Chapter 9: Arrays

1. Chapter 9: Arrays JavaProgramming: From Problem Analysis to Program Design, Second Edition

2. Chapter Objectives • Learn about arrays. • Explore how to declare and manipulate data into arrays. • Understand the meaning of “array index out of bounds.” • Become familiar with the restrictions on array processing.

3. Chapter Objectives • Discover how to pass an array as a parameter to a method. • Discover how to manipulate data in a two-dimensional array. • Learn about multidimensional arrays.

4. Enter 5 numbers: 3 6 8 9 0Sum = 26Numbers is reverse = 0 9 8 6 3 Motivation Write a program to read 5 numbers, find their sum and print them in reverse order. import java.util.*; public class RevOrder{ static Scanner console = new Scanner(System.in); public static void main(String[] args){ int item0, item1, item2, item3, item4; int sum; System.out.println("Enter 5 numbers: "); item0=console.nextInt(); item1=console.nextInt(); item2=console.nextInt(); item3=console.nextInt(); item4=console.nextInt(); sum = item0+item1+item2+item3+item4; System.out.println("Sum = " + sum); System.out.print("Numbers is reverse = ");System.out.println(item4+" "+item3+" "+item2+" "+item1+" "+item0); }}

5. Motivation Write a program to read 100 numbers, find their sum and print them in reverse order ?????

6. Motivation • Note the following in the preceding program: • 5 variables must be declared. • All variables are of type int. • Variables name (num0  num4); • Instead of having to use a large number of individual and unconnected variables, we can use a single data structure that combines them.

7. Motivation • In this chapter, we will examine: • How to store collections of items and access these items using one variable name given to the collection. • The data structure that lets you do all of theses things in java is called array.

8. Array • A structured data type with a fixed number of components. • Every component is of the same type. • Components are accessed using their relative positions in the array.

9. One-Dimensional Arrays • Syntax to declare an array: • dataType[ ] arrayName; • In java, an array is an object: • It is a reference variable. • To store data we must instantiate the array object using new operator. • Syntax to instantiate an array object: arrayName = new dataType[intExp] Where intExp = number of components in array >= 0

10. One-Dimensional Arrays • Declaration with instantiation: dataType[ ] arrayName = new dataType[intExp] • Declaring more than one array of the same type: dataType[ ] arrayName1, arrayName2; • Syntax to access an array component: • arrayName[indexExp] • intExp = number of components in array >= 0 • 0 <= indexExp <= intExp

11. Array num Example9-1 • The statement: int[] num = new int[5]; • Declares and creates the array num of 5 components. • Each component is of type int. • The components are accessed as: num[0], num[1],num[2],num[3],num[4]

12. Array num int[] num = new int[5]; Why all the values = 0 ???????????

13. Array num • When an array is instantiated. Java automatically initializes its components to their default values. For example, numeric arrays are initialized to 0. • What are string, char array initialized to?

14. Alternate Ways to Declare an array • int list[];// here [] is after list not int • But take care, what is the difference here: • int alpha[], beta; • int[] gamma, delta; • It is recommended that you declare arrays as int[].

15. Array list int [] list = new int[10];

16. Array list

17. Specifying Array Size During Program Execution int arraySize; System.out.print("Enter the size of the array: "); arraySize = console.nextInt(); System.out.println(); int[] list = new int[arraySize]; • Arrays that are created during program execution are called dynamic arrays.

18. Array Initialization During Declaration double[] sales = {12.25, 32.50, 16.90, 23, 45.68}; • The values, called initial values, are placed between braces and separated by commas. • Here, sales[0]= 12.25, sales[1]= 32.50, sales[2]= 16.90, sales[3]= 23.00, and sales[4]= 45.68. • When declaring and initializing arrays, the size of the array is determined by the number of initial values within the braces. • If an array is declared and initialized simultaneously, we do not use the operator new to instantiate the array object.

19. Arrays and the Instance Variable length • A public instance variable length is associated with each array that has been instantiated. • The variable length contains the size of the array. • The variable length can be directly accessed in a program using the array name and the dot operator. • This statement creates the array list of six components and initializes the components using the values given. int[] list = {10, 20, 30, 40, 50, 60}; Here, list.length is 6.

20. Arrays and the Instance Variable length • This statement creates the array numList of 10 components and initializes each component to 0. int[] numList = new int[10]; • The value of numList.length is 10. • These statements store 5, 10, 15, and 20, respectively, in the first four components of numList. numList[0] = 5; numList[1] = 10; numList[2] = 15; numList[3] = 20; • The value of numList.length is still 10. • You can store the number of filled elements, that is, the actual number of elements, in the array in a variable, SAYnoOfElement. It is a common practice for a program to keep track of the number of filled elements in an array.

21. Processing One-Dimensional Arrays • Loops used to step through elements in array and perform operations. int[] list = new int[100]; int i; //process list[i], the (i + 1)th element of list for (i = 0; i < list.length; i++) //inputting data to list for (i = 0; i < list.length; i++) list[i] = console.nextInt(); //outputting data from list for (i = 0; i < list.length; i++) System.out.print(list[i] + " "); • Remember to start loop counter from 0 because Array index starts from 0.

22. Arrays • Some operations on arrays: • Initialize • Input data • Output stored data • Find largest/smallest/sum/average of elements Example 9_3 double[] sales = newdouble[10]; int index; double largestSale, sum, average;

23. Code to Initialize Array to Specific Value (10.00) for (index = 0; index < sales.length; index++) sales[index] = 10.00; Note: .length NOT .length() It is a variable not a method.

