CPT120 subject material (pdf file) - RMIT University

Introduction 

Introduction to Programming 

Chapter 1: Introduction 

At the completion of this weeks study, students will: 

Have an understanding of what is involved in using a programming language 

Understand the difference between methods and properties 

Have an appreciation of scenarios and the use of storyboards 

Have an understanding of the steps involved in constructing a program 

Understand how to write and run a simple program in Alice 

Co urse Materials 

Online Course Material 

Please select a subtopic to view its contents. 

The Alice Environment 

Scenarios and Storyboards 

Practical Exercises 


Additional Materials 

Reading References 

Textbook: "Learning to Program with Alice", W. Dann et al., Pearson Prentice Hall. 

Chapter 1, pp7-14 


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C ha p ter 1: Introduction 

The Alice Envi ronment 

Introduction 

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In this subject we will be looking at many aspects of a programming language; we will study 

different types of variables, IF statements, looping structures, properties and methods, 

functions and arrays. 

To accomplish these tasks, you will need three pieces of software: all free and downloadable 

from the web, or available on the textbook CD. Firstly, you will need a program called Alice. 

This is on the CD that comes with the course textbook ("Learning to program with Alice"). 

Please note this is a Windows version only, A version for Macintosh and Linux can be 

downloaded from the Alice website's download page at www.alice.org. (Note: do not download 

"Storytelling Alice".) 

The Alice download page: http://www.alice.org/index.php?page=downloads/download_alice 

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Later on in this subject, you will require the Eclipse Development platform. This is freely 

downloadable from the Eclipse Web site at www.eclipse.org. Download the Eclipse IDE for Java 

Developers; versions are available for Windows, Linux and MacOS X. 

When we will be studying the basics of Java, you will need a Java runtime environment (JRE). I 

would suggest the Sun JRE; this is freely available from Sun's Java web site at java.sun.com. 

The first program we will be using is Alice. This programming tool uses a graphical interface to 

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build a program. We accomplish this by dragging and dropping items in order to 'build' the 

code. Then, by clicking on the run button, you can see instantly what your program does. We 

will use Alice to demonstrate all of the concepts we will be learning as part of this course. 

Note though, that Alice is not a commercial programming language. Alice is a language that 

was designed to be a teaching tool for introductory programming. The other language we will 

be using, Java, is a commercial programming language and uses a written syntax to program 

it. 

Why Learn Programming? 

The reasons why people learn how to write programs for computers are many and varied. 

Programming is a profitable occupation; many programmers have jobs that are highly paid. 

You may also need an understand the basic concepts of programming in order to work with 

programmers. If you understand how to use even the basic methods of programming, it will be 

much easier for you to describe to a programmer exactly what you require. Programming gives 

you experience in following a much more logical way of thinking. 

When we are starting to design a program, we must look at the logical steps required to 

complete the task. These steps must be small enough so that they can be converted into 

logical steps in our program code. On a personal level, when we write code it is an expression 

of ourselves: just as an author, when writing a book, is expressing themselves in words, a 

programmer is expressing their ideas and logic in the way they design and build their 

programs. 

The World and Objects 

Any 'world' consists of objects. Think of the room you are in now; what objects are there? 

Firstly there's you, maybe a chair, a desk, the computer, the mouse, the keyboard, a light on 

the ceiling, a light switch on the wall, and lots of other objects. Programs must be able to 

model these objects. By this, we mean we must be able to describe the qualities and attributes 

of an object. 

For example, we must be able to describe the objects' color, or the fact that the object has two 

arms. We also then have to provide methods to manipulate these qualities or attributes. With 

the previous examples, we must for instance have a way to be able to change the color of an 

object, or to raise the object's left arm. With Alice, we are able to do this directly. We can, and 

often have to, define much information about an object. We call this information the object's 

properties (qualities and attributes). 

Take, for instance, a chair (we'll assume it's an office chair). 

What parts has the chair got? The legs with castors, the seat, and the chair back. 

What qualities has this chair? Its height, color, material it's made of, and so on. 

