Laboratory 10 Arrays Week of Wednesday, 9 November, 2005

Objective

A programmer often needs to represent a group of values or objects together in a list. The basic Java list mechanism is the array. When you use a list, you often need to be able to find values in the list. Therefore, we review several search algorithms.

Key Concepts

• One-dimensional arrays
• Subscripting

Getting Started

Using the procedures in the previous laboratories, copy the files Sum.java, ArrayFun.java, Descriptions.java, and DescriptionGenerator.java for your manipulation (right-click and select "save as...").

Basics

An array in Java is a list of elements. The list is an object. The individual elements can be accessed through subscripting using the bracket operator []. If a list is composed of n values, then the first element has a subscript value of 0, the second element has a subscript value of 1, and so on with the last element having a subscript value of n-1.

To assist programmers, an array has a public data field named length that specifies the number of elements in the list.

Another form of array assistance is the automatic detection by Java of illegal subscripting – subscripts that are too big or too little. If a subscript is determined to be invalid, an exception of type IndexOutOfBoundsException is generated. As it is with other unhandled exceptions, if IndexOutOfBoundsException occurs and is not handled explicitly by the program, then the program terminates.

The IndexOutOfBoundsException prevents your program from inappropriately accessing memory outside its allocated space. (Attempting illegal memory accesses and assignments is a common exploitation effort of virus writers.)

• Open the program file Sum.java. This program initializes two arrays a and b and produces a third array c whose values are the sums of the values of a and b.
   
• Compile and run the program to see whether its behavior matches your expectations.
   
• Correct the errors in the program.
   
• Recompile and re-execute the program. Check the sums to ensure that the program ran correctly.
   
• Submit the completed program Sum.java.

When writing programs in the future pay attention to the fact that when using a for loop in Java to iterate through an array, the body of loop should typically only iterate when loop indices are in the range 0 to n-1, where n is the size of the array. This design accommodates the fact that Java numbers array indices from 0 to n-1. Hence, the typical for loop initializes its loop control variable to 0 and repeats the loop as long as the index is less than the size of the list.

Array manipulations

Open program file ArrayFun.java. This program first determines from its user the size n of a list that is to be extracted. The program then extracts n values from the standard input stream and uses them to set the values of the elements of array number[]. The program then determines and displays the average of the values in the array.

• Modify the program in the following manner
   
  • Add a new loop that determines the number of values in the list that are equal to the average.
     
  • Add a System.out.println() statement that displays the number of values in the list that are equal to the average.
     
• As a simple test case, run the program on a list of size four whose values are 2, 1, 2, and 3. Also run the program on a list of size three whose values are 4, 0, and 2.
   
• Submit the completed program ArrayFun.java.
   
• Modify the program to determine and display the minimum in the list. Assign this value to an int variable named min. The standard approach to such a task is to initialize min with the value of the first element in the list (i.e., number[0]). A loop then considers the other elements in the list. Each of these elements is tested in turn to determine whether it is less than min. If an element is found to be less than min, the value of min is updated. After the loop completes, the minimum value is definitely stored in min.
   
• Submit the completed program ArrayFun.java. Do not worry about overwriting the previous submission.
   
• Modify the program to display each element in the list. If the element is equal to the average then, the text "average" should be displayed after the value. If the element is equal to the minimum then, the text "minimum" should be displayed after the value. Although it it is pathological, it is possible for a value to be both the minimum and average.
   
• Submit the completed program ArrayFun.java. Do not worry about overwriting the previous submission.

Descriptions

For the last part of this lab, we will finish a Descriptions class, which will be used in our final game.  The class will allow us to generate a random adjective to describe a room ("dark", "haunted", etc.) and a random room type ("living room", "parlor", etc.), as well as random weapon names ("katana", "sai", etc.) and monster names ("goblin", "troll", etc.).

• Examine the code in Descriptions.java.  There are two arrays: the first is the list of adjectives, and the second is the list of room types.  The third instance variable is a Random object, used to create random numbers.  The default constructor is there, but does not need to do anything.  The other two methods (getNextAdjective() and getNextRoomType())  require completion for this part of the lab.
   
• Examine the DescriptionGenerator.java file.  This file contains just a main() method, which uses the DescriptionGenerator class.  Do you understand how this works?  If not, ask a TA.
   
• Add a few values to each of the arrays in Descriptions.java (minimum of 5 each).
   
• Next, complete the getNextRoomAdjective() method.  It currently returns null, but should return one of the adjectives in the roomAdjectives array, chosen at random.  Thus, you need to generate an appropriate random number (via the nextInt() method in the Random class), then return that element in the roomAdjectives array.
   
• Similarly, the getNextRoomType() method should do the same for the roomTypes array.  Note that the two arrays may be of different sizes, so be sure not to mix them up in your code.
   
• Similarly for the getNextMonsterName() and getNextWeaponName() methods.
   
• The Descriptions class itself can't be run, but the DescriptionGenerator class can be (as only the latter has a main() method).  Every time you run the DescriptionGenerator, it will print out twenty random rooms, each with a random description, a random monster, and a random weapon.
   
• Submit the Descriptions.java file.

Finishing Up

• Submit Sum.java, ArrayFun.java, and Descriptions.java.  As DescriptionsGenerator.java was not modified, it does not need to be submitted.
   
• Copy any files you wish to keep to your home directory.
   
• Delete all of the files you copied or created on the laboratory machine.

Homework J7

Use any remaining time in the lab to work on HW J7.