24. Code to Read Data into Array for (index = 0; index < sales.length; index++) sales[index] = console.nextDouble();

25. Code to Print Array for (index = 0; index < sales.length; index++) System.out.print(sales[index] + " ");

26. Code to Find Sum and Average of Array sum = 0; for (index = 0; index < sales.length; index++) sum = sum + sales[index]; if (sales.length != 0) average = sum / sales.length; else average = 0.0;

27. Determining Largest Element in Array maxIndex = 0; for (index = 1; index < sales.length; index++) if (sales[maxIndex] < sales[index]) maxIndex = index; largestSale = sales[maxIndex];

28. Determining Largest Element in Array

29. Determining Largest Element in Array After for loop executes, maxIndex=7 largestSale=sales[maxIndex]= 98.23. Exercises: • How to find the smallest element in an array???? • Rewrite the program that we discussed in the beginning of this chapter: the RevOrd class.

30. Array Index Out of Bounds • An array is in bounds if: 0 <= index <= arraySize – 1 • If index < 0 or index > arraySize: ArrayIndexOutOfBoundsException exception is thrown. If the program does not handle this exception, the program terminates.

31. Base address: • Base address: is the address (memory location) of the first component in an array. • Base address of list is the address of list[0]. • Value of variable list is the base address of the array= the address of list[0]. • When you pass an array as a parameter, the base address of the actual array is passed to the formal parameter.

32. Declaring Arrays as Formal Parameters to Methods General syntax to declare an array as a formal parameter: dataType[] arrayName public static void arraysAsFormalParameter(int[] listA, double[] listB, int num) { //... } int[] intList = newint[10]; double[] doubleNumList = newdouble[15]; int number; arraysAsFormalParameter(intList, doubleNumList, number);

33. The Assignment Operators and Arrays int listA={5,10,15,20,25,30,35}; int listB=new int[listA.length];

34. The Assignment Operators and Arrays • You can use the assignment operator(=) to assign listA to listB. However, the result obtained might not be what you expect. • For example: listA = listB; • You expect: element of listA are copied to listB. • That is wrong,listA is a refrence varible, both listA and listB refer to the same array.

35. The Assignment Operators and Arrays

36. The Assignment Operators and Arrays To copy listA components to listB: for (int index = 0; index < listA.length; index++) listB[index] = listA[index];

37. Relational Operators Arrays • if (listA == listB) • ... • The expression listA == listB determines if the values of listA and listB are the same, thus determining whether listA and listB refer to the same array. • To determine whether listA and listB contain the same elements, you need to compare them component by component. • You can write a method that returns true if two int arrays contain the same elements.

38. Relational Operators and Arrays boolean isEqualArrays(int[] firstArray, int[] secondArray) { if (firstArray.length != secondArray.length) return false; for (int index = 0; index < firstArray.length; index++) if (firstArray[index] != secondArray[index]) return false; return true; } if (isEqualArrays(listA, listB)) ...

39. Methods for Array Processing Example 9_5 public static void fillArray(int[] list, int noOfElements) { int index; for (index = 0; index < noOfElements; index++) list[index] = console.nextInt(); }

40. Methods for Array Processing public static void printArray(int[] list, int noOfElements) { int index; for (index = 0; index < noOfElements; index++) System.out.print(list[index] + " "); } public static int sumArray(int[] list, int noOfElements) { int index; int sum = 0; for (index = 0; index < noOfElements; index++) sum = sum + list[index]; return sum; }

41. Methods for Array Processing public static int indexLargestElement(int[] list, int noOfElements) { int index; int maxIndex = 0; for (index = 1; index < noOfElements; index++) if (list[maxIndex] < list[index]) maxIndex = index; return maxIndex; } public static void copyArray(int[] list1, int[] list2, int noOfElements) { int index; for (index = 0; index < noOfElements; index++) list2[index] = list1[index]; }

42. Methods for Array Processing You MUST Check example9_6 for calling these methods.

43. Parallel Arrays • Arrays are parallel if the corresponding components hold related information. • Example: You need to keep track of 50 students ID with their grades. Int [] studentId = new int[50]; Char[] courseGrade = new char[50]; studentId[3] and courseGrade[3] holds the data for the same student. studentId courseGrade 2345 A 4563 C 4590 C 2404 B

44. Arrays of Objects • Can use arrays to manipulate objects. • Example: Create an array named array1 with N objects of type T: T[] array1 = new T[N] • Can instantiate array1 as follows: for(int j=0; j <array1.length; j++) array1[j] = new T();

45. Array of String Objects String[] nameList = new String[5]; nameList[0] = "Amanda Green"; nameList[1] = "Vijay Arora"; nameList[2] = "Sheila Mann"; nameList[3] = "Rohit Sharma"; nameList[4] = "Mandy Johnson";

46. Array of String Objects

47. Array of String Objects • You can use String methods to work with the objects of nameList. • nameList[0].equals(“Amanda Green”) // true • nameList[4].substring(0,5) //Mandy

48. Arrays of Objects of Other Classes Clock[] arrivalTimeEmp = new Clock[100];

49. Instantiating Array Objects for (int j = 0; j < arrivalTimeEmp.length; j++) arrivalTimeEmp[j] = new Clock();

50. Instantiating Array Objects arrivalTimeEmp[49].setTime(8, 5, 10);