The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie COMP 14 Introduction to Programming Adrian Ilie July 14, 2005

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 2 Arrays An array is a list of values that can be represented by one variable Members of an array must all have the same data type Each value is stored at a specific, numbered position in the array ♦ the number corresponding to each position is called an index or subscript All arrays have a length ♦ number of elements the array can hold

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 3 Declaring Arrays type[] name; The array (element) data type Empty square brackets The array (variable) name Creates a reference variable called name that can point to an array of type elements.

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 4 Declaring Arrays Examples // array of characters char[] characterSet; // array of counters (integers) int[] counter; // array of grades (doubles) double[] grade; counter characterSet grade

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 5 Instantiating Arrays You must instantiate (create) arrays ♦ the size of an array is typically not known before run time name = new type[size]; The array (element) data type The new operator The array (variable) name The assignment operator The number of elements

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 6 Instantiating Arrays Examples // instantiate an array of counters counter = new int[5]; // instantiate the array of grades numStudents = 10; grade = new double[numStudents]; counter <= index < size

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 7 Declaration and Instantiation type[] name = new type[size]; DeclarationInstantiation

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 8 Example int[] num = new int[5];

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 9 Array Access Examples averageScore = (score[0]+score[1]+score[2])/3; numStudents = 3; totalScore = 0; for (int i = 0; i < numStudents; i++) { totalScore += score[i]; } averageScore = totalScore/numStudents; double score[] = new score[3]; score[0] = 98.3; score[1] = 57.8; score[2] = 93.4; often use loops for access

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 10 Array Length Arrays have length ♦ an internal variable called length ♦ number of elements in array ♦ access the length variable using the “dot’ notation (arrayname. length ) // loop through the array of test scores sumOfScores = 0; for (int i=0; i<scores.length; i++) { sumOfScores += scores[i]; }

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 11 int counter[] = {0, 0, 0, 0, 0}; char[] characterSet = {‘a’,’b’,’c’}; // etc. Initializing Arrays Array elements are variables too! ♦ if you don’t initialize, the contents are undefined When and how? ♦ if you don’t yet know the size initialize at run time, typically with a loop ♦ if you know how many elements perhaps use an initializer list

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 12 List the initial value for the elements of an array Items are separated by commas and the list is in braces {} The size of the array is determined by the number of items in the list int[] scores = {87, 98, 45}; Can only be used in the same statement as declaring the array NOT int[] scores; scores = {87, 98, 45}; Initializer Lists

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 13 Array Bounds Arrays have finite size If you access an element outside of the array, you’ll get an ArrayIndexOutOfBounds Exception Example: int grades[] = {99, 98, 95, 96}; System.out.println (grades[4]);

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 14 int arraySize; System.out.print ("Enter the size of the array:"); arraySize = Integer.parseInt(keyboard.readLine()); int[] list = new int[arraySize]; Specify Array Size During Program Execution Example (Assume that keyboard has already been declared and instantiated.)

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 15 Example for (int ind = 0; ind < sale.length; ind++) { sale[ind] = 10.00; } Initialize Array to Specific Value (10.00) (Assume that sale has already been declared and instantiated.)

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 16 Example for (int ind = 0; ind < sale.length; ind++) { sale[ind] = Double.parseDouble(keyboard.readLine()); } (Assume that sale has already been declared and instantiated, and that keyboard has already been declared and instantiated.) Read Data into Array

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 17 Example for (int ind = 0; ind < sale.length; ind++) { System.out.print(sale[ind] + " "); } Print Array (Assume that sale has already been declared and instantiated.)

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 18 Parallel Arrays Arrays are parallel if corresponding components hold related information String[] studentName; double[] studentGPA; For example, studentName and studentGPA are parallel if studentGPA[3] is the GPA of the student with studentName[3].

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 19 In-Class Exercises 1.Declare an array of integers called numbers 2.Declare and instantiate an array of 26 characters called alphabet

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 20 In-Class Exercises 3.Declare an array of 5 characters called grades and initialize it with the letters: A, B, C, D, F Hint: type[] name = {initialization list}; 4.Write a loop to print the contents of an array named zipCodes Hint: to access array element name[index]

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 21 In-Class Exercises 5.Write a loop to change all the values of the integer array numbers to index + 1

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 22 Exercises 1. Find Sum and Average of Array 2. Determine Largest and Smallest Elements in Array

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 23 Homework 6 (practice) Read data from file Fill arrays with data Fill objects with data

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Adrian Ilie 24 Tomorrow More Arrays ♦ arrays of objects ♦ passing arrays as parameters ♦ searching and sorting Homework 5 due Sunday midnight Bring laptops!!!