1 Arrays An array is a special kind of object that is used to store a collection of data. The data stored in an array must all be of the same type, whether primitive or reference data types. For example, we may want to store: –An array of double values –An array of int values

2 The Need for Arrays Write a Java program that asks the user for 10 exam scores and displays the average. What if you are also required to display all the scores which are higher than the average? What if you are to record 100 scores?

3 The Problem: We need to keep track of each score that is entered by the user in an efficient manner. import java.util.; public class ExamScoresForLoop { public static void main(String[] args) { Scanner keyboard = new Scanner (System.in); double score; double total = 0.0; for(int i = 1; i <= 10; i++) { System.out.println("Enter score " + i +":"); score = keyboard.nextDouble(); total = total + score; } System.out.println("The average score is " + total/10); // how to display scores which are greater than average??? }

4 Using Arrays Instead of only storing one score: score 0x0010 We need to store a collection of scores, or what is called an array. score 0x0010 0x0h40 0x0h41 0x0h42 0x0h43 0x0h x0h40 in this case, score is considered to be a reference to the array at the memory address stored.

5 Accessing Array Elements Once an array has been declared, we can store and access data in it. We must specify the index representing the position of the data. The index always begins from 0. score Index representing position of the data score[0] score[3]

6 Declaring Arrays An array is declared by: –specifying the type of data it will hold –specifying the name of the array with an identifier –including square brackets [ ] Syntax: [] ; Examples int[] num;// an array of ints double[] score;// an array of doubles Student[] st;// an array of Student objects

7 Creating Arrays In Java, the array is an object and must be instantiated with its size. Syntax: = new [ ]; Therefore, an array may be declared and instantiated in one statement: int[] num = new int[10]; //creates an array to store 10 integers It may also be instantiated in two steps: int[] num; num = new int[10];

8 Initializer Lists An array can also be created by using an initializer list that fills in the values of the elements in the array in one line. When an initializer list is used, the new operator and size of the array are not specified. Examples: int[] num = {3, 6, 5, 7, 2, 8, 10, 20} char[] letterGrades = {'A', 'B', 'C', 'D', 'F'}

9 Bounds Checking An array that has been instantiated has a fixed size. –int[] num = new int[10] has array elements from num[0] to num[9] only. The Java interpreter will indicate an out-of- bounds error at run-time if the array index is not valid. Each array object has an attribute length that stores the size of the array, or the number of elements. Therefore an array will have valid indices from 0 to (length-1) only.

10 Traversing Arrays We can use a for loop to traverse the array For example, we may display each element of the array: for (int i = 0; i < maks; i++) { System.out.println(data[i]); } We may even use a for loop to allow the user to enter the elements of the array via interactive input.

11 Searching an Array We may need to check if an element is in the array: We can use the following logic to check whether a name that the user enters is in the array of names declared and filled earlier: –ask user for the required name –Start from i = 0 –assume that it is not found –While the required name is not found and i < length of array if the value at position i corresponds to the required name –then the name is found otherwise, –check the next element (i = i + 1)

12 Calculations We often need to perform calculations using some or all of the elements of the array: –total of all elements –the average –the element with the largest or smallest value Assume that an array of double values has been declared and filled: double[] score = new double[10];

13 Finding the Total and Average In order to find the total, we must initialize and keep track of the running total. The total is then used to calculate the average. double total = 0.0; for (int i = 0; i < score.length; i++) { total = total + score[i]; } System.out.println("The total is " + total); double avg = total / score.length; System.out.println("The average is " + avg);

14 Finding Maximum Values In order to find the maximum value in an array, we must remember that: –we usually consider an array one element at a time, in order. Assume that a double array score has been declared and filled: double maxScore = score[0] // assume max at pos 0 for (int i = 1; i < score.length; i++) { if (score[i] > maxScore) maxScore = score[i]; // new max found } System.out.println("The maximum value is " + maxScore);

15 Example We can solve the earlier problem now using arrays: Write a Java program that: –asks the user how many scores there, then allows the user to enter the scores. –After the scores have been entered, calculate the average. –Also display all the scores which are higher than the average and display the highest score.

16 Arrays and Methods Arrays can be used as parameters in methods: the entire array is passed to the method. For example, write a method findEmp that receives two parameters: –the first parameter is an array of Strings representing names of employees –the second parameter is a String representing the name of the employee required The method should return the position of the employee whose name is found based on the second parameter.

17 Arrays as Parameters The method can be defined as follows: public static int findEmp(String[] name, String wanted) { for(int i = 0; i < name.length; i++) if (wanted.equalsIgnoreCase(name[i])) return i;// found at pos i return -1;// not found }

18 Arrays as Returned Objects Arrays can also be returned by methods, as long as the array type is specified as the return type. The following method takes as input an array of Strings and returns another array with the lengths of each of the Strings in the array. public static int[] findLengths(String[] strArray) { int[] strLength = new int[strArray.length]; for (int i = 0; i < strArray.length; i++) strLength[i] = strArray[i].length(); return strLength; }

19 Initial Values of Array Elements All elements in an array are automatically initialised with default values. An array of numeric data has default elements with value 0 double[] score = new double[10]; all elements in score will have value 0.0 Arrays of boolean will be initialized to false. Arrays of reference data types will be initialized to null.