Objects are able to do things; take our chair. what things can a chair do? The whole chair can 

move on the castors, the chair seat can rotate on its legs, in the computer 'world' the chair can 

also get bigger or smaller, or change color. We call the actions that the chair may perform, the 

object's methods. 

What is a Program? 


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When we write a program, we are providing a set of instructions for the object to follow. We 

specify the methods an object (in this example, a dancer) must do and in what order the object 

must do them in. For example: 

dancer turn .25 revolutions right - the dancer would turn 90 degrees to the right 

dancer move forward 2 meter - the dancer moves forward 2 meters 

dancer turn 5 revolutions left - The dancer would pirouette to the left 

dancer move backward 1 meter 

In the above example, we have told the dancer exactly what to do. They must do exactly what 

we have specified and in the exact order. Each time we run this program the dancer will follow 

the same path and end up in exactly the same position, given that our starting position is the 

same each time. 

With our chair example, we could think of it as a movie script, the director (programmer) has 

said, "I want a dancer, I want them to start at this position, and I want them to move like this 

and end up here." Each time our director calls 'Action!', the same set of movements are carried 

out. 

Another way we can also think about programming is it being like a sports game. In a sports 

game players interact with other players around them, in the above example we could have 

said: 

I want 2 dancers starting from these positions, 

dancer1 turn .25 revolutions right - dancer1 would turn 90 degrees to the right 

if dancer1 is facing dancer2 

dancer1 turn .25 revolutions right 

dancer1 move forward 2 meter - dancer1 moves forward 2 meters 

dancer1 turn 5 revolutions left 

dancer1 move backward 1 meter 

In this example, the dancers interact with each other. If dancer1 is facing dancer2, dancer1 

turns an extra 90 degrees then continues with the rest of the program. 

The Alice System 

When you start Alice you will notice a number of distinct areas on the Alice screen. 


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In the middle at the top is the World Window. This gives you a visual indication of the current 

world. It shows you what objects are present, and their positions and sizes. This window also 

allows you to add objects and to position them. 

Next to the World Window, on the left, is the Object Tree. This window allows you to see all of 

the objects present in your world, including the camera and light. 

Below the Object Tree is the Details Area. This window shows the selected object's properties, 

methods and functions. 

Right of the Details Area is the Editor Area. This area is where you build your program, by 

dragging and dropping an object's methods from the Details Window. 

Above the Editor Window is the Events Area. This is where you specify which events can occur 

in your program, and what is to be done in response to each event. When we click on the Play 

button, our world starts, and in the Events Area we can then specify the method we want to 


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run, such as World.MyFirstAnimation. 

As we have seen previously, objects have properties (e.g. size, color, head, wing, ...) and 

methods (change size, flap wing, nod head, ...) We can see the properties of an object by 

clicking on it from the object tree, and then viewing the Properties tab of the details tab. 

In the editor area, we are able to write new methods that we can use to change an object's 

properties. As with an object's properties, we can also view an object's methods by clicking on 

the Methods tab of the details area. This area gives us a list of what the object can do. In this 

tab, we can also define extra methods for each object. 



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C ha p ter 1: Introduction 

Scenarios and St orybo ards 

Designing a Program 

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Before construction is started on any engineering work, a design is arrived at. What would 

happen if the mechanics started to build an aeroplane without first putting in hours of 

preliminary design? What if the brakes were not strong enough to stop the plane? What if they 

had only put in a very small fuel tank? It would be too late for modifications once it was in the 

air. 

Just as in designing an aircraft or a skyscraper, it is vitally important to design your program 

prior to writing the first line of code. This design process starts by looking at the tasks 

scenario. From this, we can find exactly what our program is meant to accomplish. We can also 

find from the scenario what objects are needed and what actions occur. 

A Scenario 

After traveling through space, a robot-manned craft lands on the moon. The robot 

sets up his camera to show the Robot, the Lander and some nearby rock 

formations. An alien peeks out from behind a rock and surprises the robot. the 

robot looks around and sees the alien and walks over for a closer look. The alien is 

frightened and hides behind a rock. 

