©2004 Brooks/Cole Chapter 8 Arrays. Figures ©2004 Brooks/Cole CS 119: Intro to JavaFall 2005...

©2004 Brooks/Cole

Chapter 8

Arrays

Figures ©2004 Brooks/ColeCS 119: Intro to Java Fall 2005

Sometimes we have lists of data values that all need to be treated similarly

Three Lists of Items


Creating Arrays

• An array is an indexed collection of data values of the same type.

• Arrays are reference types (like objects)

• Three steps to creating an array– Declaration creates a reference variable

– Use new to allocate memory

– Assign values to individual elements


Arrays are like Objects

• Declaring an array allocates memory for a reference

– The declaration does NOT allocate memory for the array data

• The result of the declaration double prices[];


Allocating Memory for Array

• The Results of the Allocationprice = new double[6];– All elements are initialized to 0


Example: A List of Prices

• Declarationdouble[] prices;

• Memory Allocationprices = new double[6]

• Assigning valuesprices[0] = 10.96;prices[1] = 6.43;prices[2] = 2.58;prices[3] = 0.58;prices[4] = 12.27;prices[5] = 6.39;


More Array Allocations

int [] grade = new int[5];

char [] code = new char[4];


Array Allocation

• You don't have to use a literal value for the array size– You can use any integer expression

• Examplesint [] grade = new int[NUMELS];

int [] grade = new int[ size * 2];


Array Size

• Once memory is allocated, the array has a fixed size

• Every array has a variable named length associated with it– length contains the number of elements for

which memory was allocated– get the length using

arrayName.length


Access to Array Elements

• Use [ ] to access individual elements of an array– Index can be any integer expression– Individual elements can be used anywhere that

a value of the base type can be used– Indexes go from 0 to one less that the length of

the array


Element Access Example

• grade[3]


Practice

• Declare and allocate memory for the following– a list of 100 integer years– a list of 6 character codes– a list of 32 floating point velocities

• Access the 1st, 3rd and 7th elements of the arrays above


More Practice

• What elements are output by the followingfor (m=1; m<=5; m++)

System.out.println( a[m] + " ");

for (int k=1; k< 5; k=k+2)

System.out.println( a[k] + " ");


Array Initializers

• You can initialize the elements of an array when you declare itint [] grade = {98, 87, 92, 79, 85};

char[] code

= {'s', 'a', 'm', 'p', 'l', 'e'};

• The size of the array is determined by the number of elements in the initializer list.– grade.length is 5 elements, code.length is 6


List of Prices with Initializer

• Declaration with initializerdouble[] prices = {10.96, 6.43, 2.58, 0.86, 12.27, 6.39};

• The array has 6 elements– prices.length will be 6


More Practice

• Write array declarations with initialization for the following– integer grades 89, 75, 82, 93, 78, 95, 81– floating point temperatures 78.2, 69.6, 68.5,

83.9, 55.4– character menu choices p, a, c, w, m, q

• What is the length of each of the arrays?


Using Arrays

• Once you have created an array, you can access the elements in any order

• Use loops to process all elements of the array in the same way– input values for each element– modify each element in the same way– print each element out


Arrays of Objects

• Declaration works the same as for arrays of primitive values

• Memory is allocated to store a reference to the array dataString [] names; String names[];


Allocating Memory for an Array of Strings

• Instantiation creates an array of referencesnames = new

String[4]• Each element of the array

has to be instantiated individuallynames[0] = "Joe";

names[1] = new String("Harriet");


The Assignment of Values Creates Actual Array Objects


Using arrays with methods

• Arrays can be used as parameters for methods– location of the array is copied into the method

parameter variable– the contents of the array can be modified by the

method

• An array can be returned by a method– the location of the array is returned


Arrays as Method Parameters

• Only one array exists– method gets a copy of the location


Arrays as method argumentsThe Location of the Array is Passed


Practice

• Write the header for a method called sort that takes an array of double values as a parameter and returns nothing (modifies the existing array)

• Write the header for a method called average that takes an array of doubles as a parameter and returns a double

• Write the header for a method called readArray that takes no parameters and returns an array of integers


2D Arrays

• What if we need to make a table instead of a list?– We can make arrays with two indexes

• Think of 2D array as an array whose elements are arrays


Creating 2D Arrays

• Declaration

int [][] table • Instantiation

table = new int[3][4];• Declaration with initialization

int[][]table2

= {{8, 16, 9, 52},

{3, 15, 27, 6},

{14, 25, 2, 10}};


Each Array Element Is Identified by Its Row and Column Position

• Access row 1 with table[1]• Access element 3 of row 2 with table[2][3]


Practice

• Write declaration and allocation statements for– an array of integers with 6 rows and 10

columns– an array of doubles with 10 rows and 25

columns

• Write loops needed to add two 2x3 element arrays on integers and put the results in a new array.


Representation of aThree-Dimensional Array


Higher Dimensional arrays are also possible

• Need one index for each dimension

• For n dimensions, need n nested loops to process all elements

• Higher dimensional arrays are harder to visualize


Deep Copy

• After the Call to System.arraycopy()newnums = System.arraycopy(nums)– This method creates a new array and copies the

values from the original


The Result of a Shallow Copy

newnums = nums

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©2004 Brooks/Cole Chapter 8 Arrays. Figures ©2004 Brooks/Cole CS 119: Intro to JavaFall 2005...

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