Arrays Chapter 6 Chapter 6 1

Transcription

1 Arrays 1

2 Reminders Project 4 released: due Oct 10:30 pm Project 1 regrades due by midnight tonight Project 2 grades posted on WebCT regrade requests due by Friday Oct 7 midnight (send to rnehme@cs.purdue.edu) 2

3 Exam 1 Info Exam will be returned in recitation today Solution available on the website Scores are available on WebCT Problems/scoring errors: talk to your GTA Exam 1 Stats: Mean: 73.2 Median: 77 High: 98 3

4 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]. Similar to: Automobile mycar = new Automobile( Toyota ); 4

5 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. 5

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. length is an instance variable, not a method Array_Name.length() wont work 6

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8 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++) 8

9 Array Index Out of Bounds Every index must evaluate to an integer which is not less than 0 and not greater than Array_Name.length-1. Otherwise, the index is said to be out of bounds or invalid. An out-of-bounds index will produce a runtime error. 9

10 Arrays of Objects 10

11 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. 11

12 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. 12

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14 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. 14

15 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] 15

16 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. 16

17 Selection Sort, cont. 17

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19 Swapping Elements To swap two elements a[i] and a[j], one of them must be saved temporarily. 19

20 Swapping Elements, cont. class interchange 20

21 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 twodimensional array. 21

22 Introduction to Multidimensional Arrays, cont. 22

23 Introduction to Multidimensional Arrays, cont. 23

24 Introduction to Multidimensional Arrays, cont. An array with n indices is called an ndimensional array. The arrays we considered previously, which had one index, were one-dimensional arrays. 24

25 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. 25

26 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]; 26

27 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. 27

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29 Multidimensional-Array Parameters Methods may have multidimensional-array parameters. 29

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31 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; } 31

32 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. 32

33 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++) 33

34 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. 34

35 Ragged Arrays, cont. Example int[][] b = new int[3][]; b[0] = new int[5]; b[1] = new int[7]; b[2] = new int[4]; 35

36 (optional) Graphics Supplement: Outline Part 1: Text Areas and Text Fields Part 2: Drawing Polygons and Polylines 36

37 Text Areas and Text Fields A text area is a window that can be used for text input and text output. any number of lines any number of characters per line A text field allows only one line of characters. Text area and text fields provide areas for changeable text in a GUI. 37

38 Programming Example: A Question and Answer Applet The applet accepts a question, requests some advice, and provides an answer. The applet contains some advice for the first user, but then takes the advice provided by one user, and uses it to answer the next user. 38

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40 Programming Example:, cont. 40

41 JTextArea Objects A text area is an object of the class A JTextArea can be created with JTextArea. private JTextArea thetext; thetext = new JTextArea(LINES, CHAR_PER_LINE); Typically, this occurs inside the init method. 41

42 JTextArea Objects, cont. The text in the text area can be read using String question = thetext.gettext(); The text in the text area can be set using thetext.settext( That s difficult.\n + I need some advice.\n + Give me some advice and click. ); Text can be entered in excess of the specified size, but may not be entirely visible. 42

43 JTextField Objects A text field is an object of the class A JTextField can be created with JTextField. private JTextField thetext; thetext = new JTextField(NUMBER_OF_CHARS); Typically, this occurs inside the init method. 43

44 JTextField Objects, cont. The text in the text field can be read using String question = thetext.gettext(); The text in the text area can be set using thetext.settext( That s all, folks. ); Text can be entered in excess of the specified size, but may not be entirely visible. 44

45 Drawing Polygons and Polylines The methods drawpolygon and fillpolygon allow you to draw polygons which are closed figures made of of line segments that meet at vertices but do not cross. A polyline is similar to a polygon, but uses the method drawpolyline and need not be closed. 45

46 Drawing Polygons and Polylines, cont. 46

47 Drawing Polygons and Polylines, cont. class House 47

48 Drawing Polygons and Polylines, cont. 48

49 Method drawpolygon syntax canvas.drawpolygon(array_of_xs, Array_of_ys, Number_of_Points); example private int[] xcoord = {150, 150, 200, 250, 250}; private int[] ycoord = {100, 40, 20, 40, 100}; canvas.drawpolygon(xcoord, ycoord, xcoord.length); 49

50 Method fillpolygon syntax canvas.fillpolygon(array_of_xs, Array_of_ys, Number_of_Points); example private int[] xcoord = {150, 150, 200, 250, 250}; private int[] ycoord = {100, 40, 20, 40, 100}; canvas.fillpolygon(xcoord, ycoord, xcoord.length); 50

51 Method drawpolyline syntax canvas.drawpolyline(array_of_xs, Array_of_ys, Number_of_Points); example private int[] xcoord = {150, 150, 200, 250, 250}; private int[] ycoord = {100, 40, 20, 40, 100}; canvas.drawpolyline(xcoord, ycoord, xcoord.length); 51

52 Method drawpolyline, cont. There is no automatic line segment from the last point to the first point. As a consequence, the figure typically is not closed. A polyline cannot be filled. 52

53 Summary You have learned about arrays and how to use them in Java programs. You have learned how to use array parameters and how to define methods that return an array. You have learned the proper use of an array as an instance variable. You have learned about multidimensional arrays. 53

54 Summary, cont. (optional) You have learned about text fields and text areas in applets. (optional) You have learned to draw arbitrary polygons and polylines in applets. 54