements

(31)

Print All Elements

for (int i = 0; i < myArray.length; i++)

{

System.out.print(myArray[i] + " ");

}

(32)

Sum All Elements

double total = 0;

{

total += myArray[i];

}

(33)

Find Largest Element

//should check that there is at least one element

double max = myArray[0];

{

if (myArray[i] > max)

{

max = myArray[i];

}

(34)

Shifting Elements

double temp = myArray[0];

for(int i=0; i<myArray.length-1; i++)

{

myArray[i] = myArray[i+1];

}

myArray[myArray.length-1] = temp;

(35)

Random Shuffle

•

For each array element

–

Choose random number

–

Swap array[i] and array[randomNumber]

(36)

Random Shuffle

//initialize array up here

Random random = new Random(0);

for(int i=0; i<myArray.length; i++)

{

int randomSwap = random.nextInt(myArray.length);

int temp = myArray[i];

myArray[i] = myArray[randomSwap];

myArray[randomSwap] = temp;

}

(37)

Example – RandomShuffle.java

•

RandomShuffle.java

(38)

Iterate Over All Elements

•

Iterate through all elements in order

•

Different type of for loop syntax

for(double value: myArray)

{

System.out.println(value);

}

(39)

•

Iterate through all elements in order

•

{

}

39

(40)

•

Iterate through all elements in order

•

{

}

40

(41)

•

Iterate through all elements in order

•

{

}

41

(42)

Copying Arrays

42

Array Contents

array1

Array Contents

array2

(43)

Shallow Copy

43

Array Contents

array1

Array Contents

array2

array2 = array1;

Nothing pointing to array2

array2 = garbage

(44)

Deep Copy

44

Array Contents

array1

Array Contents

array2

for(int i=0; i<array1.length; i++) {

array2[i] = array1[i]; }

(45)

Arraycopy

•

System.arraycopy

–

https://

docs.oracle.com/javase/8/docs/api/java/lang/Syste

m.html#arraycopy-java.lang.Object-int-java.lang.

Object-int-int-–

public static void arraycopy

• (Object src, int srcPos, Object dest, int destPos, int length)

•

Arrays.copyOf

–

https://

docs.oracle.com/javase/8/docs/api/java/util/Arrays.

html

(46)

Searching Arrays

•

Linear Search

–

For unsorted arrays

•

Binary Search

–

For sorted arrays

–

Arrays.binarySearch

•

https://

docs.oracle.com/javase/8/docs/api/java/util/Arrays.ht

ml

(47)

Example – SearchExamples.java

•

SearchExamples.java

(48)

Sorting Arrays

•

Arrays.sort

–

https://

docs.oracle.com/javase/8/docs/api/java/util/Arrays.

html

•

Uses dual pivot quicksort for primitive types

•

Uses Timsort for Objects

–

Merge sort + Insertion sort hybrid

(49)

Example – SortComparison.java

•

SortComparison.java

(50)

Problem

•

Given an unsorted array of numbers determine

if the array contains a given value for each

value in a large list

•

Do not modify the original array

–

It is important not to lose the original data

(51)

Problem

•

What needs to be done?

–

Can do linear search for each value

•

Will be slow for a large number of values

–

Can do binary search for each value

•

Requires the list to be sorted

(52)

Possible Solution #1

•

Copy the original array

•

Sort the copied array

•

Search for each value in the copied array

(53)

Possible Solution #2

•

Copy the array

•

Sort the copied array

•

Copy the list of values

•

Sort the copied list of values

•

Use binary search

–

Change the minimum starting element based on

the results of the previous search

(54)

Example – ValueSearch.java

•

ValueSearch.java

•

Solution for solution #1

(55)

Multidimensional Arrays

(56)

Single Array

•

Can be thought of as a list

•

{1, 2, 3, 4, 5,}

–

1

–

2

–

3

–

4

–

5

(57)

2D Array

•

Can be thought of as a table

x

2

x

3

x

4

x

5

1

2

4

8

16

32

3

9

27

81

243

4

16

64

256

1024

5

25

125

625

3125

6

36

216

1296

7776

7

49

343

2401

16807

8

64

512

4096

32768

9

81

729

6561

59049

10

100 1000 10000 100000

(58)

N-dimensional array

•

How to visualize this?