In our the above scenario we can see that the overall task is to create a story about an 

encounter between a robot and an alien. 

what are the objects? 

Robot-manned craft, Robot, rocks, Alien and the moon landscape. 

What are the actions? 

Alien peeks out from behind rock 

the Robot is surprised 

Robot looks around 

Robot sees the alien 

Robot moves towards Alien 

Alien is frightened and hides. 

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In the above example we are able to see exactly what objects we need to have in our 'world', 

the actions each object needs to be able to make and in what order the actions need to be 

done. 

When we know the who, what, when and where of our program we can then start to translate 

this into our code. We can get this information from our scenario and use it in the design of our 

program prior to writing any code. There are a number of methods we can use to design our 

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programs. these include flow charting, pseudo-code and storyboarding. 

Storyboards 

With the "visual programming" that you are now starting to learn, it is important to use a way 

of describing your world and the events that happen in it. One type of design tool is the use of 

storyboards. These allow you to depict your story scene by scene. A type of storyboard called 

visual storyboards allow us to draw each scene in the story, allowing us to show the who, what 

and when in picture form. 

Another type of storyboard is called a textual storyboard: this is very similar to the list of 

actions for the alien-robot scenario. This type of storyboard gives us a breakdown of the steps 

we need to use in our code, and is similar to a very detailed to-do list. 

Do the following steps in order 

Alien moves up 

Alien says, "Slithy toves?" 

Robot's head turns around 

Robot turns to look at Alien 

Do together 

Robot moves toward the Alien 

Robot legs walk 

Alien moves down 

A visual storyboard from the previous scenario 

It is important to start now to practice using design prior to coding, even 

with the easier scenarios in this course, this will prepare you for harder 

scenarios you may encounter. 


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Practical 1 


Chapter 1: Intr oduction 

Practical E x ercise s 

Installing Alice 

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For this session you will need to have either the CD that comes with the textbook (Learning to program with Alice). 

Please note this is a Windows version only, otherwise download a copy from the Alice web site's download page. Click 

on the file you require, and save this to your computer. 

If you are using the textbook CD on a Windows machine, this should automatically run. If not, open Windows Explorer 

and click on the CD drive icon. This will be labeled Alice 2.0. A box will then ask where you want to place the files; 

select My Desktop and also select under how much would you like to copy: Minimum. After this, click on Copy Alice 

from CD. You will then be asked if you want a shortcut placed on the desktop, Please select yes. 

For those students who have downloaded a copy from the web: 

If you are installing the Windows copy, double click on the file called Alice.zip which you have downloaded. Once the zip 

file opens, select Extract. You will then be asked where to extract to, I would suggest extracting to C:\ - the program 

will then install to a folder called Alice. You will need to create a shortcut to the files you have just extracted. 

There are two executable files for Alice. Firstly, try making a shortcut to the file called Alice (in the directory you 

extracted the zip file). If this does not run correctly, try making a shortcut to SlowAndSteadyAlice as this may run 

better. 

If you are having troubles with installing, please post a message to the discussion board. 

Your First Alice Program 

Once you have installed Alice, let's get to writing your first Alice program. 

The scenario: 

Do not download the file called "Storytelling Alice". The file we require for this subject is Alice 

2.0. 

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One day a cow was out walking in the field, suddenly it sees a snake in the grass and waves it's tail in fright. The cow 

says, "Eek!! a snake!" and then jumps up and down to frighten away the snake. The snake then slithers away from the 

cow 

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Practical 1 

What are the Objects in our scenario? 

The Cow, the Snake 

What are the actions (Storyboard) 

Cow walks towards Snake 

Cow gets near Snake 

Cow waves tail in fright 

Cow says, "Eek!! A snake!" 

Cow jumps up and down 

Snake slithers away 

Now open Alice and firstly we need the two objects. Click on add objects in the World Window, at the bottom of the 

screen is a list of objects to select from. Click on the Animals tab and scroll across to find the cow. Click and hold the 

left button on the cow and drag this to the world. Drop the cow somewhere near the middle of the world screen. 

