Chapter 9 Arrays

1. Chapter 9 • Arrays

2. Knowledge Goals • Understand the difference between atomic and composite data types • Understand the difference between unstructured and structured composite data types • Know how Java implements arrays • Know how an array is passed as an argument

3. Knowledge Goals • Understand the difference between an array and the information stored or referenced within the array • Understand the role of an array in structuring data within a problem • Understand the role of a two-dimensional array in representing a table with rows and columns • Know how a two-dimensional array is constructed as an array of arrays

4. Skill Goals • Declare and instantiate a one-dimensional array • Access and manipulate the individual components in a one-dimensional array where the elements are • Atomic types • Composite types • Use an initializer list to instantiate a one-dimensional array

5. Skill Goals • Declare a two-dimensional array • Perform fundamental operations on a two-dimensional array: • Access a component of the array • Process the array by rows • Process the array by columns • Declare a two-dimensional array as a parameter • Declare and process a multidimensional array

6. Java Data Types

7. Java Data Types Try expressing these definitions in words

8. Java Data Types • Composite data type • A data type that allows a collection of values to be associated with an identifier of that type • Unstructured data type • A collection of components that are not organized with respect to one another • Structured data type • An organized collection of components; the organization determines the means used to access individual components Is a class structured?

9. Java Data Types class Example { double field3; int field2; int field1; } • class Example • { • int field1; • int field2; • double field3; • } Changing the order does not change the access Is a class structured? Did you change your answer?

10. One-Dimensional Arrays • Data structure • The implementation of a composite data type • Note the difference between a data structure (implementation of any composite type) and a structured data type (a composite type that is structured)

11. One-Dimensional Arrays • One-dimensional array • A structured collection of components, all of the same type, that is given a single name; each component is accessed by an index that indicates the component's position within the collection Class • composite, unstructured • heterogeneous • access by field name • Array • composite, structured • homogeneous • access by position

12. One-Dimensional Arrays Declare Instantiate

13. One-Dimensional Arrays • int[] numbers = new int[4]; What type of values can be stored in each cell ?

14. One-Dimensional Arrays • float[] realNumbers = new float[10]; How do you get values into the cells ?

15. One-Dimensional Arrays • Array Initializers • int[] numbers = {4.93, -15.2, 0.5, 1.67}; Initializers do the instantiation and storing in with the declaration

16. One-Dimensional Arrays • Accessing Individual Components Indexing expression

17. One-Dimensional Arrays Indexing Expression Indexing Expression Place into which a value is stored; value is changed Place from which a value is extracted; value is not changed

18. One-Dimensional Arrays What happens if you try to access value[1000] ?

19. One-Dimensional Arrays • Out-of-bounds array index • An index that is either less than 0 or greater than the array size minus 1, causing an ArrayIndexoutOfBoundsException to be thrown • Length • A public instance variable associated with each instantiated array, accessed by array name .length Use length to avoid out-of-bounds indexes

20. One-Dimensional Arrays • Aggregate Array Operations What does the following expression return? numbers == values

21. One-Dimensional Arrays Now, what does the following expression return? numbers == values

22. One-Dimensional Arrays • System provides two useful array methods • first = second.clone(); // duplicates second import java.util.Arrays; • Arrays.equals(first, second); // item-by-item check • System.out.println(first == second); • System.out.println(Arrays.equals(first, second); What is printed?

23. More Examples What does this code segment do? totalOccupants = 0; for (int aptNo = 0; aptNo < occupants.length; aptNo++) totalOccupants = totalOccupants +occupants[aptNo];

24. More Examples What does this code fragment do ? if ((letter >= 'A' && letter <= 'Z' || letter >= 'a' && letter <= 'z')) { index = (int)Character.toUpperCase(letter) - (int)'A'; lettrCount[index] = letterCount[index] + 1; }

25. Arrays of Objects String[] groceryItems = new String[10]; for (index = 0; index < grocerItems.length; index++) { groceryItems[index] = inFile.nextLine(); }

26. Arrays of Objects • Expression Class/Type • groceryItems Reference to an array • groceryItems[0] Reference to a string • groceryItems[0].charAt() A character • groceryItems[10] Error • Base address • The memory address of the first element of the array • "Reference to" is the base address

27. Arrays of Objects • How do Array.equals and clone operate with arrays of objects? • Array.equals uses the == operator to compare arrays, so the addresses are compared • clone uses the = operator, so the addresses are copied • Array.equals(numbers, numbers.clone()) ?

