Array indexing. Before we use an array, we have to declare it. For example, int[] height=new int[11];

Transcription

1 Chapter 6 Arrays We want to organize objects or primitive data in a form that is easy to access and modify data. We will explore more about the ways to group and organize data in a later chapter, and even more in a later course. We now discuss a very simply way to put them together, an array of data. When writing a program to work with, e.g., a list of 100 piece of data, it is not practical to treat them as 100 isolated data. The array structure lets us declare them as a whole. 1

2 Array indexing An array is a list of values, each of which is stored at a specific, numbered position in the array. Those numbers corresponding to positions are referred to as index or subscript. In Java, they start at 0, thus, the data with index 5 is actually stored in the sixth position. To access a value in an array, we use the name of this array, together with its index. Thus, to get the value with index 5 in an array height, we use the expression of height[5]. Such an expression can be used whenever an integer can be used. Before we use an array, we have to declare it. For example, int[] height=new int[11]; declares an array, height, with 11 int type elements. 2

3 More examples Once the height array is declared, the following are all valid expressions: height[2]=72; height[count]=feet*12; average=(height[0]+height[1]+height[2])/3; System.out.println("The middle value is " +height[max/2]); pick=height[rand.nextint(11)]; When declaring an array, we don t need to give its size. However, once it is instantiated with the new, its size can t be changed. 3

4 A complete example The following code shows how to initialize an array, change one value, then print the whole array. public class BasicArray{ final static int LIMIT = 15; final static int MULTIPLE = 10; public static void main(string[] args){ int[] list = new int[limit]; //Initialize the array values for(int index = 0;index<LIMIT;index++) list[index] = index * MULTIPLE; list[5] = 999; // change one array value for (int index=0;index<limit;index++) System.out.print(list[index]+ " "); System.out.println (); 4

5 A couple of points 1. Technically, the bracket [] is an operator, and usually, has the highest precedence. 2. The index operator carries out automatic bound checking. Thus, if you try to print out height[23], an exception will be thrown back. 3. The size of any array is kept in a public variable, length, coming with the array. 4. Arrays can also be declared in an alternative way. For example, int height[]; Homework: Exercises 6.1, 6.2, 6.3 and

6 Another example The following code shows how to read in a list of numbers, then print them out in an reversed order. public class ReverseOrder{ public static void main (String[] args){ double[] numbers = new double[10]; System.out.println ("The size of the array: " + numbers.length); for (int index = 0; index < numbers.length; index++){ System.out.print("Enter number " + (index+1) + ": "); numbers[index] = Keyboard.readDouble(); System.out.println("The numbers in reverse order:"); for (int index=numbers.length-1;index>=0;index--) System.out.print(numbers[index] + " "); System.out.println(); 6

7 A couple of points 1. Arrays can be initialized. For example, int score=new int[4]; char lettergrades=new char[4]; score={87, 98, 78, 23; lettergrades={ A, B, C, F ; 2. Array can also be used as parameters. When an array is passed in, a copy of the reference to this array as an object is passed in. An array element can also be passed in. If the type of its element is a primitive type, a copy of its value is passed in. Homework: Exercises 6.5 and

8 Array of string objects Consider the following line String[] words=new String[25]; the variable words now becomes an array of references to String objects. The new operator instantiates this array and reserves space for them. An important point is that it does not create any String objects at this point. In the following example, an array of String objects are created, the associated string are created later, using string literals contained in the initializing list. 8

9 An example public class GradeRange{ public static void main (String[] args){ String[] grades = {"A", "A-", "B+", "B", "B-", "C+", "C", "C-", "D+", "D", "D-", "F"; int[] cutoff={95,90,87,83,80,77,73,70,67,63,60,0; for (int level = 0; level < cutoff.length; level++) System.out.println(grades[level]+"\t"+cutoff[level]); The problem with this sort of parallel array is that they may go out of sync. Thus, a better way is to construct one array of pairs, containing both the letter grade and the corresponding cutoff. Such a pair can be done as a class. Thus, we now will work with an array of objects, each of which consists of a pair. 9

