COMP 14 Introduction to Programming Miguel A. Otaduy June 1, 2004

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 01 23

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

Declaring Arrays Examples // array of characters char[] characterSet; // array of counters (integers) int[] counter; // array of grades (doubles) double[] grade; counter characterSet grade 01 23

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 01 23

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 01 23

Declaration and Instantiation type[] name = new type[size]; DeclarationInstantiation 01 23

Example int[] num = new int[5]; 01 23

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 01 23

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]; } 01 23

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 01 23

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 01 23

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]); 01 23

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.)

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.) 01 23

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 01 23

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.) 01 23

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]

In-Class Exercises 1.Declare an array of integers called numbers 2.Declare and instantiate an array of 26 characters called alphabet 01 23

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] 01 23

In-Class Exercises 5.Write a loop to change all the values of the integer array numbers to index

Exercises 1. Find Sum and Average of Array 2. Determine Largest and Smallest Elements in Array 01 23

Homework 6 (practice) Read data from file Fill arrays with data Fill objects with data

Tomorrow More Arrays –arrays of objects –passing arrays as parameters –searching and sorting Homework 5 due Wednesday midnight Homework 6 assigned Bring laptops!!!