Arrays

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Arrays

Spring 2006UVA - CS 101Aaron BloomfieldEdited for WLCS by Paul Bui

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Background Programmers often need the ability to represent a group of

values as a list List may be one-dimensional or multidimensional

Java provides arrays and the collection classes

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Example Definitions

char[] c;int[] value = new int[10];

Causes Array object variable c is un-initialized Array object variable value references a new ten element

list of integers Each of the integers is default initialized to 0

value 0 0 0 0 0

-c

…

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An array example

int[] v = new int[10];int i = 7;int j = 2;int k = 4;v[0] = 1;v[i] = 5;v[j] = v[i] + 3;v[j+1] = v[i] + v[0];v[v[j]] = 12;System.out.println(v[2]);v[k] = stdin.nextInt();

v 00 0 00 0 00 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 01 0 00 0 00 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 01 0 00 0 50 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 00 0 50 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 06 0 50 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 06 0 50 012

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 06 0 50 012

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]

8 is displayed


v 81 0 06 3 50 012

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]

Suppose 3 is extracted


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Array variable definition styles Without initialization

Type ofvalues in

list

Name oflist

Bracketsindicate arrayvariable being

defined

ElementType [ ] id;

int [] a;int a[];

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Array variable definition styles With initialization

ElementType[ ] id = new ElementType [n];

Nonnegative integer expression specifying thenumber of elements in the array

A new array of nelements

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Basic terminology List is composed of elements

Elements in a list have a common name Example: a[3] = 5; The common name is ‘a’

The list as a whole is referenced through the common name

List elements are of the same type — the base type

Elements of a list are referenced by subscripting (indexing) the common name

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Java array features Subscripts are denoted as expressions within brackets: [ ]

Base (element) type can be any type

Size of array must be specified

Index type is integer and the index range must be 0 ... n-1 Where n is the number of elements Just like Strings indexing!

Automatic bounds checking Ensures any reference to an array element is valid

Data field “.length” specifies the number of elements in the list

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Consider Segment

int[] b = new int[100];b[-1] = 0;b[100] = 0;

Causes Array variable to reference a new list of 100 integers

Each element is initialized to 0 Two exceptions to be thrown

-1 is not a valid index – too small 100 is not a valid index – too large

IndexOutOfBoundsException

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Consider

Point[] p = new Point[3];p[0] = new Point(0, 0);p[1] = new Point(1, 1);p[2] = new Point(2, 2);p[0].setX(1);p[1].setY(p[2].getY());Point vertex = new Point(4,4);p[1] = p[0];p[2] = vertex;

p

p[0] p[1] p[2]

null null null

Point: (0, 0)

p

p[0] p[1]

Point: (1, 1) Point: (2, 2)

p[2]

Point: (1, 0)

p

p[0] p[1]

Point: (1, 1) Point: (2, 2)

p[2]

Point: (1, 0)

p

p[0] p[1]

Point: (1, 2) Point: (2, 2)

p[2]

Point: (1, 0)

p

p[0] p[1]

Point: (1, 2) Point: (2, 2)

p[2]

vertex

Point: (4, 4)

Point: (1, 0)

p

p[0] p[1]

Point: (2, 2)

p[2]

vertex

Point: (4, 4)

Point: (1, 0)

p

p[0] p[1] p[2]

vertex

Point: (4, 4)

Point[] p = new Point[3];p[0] = new Point(0, 0);p[1] = new Point(1, 1);p[2] = new Point(2, 2);p[0].setX(1);p[1].setY(p[2].getY());Point vertex = new Point(4,4);p[1] = p[0];p[2] = vertex;

1111

New 2005 demotivatiors!New 2005 demotivatiors!

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Explicit initialization Syntax

ElementType[] id = { exp0 , exp1 , ... expn-1 };

id references an array of n elements. id[0] hasvalue exp0, id[1] has value exp1, and so on.

Each expi is an expression thatevaluates to type ElementType

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Explicit initialization Example

String[] puppy = { “pika”, “mila”, “arlo”, “nikki” };

int[] unit = { 1 };

Equivalent toString[] puppy = new String[4];puppy[0] = “pika"; puppy[1] = “mila";puppy[2] = “arlo"; puppy[3] = “nikki";

int[] unit = new int[1];unit[0] = 1;

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Array members Member length

Size of the arrayfor (int i = 0; i < puppy.length; ++i) {

System.out.println(puppy[i]);}

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Review of arrays Creating an array:

int[] foo = new int[10];

Accessing an array:foo[3] = 7;System.out.print (foo[1]);

Creating an array:String[] bar = new String[10];

Accessing an array:bar[3] = “qux”;System.out.println (bar[1]);

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How Java represents arrays Consider

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

a 1 2 3 4 5

+ …

Array

- length = 5

- data = 1 2 3 4 5

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More about how Java represents Arrays Consider

int[] a;int[] b = null;int[] c = new int[5];int[] d = { 1, 2, 3,

4, 5 };a = c;d = c;

1 2 3 4 5

0 0 0 0 0

a -

b null

c

d

int[] a;int[] b = null;int[] c = new int[5];int[] d = { 1, 2, 3,

4, 5 };a = c;d = c;

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What do these pictures What do these pictures mean?mean?

Light beerLight beer Dandy lionsDandy lions Assaulted Assaulted

peanutpeanut EggplantEggplant Dr. PepperDr. Pepper Pool tablePool table Tap dancersTap dancers Card sharkCard shark King of popKing of pop I PodI Pod Gator aideGator aide Knight mareKnight mare Hole milkHole milk

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Java Array Code Examples To print the array:

public void printArray(int[] data) {

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

System.out.println(data[i]);}

}

Considerint[] score = { 6, 9, 82, 11, 29, 85, 11, 28, 91 };printArray(score);

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Java Array Code Examples Returns a reversed version of the int arraypublic int[] reverse(int[] data) {

int[] clone = data.clone();for ( int i = 0; i < clone.length; ++i ) {

clone[i] = data[data.length-1-i];}return clone;

}

Considerint[] foo = { 1, 2, 3, 4, 5 };printArray( reverse (foo));

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Arrays