200701 Computing1 Unit4 Arrayconcept

Tutorial 4

Prepared by : Ng Mee Mee

Learning Objectives  

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Introducing Arrays Declaring Array Variables, Creating Arrays, and Initializing Arrays Passing Arrays to Methods

Introducing Arrays Array is a data structure that represents a collection of the same types of data. The entire array Each value has a numeric index has a single name

0 scores

1

2

3

4

5

6

7

8

9

79 87 94 82 67 98 87 81 74 91

An array of size N is indexed from zero to N-1 This array holds 10 values that are indexed from 0 to 9

Introducing Arrays, cont 

A particular value in an array is referenced using the array name followed by the index in brackets

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For example, the expression scores[2] refers to the value 94 (the 3rd value in the array)

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That expression represents a place to store a single integer and can be used wherever an integer variable can be used

Introducing Arrays,cont 

For example, an array element can be assigned a value, printed, or used in a calculation: scores[2] = 89; scores[first] = scores[first] + 2; mean = (scores[0] + scores[1])/2; System.out.println ("Top = " + scores[5]);

Introducing Arrays,cont 

The values held in an array are called array elements

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An array stores multiple values of the same type (the element type)

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The element type can be a primitive type or an object reference

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Therefore, we can create an array of integers, or an array of characters, or an array of String objects, etc.

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In Java, the array itself is an object

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Therefore the name of the array is a object reference variable, and the array itself must be instantiated

Declaring Array Variables 

Syntax to declare an array varaible: 

dataType[] arrayName;

Or 

dataType arrayName[];//this style is correct , but not preferred

Example: double[] myList; or double myList[];

Creating Arrays 

Using new operator to create an array arrayName = new dataType[arraySize];  It creates an array using new dataType[arraySize] 

Ir assigns the reference of the newly created array to the variable arrayName

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Example: double[] myList = new double[10];

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myList[0]

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references the first element in the array. myList[9] references the last element in the array.

Creating Arrays, cont

double[] myList = new double[10]; myList

reference

Array reference variable Array element at index 5

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

5.6 4.5 3.3 13.2 4 34.33 34 45.45 99.993 11123

Element value

Declaring and Creating in One Step 

datatype[] arrayname = new datatype[arraySize]; double[] myList = new double[10];

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datatype arrayname[] = new datatype[arraySize]; double myList[] = new double[10];

The Length of Arrays 

Once an array is created, its size is fixed. It cannot be changed. You can find its size using

arrayVariable.length For example, myList.length returns 10

Example: The Length of Arrays public class ArrayLength{ public static void main(String[] args){ double[ ] myList=new double[10]; System.out.println(myList.length); System.out.println(myList[9]); } }

Initializing Arrays 

Using a loop: for (int i = 0; i < myList.length; i++) myList[i] = i;

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Declaring, creating, initializing in one step: double[] myList = {1.9, 2.9, 3.4, 3.5};

This shorthand syntax must be in one statement.

Example: Initializing Arrays Using a for loop public class ArrayLength{ public static void main(String[] args){ double[ ] myList=new double[10]; for(int i=0; i < myList.length; i++){ myList[i] =i; System.out.println(myList[i]); } System.out.println("the last element of myList is " + myList[9]); } }

Example: Declaring, creating, initializing in one step public class ArrayLength{ public static void main(String[] args){ double[ ] myList={1.9,2.9,3.4,3.5}; for(int i=0; i < myList.length; i++){ System.out.println(myList[i]); } } }

Declaring, creating, initializing Using the Shorthand Notation double[] myList = {1.9, 2.9, 3.4, 3.5};

This shorthand notation is equivalent to the following statements: double[] myList = new double[4]; myList[0] = 1.9; myList[1] = 2.9; myList[2] = 3.4; myList[3] = 3.5;

Example: Declares, creates, and initializes the array myList with 4 elements public class ArrayLength{ public static void main(String[] args){ double[] myList=new double[4]; myList[0] = 1.9; myList[1] = 2.9; myList[2] = 3.4; myList[3] = 3.5; for(int i=0; i < myList.length; i++){ System.out.println(myList[i]); } } }

Passing Arrays to Methods   Java uses pass by value to pass parameters  to a method. There are important  differences between passing a value of  variables of primitive data types and  passing arrays.   For a parameter of a primitive type  value, the actual value is passed.  Changing the value of the local parameter  inside the method does not affect the  value of the variable outside the method.   For a parameter of an array type, the  value of the parameter contains a  reference to an array; this reference is  passed to the method. Any changes to the  array that occur inside the method body 

Example: Passing Arrays to Methods public class ArrayLength{ public static void main(String[ ] args){ double[ ] myList=new double[4]; myList[0] = 1.9; myList[1] = 2.9; myList[2] = 3.4; myList[3] = 3.5; printArray(myList); } public static void printArray(double[ ] myList){ for(int i=0; i < myList.length; i++){ System.out.println(myList[i]); } } }

Example: Pass by Value public class Test{ public static void main(String[] args){ int x =1; // x repersents an int values int[] y = new int[10]; // y represents an array of int values m(x,y); //invoke m with argunment x and y System.out.println("x is " + x); System.out.println("y[0] is " + y[0]); } public static void m(int number, int[] numbers){ number=1001;//assign a new value to number numbers[0]=5555;//assign a new value to numbers[0] } }