10 Filling arrays of objects As we have said many times, it takes two separate steps to create an array and then fill it. public class Tunes{ public static void main (String[] args){ CDCollection music = new CDCollection (); music.addcd ("Storm Front", "Billy Joel", 14.95, 10); music.addcd ("Come On Over", "Shania Twain", 14.95, 16); music.addcd ("Soundtrack", "Les Miserables", 17.95, 33); music.addcd ("Graceland", "Paul Simon", 13.90, 11); System.out.println (music); music.addcd ("Double Live", "Garth Brooks", 19.99, 26); music.addcd ("Greatest Hits", "Jimmy Buffet", 15.95, 13); System.out.println (music); The CDCollection class is defined in the following slides. 10

11 The CDCollection class public class CDCollection{ private CD[] collection; private int count; private double totalcost; public CDCollection (){ collection = new CD[100]; count = 0; totalcost = 0.0; public void addcd (String title, String artist, double cost, int tracks){ if (count == collection.length) increasesize(); collection[count]=new CD(title,artist,cost,tracks); totalcost += cost; count++; 11

12 public String tostring(){ NumberFormat fmt = NumberFormat.getCurrencyInstance(); String report = "&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&\n"; report += "My CD Collection\n\n"; report += "Number of CDs: " + count + "\n"; report += "Total cost: " + fmt.format(totalcost) + "\n"; report += "Average cost: " + fmt.format(totalcost/count); report += "\n\ncd List:\n\n"; for (int cd = 0; cd < count; cd++) report += collection[cd].tostring() + "\n"; return report; private void increasesize (){ CD[] temp = new CD[collection.length * 2]; for (int cd = 0; cd < collection.length; cd++) temp[cd] = collection[cd]; collection = temp; 12

13 Selection sort Sorting is the process of putting things in order. There are many ways to do sorting. We begin with a simple way, selection sort, which sorts things by successively selecting the smallest data, and putting it into its final place. public static void selectionsort (int[] numbers){ int min, temp; for (int index=0;index<numbers.length-1;index++){ min = index; for(int scan=index+1;scan<numbers.length;scan++) if(numbers[scan]<numbers[min]) min = scan; temp = numbers[min]; numbers[min] = numbers[index]; numbers[index] = temp; 13

14 How to run selection sort? Below shows how to pass in an array object to a method, particularly, how to sort a list of numbers. public class SortGrades{ public static void main (String[] args){ int[] grades={89,94,69,80,97,85,73,91,77,85,93; Sorts.selectionSort(grades); for (int index = 0; index < grades.length; index++) System.out.print (grades[index] + " "); 14

15 Insertion sort Insertion sort sorts things by inserting each and every element into the right place. In the following code, it starts with the left end and moves to the right. For every position, it goes backwards to insert the current data to its right place. public static void insertionsort (int[] numbers){ for (int index=1;index<numbers.length;index++){ int key = numbers[index]; int position = index; // shift larger values to the right while (position > 0 && numbers[position-1] > key){ numbers[position] = numbers[position-1]; position--; numbers[position] = key; 15

16 The general insertion sort The key difference between this more general insertion sort and the previous one is that now we apply it to a list of comparable objects, namely, the data within need not be int, as long as they are comparable. The class for that object has to implement the Comparable class, and more specifically, the compareto method definition will tell us the order between any two objects in that class. public static void insertionsort(comparable[] objects){ for (int index=1; index<objects.length; index++){ Comparable key = objects[index]; int position = index; while (position > 0 && objects[position-1].compareto(key)>0){ objects[position] = objects[position-1]; position--; objects[position] = key; Homework: Read the relevant part in pp. 346 and pp

17 Sort comparison There could be a few reasons to pick one sorting method over another one, such as its simplicity, the level of efficiency, the amount of space it needs, the amount of time it takes, etc.. Both insertion sort and selection sort basically take the same amount of space and time to run. More specifically, the time need for both is O(n 2 ), in the sense that both will take time proportional to n 2, where n is the size of the list. But, insertion sort is faster when the list is close to be sorted, while selection sort always take the same amount of work, no matter what. On the other hand, selection sort is easier to understand, and will often suffice in lots of cases. We will further study the difference between algorithms in a later course. 17

18 Two-dimensional arrays We have so far only looked at the one-dimensional arrays representing simple list of values. As a straightforward extension, the two-dimensional arrays represent a collection of values in two dimensions, or looked at from another angle, a list of pairs. 18

