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Solution for
Programming Exercise 9.5

THIS PAGE DISCUSSES ONE POSSIBLE SOLUTION to the following exercise from this on-line Java textbook.

Exercise 9.5: This exercises uses the class Expr, which was described in Exercise 9.4. For this exercise, you should write an applet that can graph a function, f(x), whose definition is entered by the user. The applet should have a text-input box where the user can enter an expression involving the variable x, such as x^2 or sin(x-3)/x. This expression is the definition of the function. When the user presses return in the text input box, the applet should use the contents of the text input box to construct an object of type Expr. If an error is found in the definition, then the applet should display an error message. Otherwise, it should display a graph of the function. (Note: A JTextField generates an ActionEvent when the user presses return.)

The applet will need a JPanel for displaying the graph. To keep things simple, this panel should represent a fixed region in the xy-plane, defined by -5 <= x <= 5 and -5 <= y <= 5. To draw the graph, compute a large number of points and connect them with line segments. (This method does not handle discontinuous functions properly; doing so is very hard, so you shouldn't try to do it for this exercise.) My applet divides the interval -5 <= x <= 5 into 300 subintervals and uses the 301 endpoints of these subintervals for drawing the graph. Note that the function might be undefined at one of these x-values. In that case, you have to skip that point.

A point on the graph has the form (x,y) where y is obtained by evaluating the user's expression at the given value of x. You will have to convert these real numbers to the integer coordinates of the corresponding pixel on the canvas. The formulas for the conversion are:

           a  =  (int)( (x + 5)/10 * width );
           b  =  (int)( (5 - y)/10 * height );

where a and b are the horizontal and vertical coordinates of the pixel, and width and height are the width and height of the canvas.

Here is my solution to this exercise:


My solution uses a nested class Graph for displaying the graph. This class is defined as a subclass of JPanel. It has an instance variable, func, of type Expr that represents the function to be drawn. If there is no function, then func is null. This is true when the applet is first started and when the user's input has been found to be illegal. The paintComponent() method checks the value of func to decide what to draw. If func is null, then the paint method simply draws a message on the panel stating that no function is available. Otherwise, it draws a pair of axes and the graph of the function.

The interesting work in class Graph is done in the drawFunction() method, which is called by the paintComponent() method. This function draws the graph of the function for -5 <= x <= 5. This interval on the x axis is divided into 300 subintervals. Since the length of the interval is 10, the length of each subinterval is given by dx, where dx is 10.0/300. The x values for the points that I want to plot are given by -5, -5+dx, -5+2*dx, and so on. Each x-value is obtained by adding dx to the previous value. For each x value, the y-value of the point on the graph is computed as func.value(x). As the points on the graph are computed, line segments are drawn to connect pairs of points (unless the y-value of either point is undefined). An algorithm for the drawFunction() method is:

       Let dx = 10.0 / 300;
       Let x = -5                // Get the first point
       Let y = func.value(x);
       for i = 1 to 300:
           Let prevx = x         // Save the previous point
           Let prevy = y
           Let x = x + dx        // Get the next point
           Let y = func.value(y)
           if neither y nor prevy is Double.NaN:
              draw a line segment from (prevx,prevy) to (x,y)

The method for drawing the line segment uses the conversion from real coordinates to integer pixel coordinates that is given in the exercise. By the way, more general conversion formulas can be given in the case where x extends from xmin to xmax and y extends from ymin to ymax. The general formulas are:

         a  =  (int)( (x - xmin) / (xmax - xmin) * width );
         b  =  (int)( (ymax - y) / (ymax - ymin) * height );

The formulas for a and b are of slightly different form to reflect the fact that a increases from 0 to width as x increases from xmin to xmax, while b decreases from height to 0 as y increases from ymin to ymax. You could improve the applet by adding text input boxes where the user can enter values for xmin, xmax, ymin, and ymax.

In the main applet class, the init() method lays out the components in the content pane of the applet with a BorderLayout that has a vertical gap, to allow some space between the graph and the components that are above it and below it. The "North" component is a JLabel that is used to display messages to the user. The "South" component is a JTextField where the user enters the definition of the function. And the "Center" component is the panel where the graph is drawn. The applet registers itself as a listener for ActionEvents from the JTextField. This means that when the user presses return in the JTextField, the applet's actionPerformed() method will be called.

(After viewing my applet for the first time, I was dissatisfied with the appearance of the label. There was no space between the text of the label and the gray background of the component. I decided to fix this by adding an EmptyBorder to the label to allow more space around the text where the white background of the label shows through. Borders were covered in Section 7.2.)

The actionPerformed() method just has to get the user's input string from the JTextField. It tries to use this string to construct an object of type Expr. The constructor throws an IllegalArgumentException if the string contains an error, so the constructor is called in a try statement that can catch and handle the error. If an error occurs, then the error message in the exception object is displayed in the JLabel at the top of the applet, and the graph is cleared. If no error occurs, the graph is set to display the user's function, and the JLabel is set to display the generic message, "Enter a function and press return." The code for all this is:

          Expr function;  // The user's function.
          try {
              String def = functionInput.getText();
              function = new Expr(def);
              message.setText(" Enter a function and press return.");
          catch (IllegalArgumentException e) {

The complete solution follows.