Likewise, find the snake and drag this to left-front of the world screen. Your World should look something like this. 

Click on done. 

The initial world with the two objects. 

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Now we can start to add some code. Our first task is to get the cow to walk towards the snake. Click on cow in the 

Object Tree. Below the object tree is the Details area. Fortunately a method has already been created for us, for the 

cow to walk towards another object. Click and hold the WalkTowards method and drag it to the Editor area. Drop this 

method in the box just under the line that says no variables. You will then be asked for which object to walk towards; 

select the snake. For times, select other and enter 10 (you may need to adjust this to stop the cow trampling on the 

snake -- we don't want dead snakes in our programs!) Then you will be asked for the speed, select 1. 

Now you have got the first line you can run the program, click on Play in the top left corner. (Tthis is where you may 

need to adjust the times value to get the cow to stop just short of the snake.) 


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Practical 1 

The first instruction 'WalkTowards' in place 

Now our next step is to get the cow to wag its tail in fright. There is a method for this in the details area, drag and drop 

this below the walk towards method. You will be asked for times - select 5. Now press Play to test your code. The cow 

walks towards the snake and stops and wags it's tail. 

When you place a new method in the Editor area, a green line will indicate where the new 

instruction will be placed. 

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The next action we have to get the cow to do is to say, "Eek!! A Snake!". Select cow say from the details area, and drop 

this method under the previous methods in the Editor area. You will need to select other and enter, "Eek!! A snake!". 

Test this code again: 


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After the third instruction 'Cow say' has been inserted 

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The next step is to get the cow to jump up and down. Drag cow move to the editor area and select up and .5. Next, 

select cow move again but this time select down and .5. 

Then we need to get the snake to slither away; click on snake in the object tree and select slide from the details area 

and drop this under the other methods in the editor area. 


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Practical 1 

Now click on Run. 

Please save your work as Week01-MyFirst.a2w. 

Our complete program 



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Functions 


Week 2: Methods, Interactio n and Functions 

At the completion of this weeks topic students should: 

know how to construct methods for an object in Alice 

have an understanding of what events are and how to write programs using events 

understand how to make a program interactive. 

understand when to use functions 

understand how to write functions 

understand how to use functions in a program 




Object Methods 

Adding Interactivity with Events 

Using Functions 

Tutorial and Practical Exercises 









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Chapter 2: Methods, Interaction and Functions 


Revision of last weeks material 

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Before we proceed with this weeks topic, let's have a quick look at the important points from 

last week. Many of them were quite basic, but they are the foundation from which the rest of 

the unit builds on. 

We have looked at the what and why of programming. Programming is a set of steps 

that tell our objects what to do. 

We have seen something of why programming is important to learn. 

We also looked briefly at the tools that we will be using in the unit: Alice, a teaching tool 

to help us understand the basic programming techniques, and Java, a commercial 

programming language that will give us a feel for "real programming". 

Regarding "the world" and objects: 

We found that objects have properties; qualities that they possess and attributes they 

may have. 

We saw in the example of a chair that its properties might include height, color, the 

chair's legs and the seat. We then talked about what the chair's methods; what it can 

do. Our chair may have a method that makes it roll on the castors. It may have another 

method that changes it's size or color. 

We then went on to look at, "What is a program?" 

We found that a program is a set of instructions for the computer to follow. Just like 

with a recipe to make a sponge cake the chef must follow exactly the same steps and 

use exactly the same ingredients to end up with the correct result. 

We found that we could think of our programs in a number of ways. Firstly like a movie 

script. The actors must follow exactly the same instructions each time the director 

shouts "Action". Our programs can also act like a sports game. When a player is running 

for the ball they must be aware of the other players around them. The player must then 

act accordingly to avoid the opposition players and kick the ball only to players of their 

own team. Our programs can be like this in that an object can be made aware of other 

objects around them. 

