Java - Arrays

ARRAYS

Declaring an Array Variable

•         Do not have to create an array while declaring array variable

–   <type> [] variable_name;

–   int [] prime;

–   int prime[];

•         Both syntaxes are equivalent

•         No memory allocation at this point Defining an Array

•         Define an array as follows:

–   variable_name=new <type>[N];

–   primes=new int[10];

•         Declaring and defining in the same statement:

–   int[] primes=new int[10];

•         In JAVA, int is of 4 bytes, total space=4*10=40 bytes Array Size through Input

BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); String inData;

int   num;

System.out.println("Enter a Size for Array:"); inData = stdin.readLine();

num  = Integer.parseInt( inData ); // convert inData to int

long[] primes = new long[num];

System.out.println(“Array Length=”+primes.length);

….

SAMPLE RUN:

Enter a Size for Array:

4

Array Length=4

Default Initialization

•         When array is created, array elements are initialized

–   Numeric values (int, double, etc.) to 0

–   Boolean values to false

–   Char values to ‘\u0000’ (unicode for blank character)

–   Class types to null

Accessing Array Elements

•         Index of an array is defined as

–   Positive int, byte or short values

–   Expression that results into these types

•         Any other types used for index will give error

–   long, double, etc.

–   Incase Expression results in long, then type cast to int

•         Indexing starts from 0 and ends at N-1 primes[2]=0;

int k = primes[2]; Validating Indexes

•         JAVA checks whether the index values are valid at runtime

– If index is negative or greater than the size of the array then an IndexOutOfBoundException will be thrown

–   Program will normally be terminated unless handled in the try {} catch {}

•         long[] primes = new long[20];

•         primes[25]=33;

•         ….

•         Runtime Error:

•         Exception in thread “main” java.lang.ArrayIndexOutOfBoundsException: 25

•         at MorePrimes.main(MorePrimes.java:6)

Initializing Arrays

•         Initialize and specify size of array while declaring an array variable int[] primes={2,3,5,7,11,13,17}; //7 elements

•         You can initialize array with an existing array int[] even={2,4,6,8,10};

int[] value=even;

–   One array but two array variables!

–   Both array variables refer to the same array

–   Array can be accessed through either variable name

Array Length

•         Refer to array length using length

–   A data member of array object

–   array_variable_name.length

–   for(int k=0; k<primes.length;k++)

….

•         Sample Code:

long[] primes = new long[20]; System.out.println(primes.length);

• Output: 20 Sample Program class MinAlgorithm

{

public static void main ( String[] args )

{

int[] array = { -20, 19, 1, 5, -1, 27, 19, 5 } ;

int min=array[0]; // initialize the current minimum for ( int index=0; index < array.length; index++ )

if ( array[ index ] < min ) min = array[ index ] ;

System.out.println("The minimum of this array is: " + min );

}

}

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

Multidimensional Arrays

•         A farmer has 10 farms of beans each in 5 countries, and each farm has 30 fields!

•         Three-dimensional array

long[][][] beans=new long[5][10][30]; //beans[country][farm][fields]