Introduction to Computer Programming Arrays Grouping Data We don t buy individual eggs we buy them in units of 12 dozens We often think in terms of these groups and not the individual members Examples classes baseball teams encyclopedia sets etc It is extremely helpful to be able to group data items that are closely related e g class grades on a test pay rates for a group of employees etc Declaring Arrays Instead of writing int x we can write int x new int 10 the name x refers to the collection or array of integer values which can contain up to 10 values Using An Array We can assign a value to any element in the array by specify the array by name and its index lowest index x 0 87 x 1 90 x 9 93 highest index Using An Array continued An index can be any integer or character literal constant variable or expression x 6 x 5 4 x five 34 x i 1 x i 3 This is really useful because we do not want to have to write separate statement to assign values to each array element Using a Counting Loop To Set An Array Counting loops are really useful when manipulating arrays for i 0 i 10 i x i keyb nextInt A Program To Find Class Average import java util Scanner public class ArrayStuff final static int numGrades 10 Find the class average on a test public static void main String args Our array int grades new int numGrades int average Read the grades getGrades grades Find the average average calcAverage grades Print the average and the grades printResults grades average getGrades Read in the grades public static void getGrades int grades Scanner keyb new Scanner System in int count Each time the loop read another grade into the array for count 0 count numGrades count System out println Enter a grade grades count keyb nextInt calcAverage Add up the grades divide by the number of grades to find the average public static int calcAverage int grades int count sum 0 for count 0 count numGrades count sum sum grades count return sum numGrades printResults Print the average and the grades public static void printResults int grades int mean int i System out println The grades are for i 0 i 10 i System out println grades i System out println The average is mean System out println corresponding to a grade of letterGrade mean letterGrades Translate the score into a letter grade public static char letterGrade int score if score 90 return A if score 80 return B if score 70 return C if score 60 return D return F Example A Payroll Program Let s rewrite our payroll so it can use different tax brackets which the user will input The Revised Payroll Program import java util Scanner public class Payroll of tax brackets final static int numBrackets 5 A payroll program that allows the user to enter the tax brackets public static void main String args double hours rate double gross tax net trate double taxmin new double numBrackets taxrate new double numBrackets boolean again Get the tax brackets before processing any payroll records getBrackets taxmin taxrate do Get the inputs calculate the gross hours getHours rate getRate gross getGross hours rate Calculate the tax and subtract it to get the net tax getTax gross taxmin taxrate net gross tax Write the paycheck writeCheck gross tax net rate hours Does the user want to process another record again calcAgain while again If not quit getbrackets Input the tax brackets There are two different arrays one stores the minimum gross for the tax bracket the other stores the tax rate public static void getBrackets double taxmin double taxrate Scanner keyb new Scanner System in int i for i 0 i numBrackets i System out println What is the maximum income for bracket i 1 taxmin i keyb nextDouble System out println What is the tax rate for bracket i 1 taxrate i keyb nextDouble taxrate i taxrate i 100 System out println n n getinput Input the hours worked and pay rate for each employee public static double getHours Scanner keyb new Scanner System in double hours System out println How many hours worked hours keyb nextDouble return hours public static double getRate Scanner keyb new Scanner System in double rate System out println At what rate per hour rate keyb nextDouble return rate calcgross Calculate the gross pay including any overtime public static double getGross double hours double rate if hours 40 With overtime return 40 rate 1 5 rate hours 40 else Without overtime return rate hours getTax Calculate the tax for the employee using the tax brackets public static double getTax double gross double taxmin double taxrate int i If the employee doesn t make enough for the lowest bracket the tax is zero if gross taxmin 0 return 0 0 Find the appropriate bracket for the employee for i 1 i numBrackets i if gross taxmin i return taxrate i 1 gross The employee is in the highest bracket return taxrate numBrackets 1 gross writecheck Write the paycheck public static void writeCheck double gross double tax double net double rate double hours Print the input data System out printf Hours 4 2f tRate 6 2f n hours rate Print the gross pay System out printf Gross 7 2f n gross Print the tax System out printf Tax Print the net pay System out printf Net 7 2f n tax 7 2 n n n net calcAgain Returns true if the user want to go again Returns false if not public static boolean calcAgain Scanner keyb new Scanner System in String inputString new String char answer do System out println Do you want to process another payroll record y n inputString keyb next answer inputString charAt 0 while Character toUpperCase answer Y Character toUpperCase answer N if Character toUpperCase answer Y return true else return false Sorting What if the brackets are not read in the proper order Let s sort them and get them in the right order Selection Sort The algorithm 1 Find the ith smallest number and place it in the ith slot Selection Sort Algorithm Refined The algorithm 1 Find the ith smallest number and place it in the ith slot 1 1 1 1 2 For I 0 to Size 1 Find the Ith smallest number Place it in slot I Selection Sort import java util Scanner public class Sorting final static int size 5 public static void main String args Scanner keyb new Scanner System in int i int a new int size for i 0 i size i System out println Enter a i t a i keyb nextInt sort a size for i 0 i size i System out println a i a i sort Sort an array of numbers public static void sort int x int n int i j small index temp Place the smallest number in the first position Place the second smallest in the second position and so on for i 0 i size 1 i small Integer MAX