•

Picture an array or rectangles

–

And rectangles inside each of those rectangles

•

And rectangles inside each of those rectangles

– Etc…

•

Treemap

(59)

59

One-dimensional array

One Dimensional Array (int[] x = new int[3]) x[1]

(60)

60

Two Dimensional Array (int[][] x = new int[3][3])

x[0] x[1] x[2]

x[1][1]

x[1][2]

x[2][1]

x[2][2]

x[1][0] x[2][0]

x[0][1]

x[0][2] x[0][0]

(61)

61

Three-dimensional array

x[0][1][2]

x[0][0] x[1][0] x[2][0]

x[0] x[1] x[2]

Three Dimensional Array (int[][][] x = new int[3][3][3])

(62)

Example – SpaceSniffer.exe

•

SpaceSniffer.exe

•

Program to visualize what contents are using

your disk space

•

Another example

–

https://

developers.google.com/chart/interactive/docs/galle

ry/treemap#example

(63)

Declaring and Creating

Two-dimensional Arrays

// Declare array ref var

dataType[][] refVar;

// Create array and assign its reference to variable

refVar = new dataType[10][10];

// Combine declaration and creation in one statement

dataType[][] refVar = new dataType[10][10];

// Alternative syntax

dataType refVar[][] = new dataType[10][10];

(64)

Two-Dimensional Array Example

int[][] array = new int[5][5];

64

0

1

2

3

4

0

1

0

2

0

3

0

(65)

array[1][4] = 222;

65

0

1

2

3

4

0

1

0

222

2

0

3

0

(66)

int[][] array = new int[5][];

66

(67)

array[0] = new int[5];

67

0

1

2

3

4

0

(68)

array[1] = new int[4];

68

0

1

2

3

4

0

1

0

(69)

array[2] = {1, 2, 3};

69

0

1

2

3

4

0

1

0

2

1

2

3

(70)

int[][] array = {{1, 2}, {3, 4}, {5, 6, 7}}

70

0

1

2

0

1

2

1

3

4

(71)

Useful Things to do with

multi-dimensional Arrays

•

Print all elements

•

Sum all elements

•

Find the largest element

•

Shift elements

(72)

Print All Elements

72

{

for(int j=0; j<myArray[i].length; j++)

{

System.out.print(myArray[i][j] + " ");

}

(73)

Sum All Elements

73

double total = 0;

{

total += myArray[i][j];

}

(74)

Find Largest Element

double max = myArray[0][0];

{

if (myArray[i][j] > max)

{

max = myArray[i][j];

}

(75)

Shifting Elements

double temp = myArray[0][0];

for(int i=0; i<myArray.length-1; i++)

{

double temp2 = myArray[i][0];

for(int j=0; j<myArray[i].length-1; j++)

{

myArray[i][j] = myArray[i][j+1];

}

myArray[i][myArray[i].length-1] = temp2;

}

myArray[myArray.length-1][myArray[i].length-1] = temp;

(76)

Random Shuffle 2D Array

•

Need to loop through all elements

–

Need to choose a random position to swap with

(77)

Random Shuffle 2D Array

//initialize array and “random” variable up here

for(int i=0; i<myArray.length; i++)

{

int randomI = random.nextInt(myArray.length);

int randomJ = random.nextInt(myArray.length);

int temp = myArray[i][j];

myArray[i][j] = myArray[randomI][randomJ];

myArray[randomI][randomJ] = temp;

}

https://docs.oracle.com/javase/8/docs/api/java/lang/Syste

https://docs.oracle.com/javase/8/docs/api/java/util/Arrays.

https://developers.google.com/chart/interactive/docs/galle