Chapter 9: Arrays J ava P rogramming: From Problem Analysis to Program Design, From Problem Analysis...

Chapter 9: ArraysChapter 9: Arrays

JJavaava PProgramming:rogramming:

From Problem Analysis to Program From Problem Analysis to Program

Design,Design, Second EditionSecond Edition

Java Programming: 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.

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.

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.

One-Dimensional Arrays

Syntax to instantiate an array: dataType[ ] arrayName;arrayName = new dataType[intExp]

dataType[ ] arrayName = new dataType[intExp] dataType[ ] arrayName1, arrayName2;

Syntax to access an array component: arrayName[indexExp]

intExp = number of components in array >= 0 0 <= indexExp <= intExp

int[] num = new int[5];

Array num

Array list

int arraySize;

System.out.print("Enter the size of "

+ "the array: ");

arraySize = console.nextInt();

System.out.println();

int[] list = new int[arraySize];

Specifying Array Size During Program Execution

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.

Array Initialization During Declaration

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. Here list.length is 6.

int[] list = {10, 20, 30, 40, 50, 60};

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;

You can store the number of filled elements, that is, the actual number of elements, in the array in a variable, say noOfElement. It is a common practice for a program to keep track of the number of filled elements in an array.

Arrays and the Instance Variable length

Loops used to step through elements in array and perform operations.

int[] list = new int[100];int i;

for (i = 0; i < list.length; i++) //process list[i], the (i + 1)th //element of list

for (i = 0; i < list.length; i++) list[i] = console.nextInt();

for (i = 0; i < list.length; i++) System.out.print(list[i] + " ");

Processing One-Dimensional Arrays

Arrays Some operations on arrays:

Initialize Input data Output stored data Find largest/smallest/sum/average of elements

double[] sales = new double[10];int index;double largestSale, sum, average;

Code to Initialize Array to Specific Value (10.00)

for (index = 0; index < sales.length; index++) sales[index] = 10.00;

Code to Read Data into Array

for (index = 0; index < sales.length; index++) sales[index] = console.nextDouble();

Code to Print Array

for (index = 0; index < sales.length; index++) System.out.print(sales[index] + " ");

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;

Determining Largest Element in Array

maxIndex = 0;

for (index = 1; index < sales.length; index++) if (sales[maxIndex] < sales[index]) maxIndex = index;

largestSale = sales[maxIndex];

Determining Largest Element in Array

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.

Base address: Memory location of the first component in an array.

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 = new int[10];double[] doubleNumList = new double[15];int number;

arraysAsFormalParameter(intList, doubleNumList, number);

The Assignment Operators and Arrays

for (int index = 0; index < listA.length;

index++)

listB[index] = listA[index];

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.

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))...

Methods for Array Processing

public static void fillArray(int[] list,

int noOfElements)

int index;

for (index = 0; index < noOfElements;

index++)

list[index] = console.nextInt();

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;}

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];}

Parallel Arrays

Arrays are parallel if the corresponding components hold related information.

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();

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";

Array of String Objects

Clock[] arrivalTimeEmp = new Clock[100];

Arrays of Objects

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

arrivalTimeEmp[49].setTime(8, 5, 10);

Instantiating Array Objects

Arrays and Variable Length Parameter List

The syntax to declare a variable length formal parameter (list) is:

dataType ... identifier

public static double largest(double ... numList){ double max; int index; if (numList.length != 0) { max = list[0]; for (index = 1; index < numList.length; index++) { if (max < numList [index]) max = numList [index]; } return max; } return 0.0;}

double num1 = largest(34, 56);

double num2 = largest(12.56, 84, 92);

double num3 = largest(98.32, 77, 64.67, 56);

System.out.println(largest(22.50, 67.78,

92.58, 45, 34, 56));

double[] numberList = {18. 50, 44, 56.23, 17.89

92.34, 112.0, 77, 11, 22,

86.62);

System.out.println(largest(numberList));

foreach loop

The syntax to use this for loop to process the elements of an array is:

for (dataType identifier : arrayName)

statements

identifier is a variable, and the data type of identifier is the same as the data type of the array components.

foreach loop

sum = 0; for (double num : list) sum = sum + num;

The for statement in Line 2 is read for each num in list. The identifier num is initialized to list[0]. In the next iteration, the value of num is list[1], and so on.

for (double num : numList){ if (max < num) max = num;}

Two-Dimensional Arrays Data is sometimes in table form (difficult to represent using a

one-dimensional array).