We saw in the Cow and Snake demonstration that the program allowed the cow to be 

aware of where the snake was. In the first instruction cow walk towards there must be 

some code that instructs the cow which direction to move towards the snake. 

We had a look at the different areas of the Alice screen. 

Firstly the World Window that shows us the current world and the objects in it. This 

screen also allows us to place objects in our world. 

The next area we looked at was the Object Tree this gives us a list of objects present in 

the world. 

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From the object tree we looked at the Details area. This gives us lists of the objects 

properties and methods. 

Next to the details area we found the Editor area. This is where our programs are built. 

We then looked at Scenario and storyboarding. The importance of designing your program 

before you actually code cannot be overemphasised. 

It is crucially important to know the who, what, when and where of the scenario. 

We must know the who to know what objects we require. 

We must know the what, when and where to know the steps in the scenario. 

Which method, whether it be visual or textual, you choose to describe the design is up 

to you -- but it is important that you do carry out the design phase prior to writing your 

code. 

Making New Methods 

By now you have gone through two Alice programs: the Cow and Snake demonstration, and 

the chicken and the Road demonstration with Ken in the virtual lecture. Let's go back to the 

cow and snake demo. Observe exactly what happens when we issue the command for the cow 

to walk towards the snake. Which step the cow actually makes is an instruction in itself: 

We must get the left back leg and the right front leg to move forward at the same time. 

Then we must get the right back leg and the left front leg to move forward at the same 

time. 

These instructions must be repeated for each step the cow makes. Imagine if we had to put 

each instruction, move left back leg, move right front leg, move right back leg, move left front 

leg, ... ! 

Consider a minimum of four instructions to move the cow one unit. Multiply this by the 

number of units needed to reach the snake (say 20 units), our program then takes 4 times 

20, or 80 steps to accomplish this one task. In fact there are many more steps just to get the 

cow to move: open the cow and snake demo from last week, click on cow in the object tree 

and the edit button of cow walk. Imagine having to write that number of steps each time! 

We can see from each step the cow makes, the directions are exactly the same. Move left 

back leg, move right front leg, move right back leg, move left front leg. Alice gives us a way of 

writing methods that allow us to encapsulate the steps that need to be repeated. If you click 

on the cow in the object window and look at the details window there is a button Create new 

method. We can then use this to create new methods for our cow. 

Please do the practical lesson for this week now; in this lesson we create a new method to get 

our cow to nod it's head. 


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W eek 2: Me th ods, Inte rac t i on and F unctions 

Adding Inte rac t i vity with Ev ents 

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Up to now, we have not been able to interact with our world. Our programs have started when we press Play and ended 

when the code ends. There are times, though, where we do need to interact with the program. A flight simulator 

program (for example) needs interaction from the user, for the direction and speed of the plane. 

In the Events window we can define the program's response to events that may happen during the program's 

execution. Events may be any action, such the press of a key, the click of the mouse button, or when the world starts. 

We can change our cow and snake program from this week's practical to nod the cow's head when we press the space 

bar. Open the prac file in Alice and firstly remove the three times that we have used lowerHead from the Editor area. 

You can do this in two ways: 

1. click and hold on the method and drag it to the rubbish bin at the top; or 

2. right click on it with the mouse and select delete. 

Your code should look like this: 

After deleting the 'lowerHead' methods 

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Above the Editor area is the Events window. Click on Create a new event and select When a key is pressed. Now click 

on any key and change this to space. Finally, click on nothing and change this to cow - lowerHead. 

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Adding a new event to the program 

Now click on Play. Whenever you press the space bar, the cow will nod its head. Once you have finished this 

demonstration, please save the file (week2-interactive.a2w) as we will need it later. 


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U sing Functions 

Asking Questions, Getting Answers 

We have seen previously that objects have properties. You have already used a few methods to manipulate these 

properties. In the previous example we wrote a new method to manipulate the cow's head. 

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Object functions allow us to ask a question about an object. For example, How far is the cow from the snake? Is the 

snake taller than the cow? In the Details area of the screen is another tab that shows the object's functions. We can use 

proximity functions to tell us what the object is near, or the distance to other objects. The size functions give us the 

dimensions of the object, or the relational dimensions. There are also functions for spatial relation and point of view. 