The Solution

       The SimpleGrapher applet can draw graphs of functions input by the
       user.  The user enters the definition of the function in a text
       input box.  When the user presses return, the function is graphed.
       (Unless the definition contains an error.  In that case, an error
       message is displayed.)
       The graph is drawn on a canvas which represents the region of the
       (x,y)-plane given by  -5 <= x <= 5  and  -5 <= y <= 5.  Any part of
       the graph that lies outside this region is not shown.  The graph
       is drawn by plotting 301 points and joining them with lines.  This
       does not handle discontinuous functions properly.
       This file defines two class files, SimpleGrapher.class and
       SimpleGrapher$Graph.class.  It also requires Expr.class,
       which is defined in by a separate file,
       That file contains a full description of the syntax
       of legal function definitions.
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
    public class SimpleGrapher extends JApplet implements ActionListener{
       Graph graph;  // The JPanel that will display the graph.
                     // The nested class, Graph, is defined below.
       JTextField functionInput;  // A text input box where the user enters the
                                  // definition of the function.
       JLabel message;  // A label for displaying messages to the user,
                        // including error messages when the function definition
                        // is illegal.
       public void init() {
             // Initialize the applet by creating and laying out the
             // components.  The applet listens for ActionEvents from
             // the text input box, so that it can respond when the
             // user presses return.
          getContentPane().setLayout(new BorderLayout(2,2));
          graph = new Graph();
          getContentPane().add(graph, BorderLayout.CENTER);
          message = new JLabel(" Enter a function and press return");
          message.setBorder( BorderFactory.createEmptyBorder(5,0,5,0) );
          getContentPane().add(message, BorderLayout.NORTH);
          functionInput = new JTextField();
          getContentPane().add(functionInput, BorderLayout.SOUTH);
       }  // end init()
       public Insets getInsets() {
            // Specify a 3-pixel border around the edges of the applet.
          return new Insets(3,3,3,3);
       public void actionPerformed(ActionEvent evt) {
            // This will be called when the user presses return in
            // the text input box.  Get the user's function definition
            // from the box and use it to create a new object of type
            // Expr.  Tell the canvas to graph this function.  If the
            // definition is illegal, an IllegalArgumentException is
            // thrown by the Expr constructor.  If this happens,
            // the graph is cleared and an error message is displayed
            // in the message label.
          Expr function;  // The user's function.
          try {
              String def = functionInput.getText();
              function = new Expr(def);
              message.setText(" Enter a function and press return.");
          catch (IllegalArgumentException e) {
       }  // end actionPerformed()
       //-------------------------- Nested class ----------------------------
       static class Graph extends JPanel {
          // A object of this class can display the graph of a function
          // on the region of the (x,y)-plane given by -5 <= x <= 5 and
          // -5 <= y <= 5.  The graph is drawn very simply, by plotting
          // 301 points and connecting them with line segments.
          Expr func;  // The definition of the function that is to be graphed.
                      // If the value is null, no graph is drawn.
          Graph() {
                // Constructor.
             func = null;
          public void setFunction(Expr exp) {
                // Set the canvas to graph the function whose definition is
                // given by the function exp.
             func = exp;
          public void clearFunction() {
                // Set the canvas to draw no graph at all.
             func = null;
          public void paintComponent(Graphics g) {
                // Draw the graph of the function or, if func is null,
                // display a message that there is no function to be graphed.
             super.paintComponent(g); // Fill with background color, white.
             if (func == null) {
                g.drawString("No function is available.", 20, 30);
             else {
                g.drawString("y = " + func.getDefinition(), 5, 15);
          void drawAxes(Graphics g) {
                // Draw horizontal and vertical axes in the middle of the
                // canvas.  A 5-pixel border is left at the ends of the axes.
             int width = getWidth();
             int height = getHeight();
             g.drawLine(5, height/2, width-5, height/2);
             g.drawLine(width/2, 5, width/2, height-5);
          void drawFunction(Graphics g) {
               // Draw the graph of the function defined by the instance
               // variable func.  Just plot 301 points with lines
               // between them.
             double x, y;          // A point on the graph.  y is f(x).
             double prevx, prevy;  // The previous point on the graph.
             double dx;  // Difference between the x-values of consecutive
                         // points on the graph.
             dx  = 10.0 / 300;
             /* Compute the first point. */
             x = -5;
             y = func.value(x);
             /* Compute each of the other 300 points, and draw a line segment
                between each consecutive pair of points.  Note that if the
                function is undefined at one of the points in a pair, then
                the line segment is not drawn.  */
             for (int i = 1; i <= 300; i++) {
                prevx = x;           // Save the coords of the previous point.
                prevy = y;
                x += dx;            // Get the coords of the next point.
                y = func.value(x);
                if ( (! Double.isNaN(y)) && (! Double.isNaN(prevy)) ) {
                      // Draw a line segment between the two points.
                   putLine(g, prevx, prevy, x, y);
             }  // end for
          }  // end drawFunction()
          void putLine(Graphics g, double x1, double y1,
                                                     double x2, double y2) {
                // Draw a line segment from the point (x1,y1) to (x2,y2).
                // These real values must be scaled to get the integer
                // coordinates of the corresponding pixels.
             int a1, b1;   // Pixel coordinates corresponding to (x1,y1).
             int a2, b2;   // Pixel coordinates corresponding to (x2,y2).
             int width = getWidth();     // Width of the canvas.
             int height = getHeight();   // Height of the canvas.
             a1 = (int)( (x1 + 5) / 10 * width );
             b1 = (int)( (5 - y1) / 10 * height );
             a2 = (int)( (x2 + 5) / 10 * width );
             b2 = (int)( (5 - y2) / 10 * height );
             if (Math.abs(y1) < 30000 && Math.abs(y2) < 30000) {
                   // Only draw lines for reasonable y-values.
                   // This should not be necessary, I don't think,
                   // but I got a problem when y was very large.)
          }  // end putLine()
       }  // end nested class Graph
    } // end class SimpleGrapher

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