To declare/instantiate a two-dimensional array:

dataType[ ][ ] arrayName = newdataType[intExp1][intExp2];

To access a component of a two-dimensional array:arrayName[indexExp1][indexExp2];

intExp1, intExp2 >= 0 indexExp1 = row position indexExp2 = column position

Two-Dimensional Arrays

Can specify different number of columns for each row (ragged arrays).

Three ways to process two-dimensional arrays: Entire array. Particular row of array (row processing). Particular column of array (column processing).

Processing algorithms is similar to processing algorithms of one-dimensional arrays.

double[][]sales = new double[10][5];

Two-Dimensional Arrays

Accessing Two-Dimensional Array Components

Two-Dimensional Arrays: Special Cases

Two-Dimensional Arrays: Processing

Initialization

for (row = 0; row < matrix.length; row++) for (col = 0; col < matrix[row].length; col++) matrix[row][col] = 10;

for (row = 0; row < matrix.length; row++){ for (col = 0; col < matrix[row].length; col++) System.out.printf("%7d", matrix[row][col]); System.out.println();}

for (row = 0; row < matrix.length; row++) for (col = 0; col < matrix[row].length; col++) matrix[row][col] = console.nextInt();

Sum by Row

for (row = 0; row < matrix.length; row++){ sum = 0; for (col = 0; col < matrix[row].length; col++) sum = sum + matrix[row][col]; System.out.println("Sum of row " + (row + 1) + " = "+ sum);}

Sum by Column

for (col = 0; col < matrix[0].length; col++){ sum = 0; for (row = 0; row < matrix.length; row++) sum = sum + matrix[row][col]; System.out.println("Sum of column " + (col + 1) + " = " + sum);}

Largest Element in Each Row

for (row = 0; row < matrix.length; row++){ largest = matrix[row][0]; for (col = 1; col < matrix[row].length; col++) if (largest < matrix[row][col]) largest = matrix[row][col]; System.out.println("The largest element of row " + (row + 1) + " = " + largest);}

Largest Element in Each Column

for (col = 0; col < matrix[0].length; col++){ largest = matrix[0][col]; for (row = 1; row < matrix.length; row++) if (largest < matrix[row][col]) largest = matrix[row][col]; System.out.println("The largest element of col " + (col + 1) + " = " + largest);}

Multidimensional Arrays

Can define three-dimensional arrays or n-dimensional arrays (n can be any number).

Syntax to declare and instantiate array:

dataType[][]…[] arrayName = new dataType[intExp1][intExp2]…[intExpn];

Syntax to access component:

arrayName[indexExp1][indexExp2]…[indexExpn]

intExp1, intExp2, ..., intExpn = positive integers indexExp1,indexExp2, ..., indexExpn = non-negative

integers

Loops to Process Multidimensional Arrays

double[][][] carDealers = new double[10][5][7];

For (i = 0; i < 10; i++) for (j = 0; j < 5; j++) for (k = 0; k < 7; k++) carDealers[i][j][k] = 10.00;

Programming Example: Text Processing

Program: Reads given text; outputs the text as is; prints number of lines and number of times each letter appears in text.

Input: File containing text to be processed. Output: File containing text, number of lines,

number of times each letter appears in text.

Programming Example Solution: Text Processing

An array of 26 representing the letters in the alphabet. Three methods:

copyText characterCount writeTotal

Value in appropriate index is incremented using methods and depends on character read from text.

Chapter Summary Arrays

Definition Uses

Different arrays One-dimensional Two-dimensional Multidimensional (n-dimensional) Arrays of objects Parallel arrays

Chapter Summary

Declaring arrays Instantiating arrays Processing arrays

Entire array Row processing Column processing

Common operations and methods performed on arrays Manipulating data in arrays

Chapter 9: Arrays J ava P rogramming: From Problem Analysis to Program Design, From Problem Analysis...

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