28. Arrays of Objects Date[] bigEvents = new Date[10]; bigEvents Expression Class/Type bigEvents Array bigEvents[0] Date bigEvents[0].month String bigEvents[0].day int bigEvents[0].year int bigEvents[0].month.charAt(0) char Date [ 0 ] [ 1 ] . . . [ 9 ] . . . Date . . . Date . . .

29. Arrays of Objects • length is the number of slots assigned to the array • What if the array doesn’t have valid data in each of these slots? • Keep a counter of how many slots have valid data and use this counter when processing the array More about this type of processing in Chapter 11

30. Arrays of Objects • The Vector class • A class available in java.util that offers functionality similar to that of a one-dimensional array of objects • Objects of class Vector can grow and shrink; the size is not fixed for its lifetime • However, internal resizing operations are time consuming What do you think is the Vector class data structure?

31. Arrays of Objects • public static double average(int[] grades) • // Calculates and returns the average grade in an • // array of grades. • // Assumption: All array slots have valid data. • { • int total = 0; • for (int i = 0; i < grades.length; i++) • total = total + grades[i]; • return (double) total / (double) grades.length; • } What is passed as an argument?

32. Two-Dimensional Arrays Two- dimensional arrays can be used to represent tables such as this map

33. Two-Dimensional Arrays • Two-dimensional array • A collection of homogeneous components, structured in two dimensions (referred to as rows and columns); each component is accessed by a pair of indexes representing the component’s position within each dimension

34. Two-Dimensional Arrays

35. Two-Dimensional Arrays

36. Two-Dimensional Arrays Can you predict how each item is accessed?

37. Two-Dimensional Arrays

38. Two-Dimensional Arrays Actual Java implementation

39. Two-Dimensional Arrays hiTemp.length is the number of rows hiTemp[2].length is the number of columns in row two

40. Two-Dimensional Arrays int[][] hiTemp = new int[6][12]; for (int col = 0; col < hiTemp[2].length; col++) System.out.println(data[2][col]); What is printed?

41. Two-Dimensional Arrays Subarray processing by row

42. Two-Dimensional Arrays • for (int row = 0; row < rowsFilled; row++) • { • // Array is not ragged • total = 0; • for (int col = 0; col < colsFilled; col++) • total = total + data[row][col]; • outFile.println("Row sum: " + total); • }

43. Two-Dimensional Arrays Subarray processing by column

44. Two-Dimensional Arrays • for (int col = 0; col < colsFilled; col++) • { • // Array is not ragged • total = 0; • for (int row = 0; row < rowsFilled; row++) • total = total + data[row][col]; • outFile.println("Column sum: " + total); • }

45. Two-Dimensional Arrays • When processing by row, • the outer loop is ______ (row, column)? • the inner loop is ______ (row, column)? • When processing by column, • the outer loop is ______ (row, column)? • the inner loop is ______ (row, column)?

46. Two-Dimensional Arrays Initializer Lists • int[][] hits = {{ 2, 1, 0, 3, 2 }, • { 1, 1, 2, 3 }, • { 1, 0, 0, 0, 0 }, • { 0, 1, 2, 1, 1 }}; hits [0] [1] [2] [3] [4] 2 1 0 3 2 1 1 2 3 1 0 0 0 0 0 1 2 1 1

47. Three-Dimensional Arrays Array A collection of homogeneous components ordered on N dimensions (N>=1); each component is accessed by N indexes, each of which represents the component's position within that dimension

48. Extras I loved to give talks to young people; I used colored wires to demonstrate different computer speeds Who am I?

49. Extras - GUI Track • Rather than using JOptionPane, you can build your own window using class JFrame JFrame outFrame = new JFrame(); outFrame consists of the outside of the window frame that holds controls and the content pane which shows the content

50. Extras - GUI Track