Let's go back to our cow and snake program from this week's practical lesson. This time it's getting a bit dangerous for 

the poor cow. When the cow gets near the snake, this time we'll get the snake to turn round and levitate (what next - a 

levitating snake) to look the cow in the eyes. 

Open the CowAndSnake program from this weeks practical. Firstly you will need to remove all code below the line 

where the cow says "Eek!! A snake!". 

Our starting point for the Functions demo 

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At this point we need to get the snake to spin around and face the cow. Select snake from the Object Tree and then 

select snake turn from the Details area. Enter left and 0.5 revolutions. Now press Play and test your program. 

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Getting the snake to spin around 

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Next, we need to get the snake to look the cow in the eye. Select snake move and select up. Select any value for 

height. Now we need a function that will give us the required height. Click on cow in the Object Tree, and functions in 

the Details area. Scroll down and select cow's height, and drop this on the height area of snake move. 


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Using a function to get the required height 

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We can select many different parts of the object to get their height. In this case, select: cow - neck - the entire neck. 


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Select the entire neck 

Now press Play and test the program. In this example, we have returned the value of the height of the cow's neck from 

a function. We then have asked the snake to move to this height. Save your program as "Week2-functions.a2w". 



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Practical 2 




Please open up the cow and snake demo from last weeks practice. I have changed the scenario for this week. 

The Scenario 

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One day a cow was out walking in the field, it stops and waves its tail. The cow then looks down and sees the snake. 

The cow then says, "Eek!! A snake!", and jumps up and down to frighten away the snake. The snake slithers away from 

the cow. The cow looks down again, nods its head and says, "the snake has gone." 

What are the objects in our scenario? 

The Cow and the Snake 

What are the actions? (Storyboard) 


Cow gets near Snake 

Cow waves tail 

Cow looks down and sees the Snake 




Cow looks down 

Cow nods head 

Cow says, "the snake has gone." 

In the above scenario, the cow nods its head three times: once to look down and see the snake, then another two to 

make sure the snake has gone. As we saw earlier, when we are repeating the same code in a program one way of 

dealing with it is to write a new method. In this way, we can place the code in the method and use it each time we want 

to perform the action. 

Building the Program 

In the Object Tree, click on the cow. Click on Create a new method and give it a name, such as lowerHead. 


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Practical 2 

Our new lowerHead method 

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We need to get the cow to turn its head forward, wait briefly, then turn its head backward; but we need this action to 

be done with the neck as the hinge. Our first task is to get the head to rotate forward. Click on the + symbol next to 

the cow in the Object Tree. We only need to select the cow's head. The cow's head is part of the neck so click on the + 

next to neck. Click on head in the Object Tree. 


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Practical 2 

Selecting the cow's head 

Now drag head turn to the Editor area. Select forward and click on other, enter .15 and click OK. 

Select 'other' 


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Practical 2 

Press the clear button and enter .15 

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We need this rotation to be about the neck. Select more at the end of this instruction. Select asSeenBy - Cow - neck 

and click on the entire neck. 


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Practical 2 

Next, select wait from below the Edit area, and place this below the head turn method. Give the wait method a value of 

0.5 seconds. Lastly, repeat the head turn instruction, but with the direction of backward. 

We are now ready to use our new method. Click on the world.my first method tab and insert the lowerHead method 

under the cow.tailSwish method in the Editor area. Now, the cow should stop and look down to see the snake. You also 

need to insert the lowerHead method twice after the snake.slide method. Then get the cow to say "The snake has 

gone." 

Save your program as "week2-prac.a2w". 

Practical Exe rcise 2 

The Scenario 

A hare is standing in front of a large fan. The white (high) button presses by itself and the fan turns on. The breeze is 

so strong, the hare's ears are blown backward. However, the fan soon turns off and the hare's ears return to their 

original position. 

What are the objects in the scenario? 

