Arrays

Ar ra ys

Java Programming: From Problem Analysis to Program Design, Second Edition

Learning Objectives  Revise about arrays.  Explore how to declare and manipulate data into arrays.  Become familiar with the restrictions on array processing.  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. Java Programming: From Problem Analysis to Program Design, Second Edition

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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.

Java Programming: From Problem Analysis to Program Design, Second Edition

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One-Dimensional Arrays  Syntax to instantiate an array:  dataType[ arrayName  dataType[  dataType[

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

 Syntax to access an array component:  arrayName[indexExp]  intExp = number of components in array >= 0  0 <= indexExp <= intExp Java Programming: From Problem Analysis to Program Design, Second Edition

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Array num int[] num = new int[5];

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Array list

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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.

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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. Here list.length is 6. int[] list = {10, 20, 30, 40, 50, 60};

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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.

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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;

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Code to Initialize Array to Specific Value (10.00) for (index = 0; index < sales.length; index++) sales[index] = 10.00;

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

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

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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;

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

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Determining Largest Element in Array

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

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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; } Java Programming: From Problem Analysis to Program Design, Second Edition

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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]; } Java Programming: From Problem Analysis to Program Design, Second Edition

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Array of String Objects String[] nameList = new String[5]; nameList[0] nameList[1] nameList[2] nameList[3]

= = = =

"Amanda Green"; "Vijay Arora"; "Sheila Mann"; "Rohit Sharma";

nameList[4] = "Mandy Johnson";

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Array of String Objects

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Arrays of Objects Clock[] arrivalTimeEmp = new Clock[100];

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

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

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Arrays and Variable Length Parameter List  The syntax to declare a variable length formal parameter (list) is: dataType ... identifier

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Arrays and Variable Length Parameter List 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; }

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Arrays and Variable Length Parameter List 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));

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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 = new dataType[intExp1][intExp2];  To access a component of a two-dimensional array: arrayName[indexExp1][indexExp2];  intExp1, intExp2 >= 0  indexExp1 = row position  indexExp2 = column position Java Programming: From Problem Analysis to Program Design, Second Edition

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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.

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Two-Dimensional Arrays double[][]sales = new double[10][5];

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Accessing Two-Dimensional Array Components

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Two-Dimensional Arrays: Special Cases

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Two-Dimensional Arrays: Processing Initialization for (row = 0; row < matrix.length; row++) for (col = 0; col < matrix[row].length; col++) matrix[row][col] = 10;

Print 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(); } Java Programming: From Problem Analysis to Program Design, Second Edition

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Two-Dimensional Arrays: Processing Input 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);

}

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Two-Dimensional Arrays: Processing 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);

}

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Two-Dimensional Arrays: Processing 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);

}

Java Programming: From Problem Analysis to Program Design, Second Edition

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Two-Dimensional Arrays: Processing 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);

}

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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

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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;

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Summary  Arrays: Definition & Uses  Different arrays:  One-dimensional,  Two-dimensional, Multidimensional (ndimensional), Arrays of objects

   

Declaring arrays Instantiating arrays Processing arrays Manipulating data in arrays

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