Arrays I Savitch Chapter 6.1: Introduction to Arrays

Finding the Average int count; double sum; Scanner keyboard = new Scanner(System.in); System.out.println(“enter 7 integers”); sum = 0 for (count=0; count<7; count++) sum = sum + keyboard.nextDouble( ); System.out.println(“Average is” + sum/7);

Introduction to Arrays To calculate the average of a series of numbers we need just one variable to store the value of the numbers. However to calculate –the average, and also –the number of items below the average All the numbers must be stored in separate variables

Introduction to Arrays, cont. Arrays satisfy this need. An array is an ordered collection of variables of the same type. An array is also a class with –predefined methods and –instance variables

Array Details, cont. The type of the elements is called the base type. The base type of an array can be any type (i.e. primitive or class type). The number of elements is the length or size of the array.

Creating and Accessing Arrays example double[] temperature = new double[7]; is like declaring seven variables of type double, named temperature[0], temperature[1], temperature[2], temperature[3], temperature[4], temperature[5], temperature[6].

Creating and Accessing Arrays, cont. These variables can be used just like any other variables of type double. examples temperature[3] = 32.0; temperature[6] = temperature[3] + 5; System.out.println(temperature[6]); temperature[index+1] = 66.5; These variables are called indexed variables, elements, or subscripted variables.

Array Declaration Syntax syntax for creating an array Base_Type[] Array_Name =new Base_Type[Length]; example int[] pressure = new int[100]; or int[] pressure; pressure = new int[100];

Array Terminology

Brackets [] Brackets [] serve three purposes: –creating the type name example: int[] pressure; –creating the new array pressure = new int[100]; –naming an indexed variable of the array pressure[3] = keyboard.nextInt();

Classes, Objects, and Methods In some ways array resembles a class type (like String). String str; int[] pressure; Recall that class types have method For example the string class has the method charAt(N) This can be used by any instance of that class. str.charAt(5)

Instance Variables Apart from methods, classes can also have variables. These variables belong to each instance of the class. Hence they are called instance variables.

The length Instance Variable An array has only one public instance variable, namely length. The length variable stores the number of elements the array can hold. Using Array_Name.length typically produces clearer code than using an integer literal.

Indices and length The indices of an array start with 0 and end with Array_Name.length-1. When a for loop is used to step through an array, the loop control variable should start at 0 and end at length-1. example for (lcv = 0; lcv < temperature.length; lcv++)

Array Index Out of Bounds Every index must evaluate to an integer which is not less than 0 and not greater than Array_Name.length-1. Otherwise, the index is said to be out of bounds or invalid. An out-of-bounds index will produce a run- time error.

Incomplete Array Processing Loops fail to process an entire array correctly when they –begin with an index other than 0 –end with an index other than length-1. Examples for (i = 1; i < oops.length-1; index++) for (i = 1; i <= oops.length; index++) for (i = 0; i <= oops.length; index++) for (i = 0; i < oops.length-1; index++)

Initializing Arrays An array can be initialized at the time it is declared using an initialiser list. example double[] reading = {3,3, 15.8, 9.7}; –The size of the array is determined by the number of values in the initializer list.

Initializing Arrays, cont. Uninitialized array elements are set to the default value of the base type. The default value of the base type integer is 0. However, it’s better to use either an initializer list or a for loop. int[] count = new int[100]; for (int i = 0, i < count.length, i++) a[i] = 0;

Class Exercise class Exercise1 { public static void main ( String[] args ) { int[] val = {0, 1, 2, 3}; sum =.... System.out.println("Sum of all numbers = "); System.out.println(sum); } Complete the assignment statement so that it computes the sum of the numbers in the array.