Arrays

1. Arrays

2. How used??? • With loops • while… • do…while • for…

3. Why used??? • To accommodate large volumes of data in memory at once, so that analysis can be performed on them

4. Declaring an Array Double[] salesFigure; Long[] idNum; Employee oneWorker;

5. After you declare an array… • You still need to set aside memory space oneWorker = new Employee; • The statement above actually creates the oneWorker object and sets aside memory space for it.

6. We do something similar for arrays double[] salesFigure; • To declare it. Then… salesFigure = new double[20]; • Or, in a single statement… double[] salesFigure = new double[20]; • In the above statement, you are both declaring and creating the array salesFigure[] in which 20 storage locations are set aside for it

7. Note that this is different from C++ double salesFigure [20]; • Is acceptable in C++, but causes a compiler error in Java

8. Distinguishing between storage locations • You use a subscript to distinguish between storage locations • A subscript is an integer contained within brackets that indicates one of the arrays’ storage locations • Legal subscripts would be 0 through 19 • Many programming languages begin with 1 as the first subscript—not so, Java salesFigure[0] salesFigure[1] salesFigure[2] salesFigure[3]

9. Array names.. • Contain references or memory addresses, not the actual contents or value • This is true of all Java objects • This is in contrast to a variable that was a primitive type; such variables actually hold the value

10. Initializing an Array Int[] someNums = new int[10]; • Here, someNums contains a reference or pointer address to the starting location of the array • The array is initialized to all 0’s because it is a numeric array • Char array elements are initialized to ‘\u0000’ and boolean array elements are initialized to false

11. Initializations occur at… • Compile time whereas assignments occur at execution time Int[] someNums = new int[10]; • Causes 10 0’s to be placed in the ten storage locations, whereas someNums[0] = 46; • Causes the first such storage location to be assigned the integer value 46 at execution time

12. Nonzero initializations Int[] tenMult = {10, 20, 30, 40, 50, 60}; • The size of the array is determined by the number of elements you put in the list. This causes the array tenMult[] to be declared and created and initialized • Here you did not need to use the keyword new or the name of the constructor

13. The power of arrays becomes • Apparent when you use subscripts that are variables rather than subscripts that are constants • Consider tenMult[]

14. Subscripts starting from 0… • For convenience, you can think of these subscripts as offsets from the starting location of the array. • Int[] tenMult = {10, 20, 30, 40, 50, 60}; tenMult[0] = +=3; tenMult[1] = +=3; tenMult[2] = +=3; tenMult[3] = +=3; tenMult[4] = +=3; What is tenmult[3] now???

15. The same assignments using for.. for(i=0; i < 5; i++) tenMult[i] += 3; • Does the same thing as the previous five statements • Can also use a symbolic constant as follows: final int FIVE = 5; for(i=0; i < FIVE; i++) tenMult[i] += 3;

16. The following is also common… for(i=0; i < tenMult.length; i++) tenMult[i] += 3; • Here length is not a method, but a field, an instance variable that is automatically assigned for every array you create for(i=0; i < tenMult.length(); i++) tenMult[i] += 3; • Is Wrong!!!

17. Enhanced for loop for(int sub = 0; sub < scoreArray.length; ++sub) System.out.println(scoreArray[sub]); For(int val : scoreArray) system.out.println(val); The second statement does the same thing as the first

18. Using Subscripts with an Array • Is the only way to go for large arrays

19. Declaring an Array of Objects Instead of … Employee painter, electrician, plumber; Employee bricklayer, roofer, carpetman; Employee[] emp = new employee[6] Here an array named emp holds six employee objects However, these statements do not actually construct those Employee objects

20. The above assumes… Public class Employee { private int empNum; private double empSal; Employee (int e, double s) { empNum = e; empSal = s; } public int getEmpNum() { return empNum; } public double getSalary() { return empSal; } }

21. Instead…. You must call the constructor six times final int NUM_EMPLOYEES = 6; final double PAYRATE = 6.35; For(int x = 0; x < NUM_EMPLOYEES; ++x) emp[x] = new Employee(101+x, PAYRATE); For(int x = 0; x < NUM_EMPLOYEES; ++x) system.out.println(emp[x].getEmpNum() + “ “ + emp[x].getSalary());

