Lab 6: Array Example

Overview

This week, you're you're writing an applet that does the following: generates an array of numbers, then finds three statistics about those numbers: minimum, maximum, and average. You will use a RandomIntGenerator to fill the array with random integers, but you'll also give the user control over the size of the array and the lowest and highest numbers the RandomIntGenerator is allowed to produce.

Writing the Applet

You'll need to refer to both your lecture notes and your previous labs to get information on how to write this one. Here's an outline of the steps you'll need to take. Take some time to work through the problems before you consult a TA. The rough spots are in there intentionally.

• Log in as usual, change into your public_html directory, and open a new file named ArrayExample.java
• Add the stuff that goes at the beginning of every lab, the import lines and the public class... line. The name of this class will be ArrayExample.
• Declarations: you'll need to declare:
  • one int array
  • six IntFields: one for the user to enter the lower bound for randomly generated numbers, one for the higher bound, one for the user to enter the size of the array, one for you (not the user) to display the minimum number actually in the array, one for the maximum such number, and one for the (rounded) average of the numbers in the array.
  • one Button, that the user can press to run your program after s/he has entered all the necessary info
• Begin the init() method as usual. In it, you'll need to create, label, and add to the screen each of the IntFields you created, as well as your Button. You add a label to an IntField with the same method you use to do it on a Button. For instance if your IntField is named avgField, this is how you'd do it:
  
  avgField.setLabel("Average:");
  
  This puts the word "Average:" on the screen, to the left of the IntField itself.

  Make sure you add an ActionListener to your Button.

  Don't do anything with the array of ints in in the init() method. Since the user is controlling its size, we have to wait until the actionPerformed() method before we mess with it.

• Now begin the actionPeformed method in the same way as usual. When you're done with the preliminaries, think about what this particular applet needs to do.

  Since there's only one Button, you really don't have to use an if statement to find out which Button the user clicked, but we will anyway. So if the user clicks the Button, the first thing we need to do is create the array of ints we declared at the top. The tricky part is making it the right size, since that size has been specified by the user. Here's an example of a line of code that does this right, for an array named numbers, and with the user having specified the array's size in an IntField named arraySizeField:
  
  numbers = new int[arraySizeField.getInt()];
  
  You should be able to figure that line out, if you refer to the notes from lecture.

  Now, the hard part. There will be two while loops in your program. The first one will fill up the newly created array with randomly generated numbers. Those numbers have to fall between the lower and upper bounds given by the user. Here's an example of how that would be done, again with an array named numbers and IntFields named lowBoundField and highBoundField. This example shows only how to put such a randomly generated number in the first slot of an array: your loop will have to repeat this for every slot.
  
  numbers[0] = RandomIntGenerator.getInt(lowBoundField.getInt(), highBoundField.getInt());

  When you're writing this loop, keep in mind that you'll need an auxiliary int to keep track of which array slot you're working on at all times. If you don't know what that means, then you need to refer to the lecture notes.

  The second loop will be the one where you find the statistics on the numbers in the array. If you're careful, you can do all this with one loop through the array. You'll need three auxiliary ints, one called sum, one called lowestSoFar and one called highestSoFar.

  sum needs to be set to 0 before you start the loop. Inside the loop, you need to add the value of each element in the array to sum.

  lowestSoFar and highestSoFar should both be set to numbers[0] before you start the loop. In order to find the minimum and maximum numbers in the array, you have to keep track of the lowest and highest numbers you've found so far, and at the beginning, the first number in the array is both the lowest and highest number you know. It's the only number you know.

  The second while loop needs to contain four basic parts:

  • add each element of the array to sum
  • check to see if each element is lower than lowestSoFar. When you find one that is, set lowestSoFar to that value.
  • check to see if each element is higher than highestSoFar When you find one that is, set highestSoFar to that value.
  • increment the counter that you're using to keep track of where you are in the array (this step is common to every while loop you'll write, so if you didn't do it in the first one, you're in trouble).

  Now you're almost done: you need to display your results in avgField, minField, and maxField. Of course before you put anything in avgField, you need to convert sum to an average by dividing it by the number of elements in the array, or arraySizeField.getInt.

Make your ArrayExample.html page to display the applet, and test it out until it's bug-free. The interesting things about this applet are the summary statistics, of course - the user can play with the size and bounds of the array, and see how big it has to get before the average becomes identical to the median of the two boundaries, and before the min and max become identical to the boundaries. You can get extra credit for putting some text above your applet explaining this phenomenon, but you can't get help on the extra credit from the TAs.

Don't forget to submit your .ajva file as usual (before next week's lab.)