19 Declaration, etc. When declaring it, we now have to use two sets of brackets. For example, int [][] table; In accessing a value in such an array, we also have to give two indices to identify it, one for the row, and the other for the column. By the same token, we can declare multi-dimensional arrays, which can be used, e.g., to represent the number of students attending colleges across USA, we could use a 4D array. 19

20 An example The following piece of code declares and creates a 2D array, fills it with increasing integers, then prints them out. public class TwoDArray{ public static void main(string[] args){ int[][] table=new int[5][10]; for (int row=0; row<table.length; row++) for (int col=0; col<table[row].length; col++) table[row][col] = row * 10 + col; for (int row=0; row < table.length; row++){ for (int col=0; col < table[row].length; col++) System.out.print (table[row][col] + "\t"); System.out.println(); Question: What should the output look like? 20

21 Polygons and polylines Arrays are helpful when drawing complex shapes. A polygon is a multi-sided shape that is defined in Java using a series (list) of (x, y) points that indicate the vertices of the polygon. An array is certainly very appropriate to represent such a list of pairs. Technically, a polygon is drawn, filled or unfilled, using methods of the Graphics class, just like what we did with rectangles and ovals. Those specific methods are called drawpolygon and fillpolygon, respectively. Each of them takes three parameters, the list of x coordinates, that for the associated y, and the number of points, i.e., the size of those two lists. Finally, there is always a line drawn between the first and the last point. 21

22 A polyline is similar to the polygon as a series of segments represented with a series of points. But, the segment between the first and the last point is not automatically drawn. It is not closed, thus can t be filled. There is hence only one method, drawpolyline, which takes the same parameters as the drawpolygon does. The next slide shows an applet that draws a rocket, and its window. The first point in the array represents the tip of the rocket, the rest just goes clockwise from the tip. Both polygons are filled. 22

23 A rocket with a window import javax.swing.japplet; import java.awt.*; public class Rocket extends JApplet{ private final int APPLET_WIDTH = 200; private final int APPLET_HEIGHT = 200; private int[] xrocket={100,120,120,130,130,70,70,80,80; private int[] yrocket={15,40,115,125,150,150,125,115,40; private int[] xwindow={95,105,110,90; private int[] ywindow={45,45,70,70; private int[] xflame={70,70,75,80,90,100,110, 115,120,130,130; private int[] yflame={155,170,165,190,170,175, 160,185,160,175,155; public void init(){ setbackground (Color.black); setsize (APPLET_WIDTH, APPLET_HEIGHT); public void paint (Graphics page){ page.setcolor (Color.cyan); page.fillpolygon (xrocket, yrocket, xrocket.length); page.setcolor (Color.gray); page.fillpolygon (xwindow, ywindow, xwindow.length); page.setcolor (Color.red); page.drawpolyline (xflame, yflame, xflame.length); 23

24 More buttons We have so far discussed the push button component: once pushed, an action event is generated, which we can respond with a listener. Let s check out some other buttons. A check box is a button that can be toggled on or off using the mouse, indicating that a particular boolean condition is set or unset. For example, a check box labeled Collate can be used to indicate whether the output of a print job should be collated. If there are multiple check boxes created in your program, all of them are independent of each other. 24

25 Simple font changer The following piece changes the font of the text displayed in a text field. import javax.swing.*; public class StyleOptions{ public static void main (String[] args){ JFrame styleframe = new JFrame ("Style Options"); styleframe.setdefaultcloseoperation(jframe.exit_on_close); StyleGUI gui = new StyleGUI(); styleframe.getcontentpane().add(gui.getpanel()); styleframe.pack(); styleframe.show(); The StyleGUI class is given in the next slide. 25

26 import javax.swing.*; import java.awt.*; import java.awt.event.*; The StyleGUI class public class StyleGUI{ private final int WIDTH=300,HEIGHT=100,FONT_SIZE=36; private JLabel saying; private JCheckBox bold, italic; private JPanel primary; public StyleGUI(){ saying = new JLabel ("Say it with style!"); saying.setfont(new Font("Helvetica",Font.PLAIN,FONT_SIZE)); bold = new JCheckBox ("Bold"); bold.setbackground (Color.cyan); italic = new JCheckBox ("Italic"); italic.setbackground (Color.cyan); StyleListener listener = new StyleListener(); bold.additemlistener (listener); italic.additemlistener (listener); primary = new JPanel(); primary.add (saying); primary.add (bold); primary.add (italic); primary.setbackground(color.cyan); primary.setpreferredsize(new Dimension(WIDTH,HEIGHT)); 26