22. Searching an Array for an Exact Match

23. Searching an Array for a Range Match

24. Passing Arrays to Methods • If you pass an entire array to a method, you are passing by reference, meaning that the starting address of the array is passed to the method and the method can change every element in the array • If you pass a specific element of an array, it is like passing a primitive element and is a pass by value, meaning the actually value is transferred to the method, so the method cannot change the original variable value

25. Creating Arrays of Strings String [] deptName = {“Accounting”, “Human Resources”, “Sales”}; for(int a=0; a<deptName.length; ++a) System.out.println(deptName[a]);

26. Sorting Array Elements • The algorithm must do a comparison between the ith element in the list and the i+1st element in the list (adjacent element in the list) • If the pair of elements are not in the correct order, the elements must be swapped: temp = list[i]; List[i] = list[i+1]; List[i+1] = temp;

27. More sorting • When numbers are sorted from smallest to largest, the order is ascending order • When numbers are sorted from largest to smallest, the order is descending order

28. Bubble sort int[] someNums = {88, 33, 99, 22, 54}; int b, temp For(b=0; b<someNums.length-1; ++b) if(someNums[b] > someNums[b+1] { temp = someNums[b]; someNums[b] = someNums[b+1]; someNums[b+1] = temp; }

29. Using Two-Dimensional and Multidimensional Arrays Employee[] someEmps = new Employee[5]; The above statement creates an array of five employee objects

30. Sorting Arrays of Objects

31. Using the Arrays Class • You can use arrays to store all kinds of objects.

32. Two dimensional arrays Int[] [] someNumbers = new int[3][4]; Creates a two-dimensional array of ints with a total of 12 elements Int[] [] someNumbers = {{9,10,11,12}, {1,2,3,4}, {5,6,7,8}}; Creates and initializes a two-dimensional array of ints with 12 elements

33. A two dimensional array • is also called a matrix or table • Can create even higher dimensional arrays

34. That’s it

36. import java.util.*; import javax.swing.*; public class ArraysDemo2 { public static void main(String[] args) { char[] grades = {'A', 'B', 'C', 'D', 'F'}; String entry; char myGrade; int position; entry = JOptionPane.showInputDialog(null, "Enter student grade"); myGrade= entry.charAt(0); position = Arrays.binarySearch(grades, myGrade); if(position >= 0) JOptionPane.showMessageDialog(null, "Position of " + myGrade + " is " + position); else JOptionPane.showMessageDialog(null, "Invalid grade"); System.exit(0); } }

37. import javax.swing.*; public class FindDiscount { public static void main(String[] args) { final int NUM_RANGES = 5; int[] discountRangeLimit = { 1, 13, 50, 100, 200}; double[] discountRate = {0.00, 0.10, 0.14, 0.18, 0.20}; double customerDiscount; String strNumOrdered; int numOrdered; int sub = NUM_RANGES - 1; strNumOrdered = JOptionPane.showInputDialog(null, "How many items are ordered?"); numOrdered = Integer.parseInt(strNumOrdered); while(sub >= 0 && numOrdered < discountRangeLimit[sub]) --sub; customerDiscount = discountRate[sub]; JOptionPane.showMessageDialog(null, "Discount rate for " + numOrdered + " items is " + customerDiscount); System.exit(0); } • }

38. public class PassArray { public static void main(String[] args) { final int NUM_ELEMENTS = 4; int[] someNums = {5, 10, 15, 20}; int x; System.out.print("At start of main: "); for(x = 0; x < NUM_ELEMENTS; ++x) System.out.print(" " + someNums[x] ); System.out.println(); methodGetsArray(someNums, NUM_ELEMENTS); System.out.print("At end of main: "); for(x = 0; x < NUM_ELEMENTS; ++x) System.out.print(" " + someNums[x]); System.out.println(); } public static void methodGetsArray(int arr[], int numEls) { int x; System.out.print("At start of method arr holds: "); for(x = 0; x < numEls; ++x) System.out.print(" " + arr[x] ); System.out.println(); for(x = 0; x < numEls; ++x) arr[x] = 888; System.out.print(" and at end of method arr holds: "); for(x = 0; x < numEls; ++x) System.out.print(" " + arr[x] ); System.out.println(); } • }

