Arrays-1For use of IST410 Students only

Arrays

Arrays-2For use of IST410 Students only

Objectives

Declaring arrays Instantiating arrays Using arrays as parameters One and multi-dimensional arrays A few array utilities

For use of IST410 Students only Arrays-3

Arrays: Introduction

Concept of array is not unique to Java; all modern programming languages support arrays

Arrays exist in C, C++, Pascal, and other languages In COBOL, tables with exactly one field type are arrays Suppose we are interested in representing the result of a

survey that has exactly one question and 200 responses We could declare 200 variables: resp0, resp1, resp2,...,resp199 Or, we could declare an array with 200 elements, each element

representing a response If the name of the array is resp, then its elements are resp[0],

resp[1], resp[2],...,resp[199] Notice that indexes go from 0 to 199, one less than the array size

For use of IST410 Students only Arrays-4

Arrays: Introduction

An array index need not be a constant, it can be a variable or an expression that yields an integer value

int i;// other statementsi = 118;resp[i] = 20; //20 is being assigned to the 119th

// element of the resp array The data type of an array can be any legal data type i.e

both primitive and object types Arrays group objects of the same data type i.e all elements

of an array are of same type

For use of IST410 Students only Arrays-5

Declaring Arrays

In Java, arrays are objects Array objects are created in two steps:

Declare an array reference Construct the array instance

Declarationint m[]; or int [] m;Point p[]; or Point [] p;

Declaration leads to space allocation for one reference variable: space for m in the case of the int array and space for p for the other

Space is allocated for elements of the array only after the array is instantiated

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Instantiating Arrays

Arrays need to be instantiated before they can be used Arrays with primitive data types

m = new int[20];

Array of objectsp = new Point[20];

p[0] = new Point(); // objects need to be constructed

m 0 1 2 19

p 0 1 2 19

space for array elements

Object references only

Arrays-7For use of IST410 Students only

Instantiating Arrays

Array elements are automatically initialized when the array is instantiated

The initial value depends on the data type of the array elements

values of m are set to 0 since the type is intvalues of p are set to null since these are references

An array can be explicitly initialized at declarationint [] m = {1, 23, -2, 0, 12};Point [] p = { new Point(1,1), new Point(2,3),

new Point(3,5)};String [] s = {“IST203”,”IST211”,”IST221”};

Arrays-8For use of IST410 Students only

Traversing Arrays

Arrays are often traversed using loops We can set up a loop to traverse through an array

for (int i = 0; i < 20; i++) {System.out.println(“Element

“+i+”=“+m[i]); } Use of fixed numbers like 20 is potentially error prone

since it is illegal to exceed the boundary of an array All arrays know their lengths; we can use this feature to

traverse any array

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Traversing Arrays

The count of array elements is stored in the array’s length attribute

We can traverse the array using the value of length to control for array boundaries

for (int i = 0; i < m.length; i++) {System.out.println(“Element

“+i+”=“+m[i]); } Traversing an array using the length attribute is a safe

technique

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Copying Arrays

Need to copy an array arises in many applications We can try to copy an array using the following

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

Notice that we did not duplicate the array, but copied the reference only; both a and b refer to the same array

b

a 1 2 3 4 5

Arrays-11For use of IST410 Students only

Copying Arrays

Arrays can be copied using System.arraycopy

System.arraycopy(a, 0, b, 0, a.length);

The general syntax of arraycopySystem.arraycopy(source array, source index, target array, target index, # of elements);

a

b

1 2 3 4 5

1 2 3 4 5

Arrays-12For use of IST410 Students only

Comparing Arrays

Consider the following two arrays int [] b = {1,2,3,4,5};

int [] c = {1,2,3,4,5};if (b = = c) System.out.println(”b and c are same arrays");else System.out.println(”b and c are different arrays");

Unfortunately, in the above statement, b would be found not equal to c

We are comparing references; b and c refer to TWO DIFFERENT objects, hence unequal

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Comparing Arrays

We can compare arrays using the equals method import java.util.Arrays;

int [] b = {1,2,3,4,5};int [] c = {1,2,3,4,5};if (Arrays.equals(b,c)) System.out.println(”b and c are same arrays");else System.out.println(”b and c are different arrays");

Contents of arrays are compared when Arrays.equals is used

java.util.Arrays provides a number of overloaded equals method to compare arrays of different types

