The answers to the other two exercises build on each other. We will only examine the combined answer:

More on Drawing

The hair has been drawn as a beard. That is because java uses a default coordinate system which is upside-down compared to the one commonly used in when laying out drawings by hand or drawing graphics in a physics class. The point with the coordinates (0,0) is at the top left and increasing y-coordinates go down the screen. This arises from the natural conflict between laying out images, for which it is common to place (0,0) at the bottom left corner, and laying out text, in which the first character of the first line is normally placed at the top left. Unless we introduce special transformations, when writing java, we have to use the text-oriented convention.

On most computers, the applet will show a disturbing flicker of partially drawn images. In the next applet, we cure the inverted image by changing a + to a -, and we cure the flicker by drawing using a Graphics object in memory and then transferring all the pixels at once to the displayed Graphics object:

Arrays

We began with a motivational example.  Suppose you want to calculate the average of a list of numbers (int or double) followed by a calculation of the standard deviation.  You quickly realize that by the time you're ready to do the standard deviation, you only have access to the last number in the list.  The solution is provided by arrays since they allow you to name a collection of memory locations with one identifier.  Subscripts or indexes then allow you to access each individual memory location.  The following example illustrates this.

//File:    NumberList.java
//This class will gather together in one place
//functions (or methods) which process lists of
//numbers (int or double).

import iostuff.*;
public class NumberList
{
    //The following two methods calculate the average of a list
    //of int's or double's respectively
    public static int average (int [] a)
    {
        int sum=0;
        for (int k=0; k<a.length; k++)
        {
            sum = sum + a [k];
        }
        return sum/a.length;
    }
   
    public static double average (double [] a)
    {
        double sum=0;
        for (int k=0; k<a.length; k++)
        {
            sum = sum + a [k];
        }
        return sum/a.length;
    }
}

//File:    Average.java
import NumberList;
import iostuff.*;
import java.text.DecimalFormat;
public class Average
{
    public static void main (String [] args)
    {
        double number [];
   
        System.out.print ("How many numbers? ");
        int n = Keyboard.readInt();
   
        number = new double [n];
       
       //inputs the numbers into the array
        for (int k=0; k<n; k++)
        {
            System.out.print("Next score please: ");
            number [k]= Keyboard.readInt();
        }

        //The following code displays the list to the screen
        for (int k=0; k<number.length; k++)
        {
            System.out.println (number[k]);
        }
        System.out.println();

        double avg = NumberList.average(number);
       
        DecimalFormat output = new DecimalFormat("0.00");
        System.out.println ("The average is: " + output.format(avg));
    }
}

Note that arrays "know" their length.  That is, it is not necessary to pass the number of items as a parameter to any of the methods in NumberList.   Instead, when that information is required, the length of the array is retrieved from the array itself; e.g., a.length or number.length.

Lab Exercise 1.  Add a method that displays a list of int's to the NumberList class.  The signature for the method will look like:

public static void display (int [] a)

HINT:  This method should incorporate the section of Average.java (above) that "displays the list to the screen".

Lab Exercise 2.  Add a method that calculates the standard deviation of a list of int's to the NumberList class.  The signature for the method will look like:

public static int sd (int [] a, int average)

Feel free to consult Prof. Steinmetz's version if you get exasperated.

Adapted from Bill Steinmetz's course summary by H. J. Bernstein, 12 Nov 01

Back to CSC 012 Home Page