39. import javax.swing.*; public class SearchList { public static void main(String[] args) { String[] deptName = {"Accounting", "Human Resources", "Sales"}; String dept; int x; boolean deptWasFound = false; dept = JOptionPane.showInputDialog(null, "Enter a department name"); for(x = 0; x < deptName.length; ++x) if(dept.equals(deptName[x])) deptWasFound = true; if(deptWasFound) JOptionPane.showMessageDialog(null, dept + " was found in the list"); else JOptionPane.showMessageDialog(null, dept + " was not found in the list"); System.exit(0); } }

40. import java.util.*; public class ArraysDemo { public static void main(String[] args) { int[] myScores = new int[5]; display("Original array: ", myScores); Arrays.fill(myScores, 8); display("After filling with 8s: ", myScores); myScores[2] = 6; myScores[4] = 3; display("After changing two values: ", myScores); Arrays.sort(myScores); display("After sorting: ", myScores); } public static void display(String message, int[] array) { int sz = array.length; System.out.print(message); for(int x = 0; x < sz; ++x) System.out.print(array[x] + " "); System.out.println(); } }

41. import javax.swing.*; public class FindPrice { public static void main(String[] args) { final int NUMBER_OF_ITEMS = 10; int[] validValues = {101, 108, 201, 213, 266, 304, 311, 409, 411, 412}; double[] prices = {0.29, 1.23, 3.50, 0.69, 6.79, 3.19, 0.99, 0.89, 1.26, 8.00}; String strItem; int itemOrdered; double itemPrice = 0.0; boolean validItem = false; strItem = JOptionPane.showInputDialog(null, "Enter the item number you want to order"); itemOrdered = Integer.parseInt(strItem); for (int x = 0; x < NUMBER_OF_ITEMS; ++x) { if(itemOrdered == validValues[x]) { validItem = true; itemPrice = prices[x]; } } if(validItem) JOptionPane.showMessageDialog(null, "The price for item " + itemOrdered + " is $" + itemPrice); else JOptionPane.showMessageDialog(null, "Sorry - invalid item entered"); System.exit(0); } • }

42. import javax.swing.*; import java.awt.*; import java.awt.event.*; public class JHello7 extends JApplet implements ActionListener { JLabel greeting = new JLabel("Hello. Who are you?"); Font headlineFont = new Font("Helvetica", Font.BOLD, 36); JTextField answer = new JTextField(10); JButton pressMe = new JButton("Press me"); JLabel personalGreeting = new JLabel(""); Container con = getContentPane(); public void init() { greeting.setFont(headlineFont); personalGreeting.setFont(headlineFont); con.add(greeting); con.add(answer); con.add(pressMe); con.setLayout(new FlowLayout()); pressMe.addActionListener(this); answer.addActionListener(this); } public void actionPerformed(ActionEvent e) { String name = answer.getText(); personalGreeting.setText("Hello, " + name); con.add(personalGreeting); validate(); } • }

43. import javax.swing.*; import java.awt.*; import java.awt.event.*; public class JHello8 extends JApplet implements ActionListener { JLabel greeting = new JLabel("Hello. Who are you?"); Font headlineFont = new Font("Helvetica", Font.BOLD, 36); JTextField answer = new JTextField(10); JButton pressMe = new JButton("Press me"); JLabel personalGreeting = new JLabel(""); Container con = getContentPane(); public void init() { greeting.setFont(headlineFont); personalGreeting.setFont(headlineFont); con.add(greeting); con.add(answer); con.add(pressMe); con.setLayout(new FlowLayout()); pressMe.addActionListener(this); answer.addActionListener(this); } public void actionPerformed(ActionEvent e) { remove(pressMe); remove(answer); String name = answer.getText(); personalGreeting.setText("Hello, " + name); con.add(personalGreeting); validate(); } }

44. public class PassArrayElement { public static void main(String[] args) { final int NUM_ELEMENTS = 4; int[] someNums = {5, 10, 15, 20}; int x; System.out.print("At start of main: "); for(x = 0; x < NUM_ELEMENTS; ++x) System.out.print(" " + someNums[x] ); System.out.println(); for(x = 0; x < NUM_ELEMENTS; ++x) methodGetsOneInt(someNums[x]); System.out.print("At end of main: "); for(x = 0; x < NUM_ELEMENTS; ++x) System.out.print(" " + someNums[x]); System.out.println(); } public static void methodGetsOneInt(int one) { System.out.print("At start of method one is: " + one); one = 999; System.out.println(" and at end of method one is: " + one); } }