27 The StyleGUI class (Con d) public JPanel getpanel(){ return primary; private class StyleListener implements ItemListener{ public void itemstatechanged (ItemEvent event){ int style = Font.PLAIN; if (bold.isselected()) style = Font.BOLD; if (italic.isselected()) style += Font.ITALIC; saying.setfont(new Font("Helvetica",style,FONT_SIZE)); In this program, we also use the Font class, part of the Java standard library. A Font object is defined by the font name, the font style, and the font size. The name established the basic characteristic of the characters, such as Times, Helvetica, etc.. The style of a Java font can be plain, bold, italic, or bold and italic combined. The size simply decides how big the characters will look like. 27

28 Radio buttons A radio button is used with other radio buttons to provide a set of mutually exclusive options. Thus, unlike the check box, it is not meaningful by itself, and has to be used with other radio buttons. Only one option among that group is valid. In other words, at any time, exactly one radio button is on, and the rest is automatically toggle off. 28

29 Which quote do you like? The following piece of code displays a label and a set of associated radio buttons. The radio button decides which quote is to be displayed in the label. Question: What do we have to use radio button, but not a check box, in this case? Answer: Since we only want to put one quote up there. If we use check box, multiple quotes can be put there, just like multiple fonts can be added in the previous case. 29

30 Which quote do you like? The structure of the following is the same as that for the StyleOptions. import javax.swing.*; public class QuoteOptions{ public static void main (String[] args){ JFrame quoteframe = new JFrame ("Quote Options"); quoteframe.setdefaultcloseoperation (JFrame.EXIT_ON_CLOSE); QuoteGUI gui = new QuoteGUI(); quoteframe.getcontentpane().add (gui.getpanel()); quoteframe.pack(); quoteframe.show(); 30

31 The QuoteGUI class public class QuoteGUI{ private final int WIDTH = 300, HEIGHT = 100; private JPanel primary; private JLabel quote; private JRadioButton comedy,philosophy,carpentry; private String comedyquote="take my wife,please."; private String philosophyquote="i think,therefore I am."; private String carpentryquote="measure twice.cut once."; public QuoteGUI(){ quote = new JLabel (comedyquote); quote.setfont(new Font("Helvetica",Font.BOLD,24)); comedy = new JRadioButton ("Comedy", true); comedy.setbackground (Color.green); philosophy = new JRadioButton ("Philosophy"); philosophy.setbackground (Color.green); carpentry = new JRadioButton ("Carpentry"); carpentry.setbackground (Color.green); ButtonGroup group=new ButtonGroup(); group.add (comedy); group.add (philosophy); group.add (carpentry); QuoteListener listener = new QuoteListener(); comedy.addactionlistener (listener); philosophy.addactionlistener (listener); carpentry.addactionlistener (listener); primary = new JPanel(); primary.add (quote); primary.add (comedy); primary.add (philosophy); primary.add (carpentry); primary.setbackground (Color.green); primary.setpreferredsize(new Dimension(WIDTH,HEIGHT)); 31

32 The QuoteGUI class (Con d) public JPanel getpanel(){ return primary; private class QuoteListener implements ActionListener{ public void actionperformed (ActionEvent event){ Object source = event.getsource(); if (source == comedy) quote.settext (comedyquote); else if (source == philosophy) quote.settext (philosophyquote); else quote.settext (carpentryquote); 32

33 A couple of points 1. Check box and radio button, unlike push button, are toggle buttons, namely, either on or off. But, check boxes are independent of each other; while radio buttons work in a group, only one of them is on and the rest is off. 2. A button group is simply a way to define a group of buttons that will work together, thus, besides adding in, logically, into this group; all the buttons also have to be added physically into a panel to be displayed. 33

34 3. A radio button produces an action event when it is selected. The actionperformed method of the listener firstly determines the source of the event using the getsource method, and then compares it with the three radio button objects, before takes appropriate actions. Question: Why do we use == to compare source and the objects? Answer: The comparator == compares for references, thus truly identifies the source of the action. Homework: Exercise