Arrays

1. Chapter 6 Arrays Chapter 6

2. Chapter 6 Announcements • Project 3 is done! • Project 4 is out! (Due in one week) • We have reached the point where you must start on time to finish projects • Cancelled labs will be made available • Grades will be prorated • Should do them on your own

3. Chapter 6 Exam: Programming Q. 1 public int dayOfYear() { int count =0; for(int i =1; i <= month -1; i++){ count += numDays( i ); } count += day; return count; } public int compareTo( Date other ) { if( month == other.getMonth()){ return day - other.getDay(); }else{ return month - other.getMonth(); } }

4. Chapter 6 Exam: Programming Q. 2 import java.util.Scanner; publicclass Padovan { publicstaticvoid main(String[] args) { Scanner s =new Scanner(System.in); System.out.print("k: "); int k = s.nextInt(); int p0 =1, p1 =1, p2 =2; if(k <1)return; System.out.print(p0 +" "+ p1 +" "+ p2); int pnew = p0 + p1; while(pnew <= k) { System.out.print(" "+ pnew); p0 = p1; p1 = p2; p2 = pnew; pnew = p0 + p1; } } }

5. Chapter 6 Exam: Programming Q. 3 publicclass Tokenizer{ publicstaticfinalchar DELIMITER =','; publicstaticvoid main(String args[]){ Tokenizer t =new Tokenizer(); Scanner keyboard =new Scanner(System.in); System.out.println("Enter a string:"); String input = keyboard.nextLine(); t.tokenize(input); } publicvoid tokenize(String input){ System.out.println("Tokens:"); for(int i =0; i < input.length(); i++){ if(input.charAt(i)== DELIMITER){ System.out.println(); } else{ System.out.print(input.charAt(i)); } } } }

6. Chapter 6 Creating and Accessing Arrays • example double[] temperature = new double[7]; is like declaring seven variables of type double, named temperature[0], temperature[1], temperature[2], temperature[3], temperature[4], temperature[5], temperature[6].

7. Chapter 6 Creating and Accessing Arrays, cont. • These variables can be used just like any other variables of type double. • examples temperature[3] = 32.0; temperature[6] = temperature[3] + 5; System.out.println(temperature[6]); temperature[index] = 66.5; • These variables are called indexed variables, elements, or subscripted variables.

8. Chapter 6 The length Instance Variable • An array has only one public instance variable, length. • The length variable stores the number of elements the array can hold. • Using Array_Name.length typically produces clearer code than using an integer literal.

9. Chapter 6 The length Instance Variable, cont • class ArrayOfTemperatures2

10. Chapter 6 Indices and length • The indices of an array start with 0 and end with Array_Name.length-1. • When a for loop is used to step through an array, the loop control variable should start at 0 and end at length-1. • example for (lcv = 0; lcv < temperature.length; lcv++)

11. Chapter 6 Initializing Arrays • An array can be initialized at the time it is declared. • example double[] reading = {3, 3, 15.8, 9.7}; • The size of the array is determined by the number of values in the initializer list.

12. Chapter 6 Initializing Arrays, cont. • Uninitialized array elements are set to the default value of the base type. • However, it’s better to use either an initializer list or a for loop. int[] count = new int[100]; for (int i = 0, i < count.length, i++) { count[i] = 0; }

13. Chapter 6 Case Study: Using an Array as an Instance Variable, cont. • class SalesReporter

14. Chapter 6 Indexed Variables as Method Arguments • An indexed variable can be used anywhere that any other variable of the base type of the array can be used. • Hence, an indexed variable can be an argument to a method.

15. Chapter 6 Indexed Variables as Method Arguments, cont. • class ArgumentDemo

16. Chapter 6 Entire Arrays as Method Arguments • An entire array can be used as a single argument passed to a method. • example double[] a = new double[10]; SampleClass.change(a); ... public static void change(double[] d) • No brackets accompany the argument. • Method change accepts an array of any size.

17. Chapter 6 Arguments for the Method main • Recall the heading for method main: public static void main(String[] args) • Method main takes an array of String values as its argument.

18. Chapter 6 Arguments for the Method main, cont. • An array of String values can be provided in the command line. • example java TestProgram Mary Lou • args[0]is set to“Mary” • args[1]is set to“Lou” System.out.println(“Hello “ + args[0] + “ “ + args[1]); printsHello Mary Lou.

19. Chapter 6 Use of = and == with Arrays • The assignment operator = and the equality operator ==, when used with arrays, behave the same as when used with other objects. • The assignment operator creates an alias, not a copy of the array. • The equality operator determines if two references contain the same memory address, not if two arrays contain the same values.

20. Chapter 6 Making a Copy of an Array • example int[] a = new int[50]; int[] b = new int[50]; ... for (int j = 0; j < a.length; j++) b[j] = a[j];

21. Chapter 6 Determining the “Equality” of Two Arrays • To determine if two arrays at different memory locations contain the same elements in the same order, define an equals method which determines if • both arrays have the same number of elements • each element in the first array is the same as the corresponding element in the second array.

