ARRAYSinJavaProgramming_Introduction.ppt

Arrays in JAVA
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
Array is collection of related data items

Creating an array
 Declare an array
 Create memory location
 Putting values to memory locations

Declaring an Array Variable

Do not have to create an array while declaring array
variable
 <type> [ ] variable_name;
 Double[ ] myList;
 double myList[ ];

Both syntaxes are equivalent

No memory allocation at this point

Defining an Array
Define an array as follows:
 variable_name=new <type>[arraySize];
 Number = new int[5];
 Mylist = new int[10];
It creates an array using new dataType[arraySize];

It assigns the reference of the newly created array to the variable
variable_name.

dataType arrayname[ ] = {list of values};

Int a [ ]={1,2,3,4,5,6,7,};

Array index starts from 0 to arraySize-1;

int is of 4 bytes, total space=4*10=40 bytes
Declaring and defining in the same statement:
 Double [ ] mylist = new double[10];


What happens if we define diffrent type…

We define
 Int[ ] a=new long[20];
incompatible types
found: long[]
required: int[]

The right hand side defines an array, and thus the array variable should
refer to the same type of array
Example:
int prime[100];
error=> ']' expected
long primes[20];

The C++ style is not permitted in JAVA syntax
long[] primes = new long[20];
primes[25]=33;
Runtime Error:Exception in thread “main”
java.lang.ArrayIndexOutOfBoundsException

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

Example for array
public class TestArray {
public static void main(String[] args) {
double[] myList = {1.9, 2.9, 3.4, 3.5};
// Print all the array elements
for (double element: myList) {
System.out.println(element);
}
}
}
Otput:
1.9
2.9
3.4
3.5

Reusing Array Variables

int[] primes=new int[10];
……
primes=new int[50];

Previous array will be discarded

Cannot alter the type of array

Demonstration
long[] primes = new long[20];
primes[0] = 2;
primes[1] = 3;
System.out.println(primes[0]);
System.out.println(primes[1]);
Output:
2
3

Array Length

Refer to array length using length() method
 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

If number of elements in the array are changed,
JAVA will automatically change the length attribute!

Sample Program
class MinArray
{
public static void main ( String[] args )
{
int[] array = { 20, 19, 1, 5, 71, 27, 19, 95 } ;
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 );
}
}
*Program taken from: http://chortle.ccsu.edu/CS151/Notes/chap47/ch47_10.html

Two dimenssional array

Representing 2D arrays
 Int myarray[][];
 Myarray = new int[3][4];
 Int myarray [][] = new int[3][4];

Example

Int myarray[2][3]={0,0,0,1,1,1};
2 columns and 3 rows
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ARRAYSinJavaProgramming_Introduction.ppt

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