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Multidimensional Arrays

Java arrays can be multidimensional A multidimensional array is an array of arrays For example, an array that has 3 rows and 5 columns can

be shown as the following

We need 2 different indexes to designate an individual cell

a[0] -----> a[0][0] a[0][1] a[0][2] a[0][3] a[0][4]

a[1] -----> a[1][0] a[1][1] a[1][2] a[1][3] a[1][4]

a[2] -----> a[2][0] a[2][1] a[2][2] a[2][3] a[2][4]

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Multidimensional Arrays

As in the case of one dimensional arrays, creation of a multidimensional array is a two-step process

int [][] p;p = new int[4][];

Since the second dimension of p is undefined, elements of p need to be defined one at a time

p[0] = new int[5];p[1] = new int[5]; and so on

The size of a multidimensional array may be completely defined at declarationint [][] p = new int[4][5]; // 4 rows and 5 columns

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Multidimensional Arrays

It is illegal to leave first dimension empty in the declaration of a multidimensional array

int [][] m = new int[][5]; // does not compile When instantiated, elements of an array are automatically

initialized to the appropriate default value based on the array’s data type

In our example, p is an int array. All elements of p are initially set to 0.

For an array of objects, elements would be initialized to null

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Multidimensional Arrays

Multidimensional arrays can be initialized at declarationint [][] m = { {1,2,3}, {4,5,6,7}, {8,9,10,11}};

A multidimensional array of objects is declared using the familiar syntax

Point [][] pt = new Point[5][10]; Each cell of pt then is a reference and and objects

themselves need to be instantiatedpt[0][0] = new Point();

pt[0][1] = new Point();

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Multidimensional Arrays

A multidimensional array need not be rectangular For example, a twodimensional array can be non-

rectangular as shown below

int [][] m = new int[4][]; m[0] = new int[5];

m[1] = new int[6]; m[2] = new int[10]; m[3] = new int[12];

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Multidimensional Arrays

A multidimensional array can be traversed using length attributefor (i = 0; i < m.length; i++)

for (j = 0; j < m[i].length; j++)System.out.println(“Element = “+m[i][j]);

Notice that so long as the length attribute of an array is used, traversing a non-rectangular array is not a problem

Arrays-20For use of IST410 Students only

Arrays as Parameters

public class ArraySearch { //ArraySearch.java public static void main(String args[]) { char [] c = {'A','C','D','E','F','H','K','J'}; int index = ArraySearch.find(c,'F'); if (index == -1)

System.out.println("Search character not found"); else

System.out.println("F is at index position "+index); } public static int find(char[]t, char s){ int index = 0; int location = -1; while (index < t.length && location == -1) {

if (t[index] == s){ location = index; } index++;

} return location; }}

Note the declaration of array parameter in the method find, and also how arrays are passed into a method

Arrays-21For use of IST410 Students only

Sorting one dimensional arrays

An array can be sorted as follows

import java.util.Arrays;int []m = {1,2,45,56,78,34,8,-1};

Arrays.sort(m);

Sorted array is in ascending order A number of overloaded sort methods are available sort method can be applied to one dimensional arrays only

Arrays-22For use of IST410 Students only

Binary search: one dimensional Arrays

Binary search for a value int []m = {1,2,45,56,78,34,8,-1}; Arrays.sort(m);int r = Arrays.binarySearch(m,8);if (r < 0)

System.out.println(“Item not found”);else

System.out.println(“Item at “+r); If the item searched for is in the list, the correct index is

returned If r is negative, the item does not exist in the list. An

index equal to (-r-1) represents the index where the value can be inserted to keep the array sorted

Arrays-23For use of IST410 Students only

Exercise

Write a MyArray class that uses a one-dimensional array of 100 elements. This array has the following methods:

a constructor that initializes the elements to a random integer between 10 and 140

a method that returns the average of all elements of the array a method that returns the sum of all elements

Write a MyTwodArray class that uses a two-dimensional array. This array has the following methods

a constructor that initializes the elements to a random character a method that takes 2 arguments: a row index and a search

character and returns the number of times this character occurs in the row

To be more challenging, take the 2 dimensional array sizes as command line arguments and test to make sure that these sizes are in fact read in as arguments before proceeding further