22. Chapter 6 Determining the “Equality” of Two Arrays, cont. • A while loop can be used. • A boolean variable match is set to true. • Each element in the first array is compared to the corresponding element in the second array. • If two elements are different, match is set to false and the while loop is exited. • Otherwise, the loop terminates with match still set to true.

23. Chapter 6 Determining the “Equality” of Two Arrays, cont. • class TestEquals

24. Chapter 6 Determining the “Equality” of Two Arrays, cont. • Common functionality is built in: • java.util.Arrays.equals( [], [] )

25. Chapter 6 Methods that Return Arrays • A method can return an array. • The mechanism is basically the same as for any other returned type. • example public static Base_Type[] Method_Name (Parameter_List) Base_Type Array_Name; … return Array_Name; • The method need not be public or static.

26. Chapter 6 Methods that Return Arrays, cont. • class ReturnArrayDemo

27. Chapter 6 Partially Filled Arrays • Sometimes you need some, but not all of the indexed variables in an array. • In such situations, it is important to keep track of how much of the array has been used and/or the index of the last entry so that only meaningful values are accessed.

28. Chapter 6 Partially Filled Arrays, cont.

29. Chapter 6 Sorting Arrays • Sometime we want numbers in an array sorted from smallest to largest, or from largest to smallest. • Sometimes we want the strings referenced by an array to be in alphabetical order. • Sorting techniques typically are easy to adapt to sort any type that can be ordered.

30. Chapter 6 Selection Sort • The selection sort arranges the values in an an array so that a[0] <= a[1] <= a[2] … <= a[a.length-1] • The selection sort places the smallest item in a[0], the next smallest item in a[1], and so on for all but the last item. for(i = 0; i <a.length-1; i++) place the ith smallest item in a[i]

31. Chapter 6 Selection Sort, cont. • Selection sort begins by finding the smallest item in the array and swapping it with the item in a[0]. • Selection sort continues by finding the smallest item in the remainder of the array and swapping it with the next item in array a. • Selection sort terminates when only one item remains.

32. Chapter 6 Selection Sort, cont.

33. Chapter 6 Selection Sort, cont. • class SelectionSort

34. Chapter 6 Swapping Elements • To swap two elements a[i] and a[j], one of them must be saved temporarily.

35. Chapter 6 Swapping Elements, cont. • class interchange

36. Chapter 6 Introduction to Multidimensional Arrays • An array with more than one index sometimes is useful. • example: savings account balances at various interest rates for various numbers of years • columns for interest rates • rows for years • This two-dimensional table calls for a two-dimensional array.

37. Chapter 6 Introduction to Multidimensional Arrays, cont.

38. Chapter 6 Introduction to Multidimensional Arrays, cont.

39. Chapter 6 Introduction to Multidimensional Arrays, cont. • An array with n indices is called an n-dimensional array. • The arrays we considered previously, which had one index, were one-dimensional arrays.

40. Chapter 6 Multidimensional-Array Basics • example declaration int[][] table = new int [10][6]; or int[][] table; table = new int[10][6]; • The number of bracket pairs in the declaration is the same as the number of indices.

41. Chapter 6 Multidimensional-Array Basics, cont. • syntax Base_Type[]…[] Array_Name = new Base_Type[Length_1]…[Length_n]; • examples char[][] page = new char [100][80]; double[][][] threeDPicture = new double[10][20][30]; SomeClass[][] entry = new SomeClass[100][80];

42. Chapter 6 Multidimensional-Array Basics, cont. • Nested for loops can be used to change the values of the elements of the array and to display the values of the elements of the array.

43. Chapter 6 Multidimensional-Array Basics, cont. • class InterestTable

44. Chapter 6 Multidimensional-Array Parameters • Methods may have multidimensional-array parameters.

45. Chapter 6 Multidimensional-Array Parameters, cont. • class InterestTable2

46. Chapter 6 Multidimensional-Array Returned Values • A method may return a multidimensional array. • example public static double[][] corner(double[][] sArray, int i) { double[][] temp = new double[i][i]; … return temp; }

47. Chapter 6 Implementation of Multidimensional Arrays • Multidimensional arrays are implemented in Java using one-dimensional arrays. • Consider int[][] table = new int[10][6]; • The array table is a one-dimensional array of length 10. • Its base type is int[]. • Therefore, it is an array of arrays.

48. Chapter 6 Implementation of Multidimensional Arrays, cont. • This permits us to use the length instance variable, instead of an integer literal, to control a for loop used to initialize or print the values of the elements of an array. • example for (r = 0; r < table.length; r++) for (c = 0; c < table[r].length; c++)

49. Chapter 6 Implementation of Multidimensional Arrays, cont. • redefinedmethod showTable

50. Chapter 6 Ragged Arrays • Since a two-dimensional array in Java is an array of arrays, each row can have a different number of elements (columns). • Arrays in which rows have different numbers of elements are called ragged arrays.