Arrays Chapter 8

What if we need to store test scores for all students in our class. We could store each test score as a unique variable: int score1 = 85; int score2 = 92; int score3 = 90; int score4 = 74; int score5 = 83; Or we could store the values in an array Test scores

 An array is a data structure used to process a collection of data that is all of the same type  An array behaves like a numbered list of variables with a uniform naming mechanism  It has a part that does not change: the name of the array  It has a part that can change: an integer in square brackets  For example, given five scores: score[0], score[1], score[2], score[3], score[4]

 An array that behaves like this collection of variables, all of type integer, can be created using one statement as follows: int[] score = new int[5];  Or using two statements: int[] score; score = new int[5];  The first statement declares the variable score to be of the array type int[]  The second statement creates an array with five numbered variables of type int and makes the variable score a name for the array

 The individual variables that together make up the array are called indexed variables  They can also be called subscripted variables or elements of the array  The number in square brackets is called an index or subscript  In Java, indices must be numbered starting with 0, and nothing else score[0], score[1], score[2], score[3], score[4]

 The number of indexed variables in an array is called the length or size of the array  When an array is created, the length of the array is given in square brackets after the array type  The indexed variables are then numbered starting with 0, and ending with the integer that is one less than the length of the array score[0], score[1], score[2], score[3], score[4]

 Test scores array score indices

 An array can be initialized when it is declared  Values for the indexed variables are enclosed in braces, and separated by commas  The array size is automatically set to the number of values in the braces int[] score = {85, 92, 90, 74, 83};  Using this shorthand notation, you have to declare, create, and initialize the array all in one statement. Using the two statement method would cause a syntax error

 Another way of initializing an array is by using a for loop: int[] score = new int[5]; int index; for (index = 0; index < score.length; index++) score[index] = 85;  If the elements of an array are not initialized explicitly, they will automatically be initialized to the default value for their base type

public class Test { public static void main(String[ ] args) { int[ ] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; } values[0] = values[1] + values[4]; }  What is created with the following code?

Declare array variable values, create an array, and assign its reference to values public class Test { public static void main(String[ ] args) { int[ ] values = new int[5]; for (int i = 1; i < 5; i++) { values[i] = i + values[i-1]; } values[0] = values[1] + values[4]; } value[0] value[1] value[2] value[3] value[4] i = 1:value[1] is 1 + value[0] = = 1 i = 2:value[2] is 2 + value[1] = = 3 i = 3:value[3] is 3 + value[2] = = 6 i = 4:value[4] is 4 + value[3] = = value[0] is value[1] + value[4] = = 11 11

Scanner input = new Scanner(System.in); System.out.print("Enter 10 values: "); for (int i = 0; i < 10; i++) myList[i] = input.nextDouble();  How do we input arrays through a scanner?

 Strings can be part of an array  Create array of Strings with 3 elements String[] deptName = {"Accounting", "Human Resources", "Sales"}; for(int a = 0; a < deptName.length; ++a) System.out.println(deptName[a]);

for (int i = 0; i < myList.length; i++) { System.out.print(myList[i] + " "); }  To print the entire array, create a loop to print each element

double total = 0; for (int i = 0; i < myList.length; i++) { total = total + myList[i]; }

double max = myList[0]; for (int i = 1; i < myList.length; i++) { if (myList[i] > max) max = myList[i]; }

 To search for a specific value, simply loop through the loop  Look at each entry and compare it with the specified value boolean found = false; int i = 0; while (i < myList.length && !found) { if (searchEntry == myList[i]) found = true; i++; } if (found) System.out.println(searchEntry + " occurs in array");

 Both array indexed variables and entire arrays can be used as arguments to methods  An indexed variable can be an argument to a method in exactly the same way that any variable of the array base type can be an argument

Java uses pass by value to pass arguments to a method. There are important differences between passing a value of variables of primitive data types and passing arrays.  For a parameter of a primitive type value, the actual value is passed. Changing the value of the local parameter inside the method does not affect the value of the variable outside the method.  For a parameter of an array type, the value of the parameter contains a reference to an array; this reference is passed to the method. Any changes to the array that occur inside the method body will affect the original array that was passed as the argument.

public class Test { public static void main(String[] args) { int[] scores = {85, 92, 90, 74, 83}; myMethod(scores); System.out.println("scores[0] is " + scores[0]); } public static void myMethod(int[] numbers) { numbers[0] = 5555; // Assign a new value to numbers[0] }

public static int[ ] reverse(int[ ] list) { int[ ] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; }  Here is a method that returns the array in reverse order