The Hare (animals) and a Fan (objects). 


High button moves down 

do together 

Fan's blades Roll (10 times) 

Hare's ears turn backward .25 revolutions 

do together 

wait 2 seconds 

Fan's blades roll (10 times) 

do together 

high button moves up 

Select the entire neck 


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Practical 2 

off button moves down 

Hare's ears turn forward .25 revolutions 

When setting up objects in the World it may be better to use the Quad view. This gives you four 

views of the work; top, front and side as well as the normal world view. Use the buttons beside 

this view to rotate each object. 

The Quad view allows you to have better control over the placement of objects 

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Firstly, you need to select the two objects. The Hare from the Animals menu and the Fan from the Objects menu. You 

will need to resize the fan. You can do this by right-clicking on the fan in the object browser, and clicking on Fan, 

methods, resize. You will need to make it about four times bigger than it is. When you are happy with the layout, click 

on done. 


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Practical 2 

Resizing the fan 

The first action we need is to get the high switch on the fan to move down. Click on the + next to fan in the object tree, 

and click on high button. Drag the method for high button, move to the editor area and select down. Select .5 meters 

for the distance. Now we need to use the methods for what happens when the on switch is pressed. 

When we need to get two or more actions happening at the same time, there is a do together 

method. Select this from the area at the bottom of the Editor area. 

Page 7 of 8 

We now need the fan's blades to roll, and the hare's ears to move back -- but we need to have these happen at the 

same time. Drag a do together method from the bar below the Editor area, and place it below the switch move down 

instruction. 

Now in the do together method we can place all of the instructions that we want to happen at the same time. We need 

the fan's blades to roll, and both of the hare's ears to turn backward. Click the + next to fan in the object tree and 

select blades. In the fan blade's methods you will find a roll method. drag this to the do together box. Select roll, other 

and enter 10. 


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Practical 2 

Select ear turn 

We now need the hare's ears to move backward. Click on the + next to hare, then neck and head. Select left ear in the 

object tree and then select turn from the method window. Select backwards and .25 revolutions. Drop this into the do 

together box. Do the same for the right ear method, now test your program. 

We now need to get the fan's blades to continue turning for a short time. Select fan, blades and drop the roll method 

below the do together box. enter other and 40 revolutions. Now you will need to move the appropriate switches and 

move the hare's ears to the original position (Hint: you may need to use another do together) 



Page 8 of 8 

Up | Site Map 

18/03/2008

Chapter 3: IF Statements 



At the completion of this weeks topic students will: 

Have an understanding of if statements 

Have an understanding of if-else Statements 

Have an understanding of nested if statements 

Be able to apply these concepts to a programming language 




IF Statements 

Nested IF Statements 


Tutorial Exercises 






Chapter 6, p161-167 



Page 1 of 1 

Up | Site Map 

Up | Site Map 

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IF Statements 



IF Statements 

Revision of Last Week's Material 

We learnt about making new methods. 

Up | Site Map 

When we needed to make our object carry out a new task or if the task was repetitive, 

like our cow walking. 

We could create a method to accomplish a single task, and include the new method in 

our program. 

We also learnt about how to make our programs interactive; that is, how to respond to 

commands from the user, like a key press or mouse click. 

Lastly, we saw how to use functions in our programs. We learnt that functions are a way 

of getting information about an object, such as its height or distance from another 

object. 

IF Statements 

We use if statements every day of our lives: 

If it is raining, I will take an umbrella. 

If the lift is there when I arrive at work, I'll catch it; else I'll walk up the stairs. 

There are many times in our daily lives where we need to make a decision based on some 

fact. Likewise, in our programs, we are constantly having to make decisions based on 

information within the program. 

Page 1 of 3 

We can think of this as the "sports game" type programming we mentioned in week one. A 

player who has the ball has constantly to make decisions; "Where is the nearest player in my 

team?", "Where is the nearest player in the other team?", "Am I close enough to the goals to 

score?", and so on. The player needs to make choices based on the answers to these 

questions. 

For example: "If I am close enough to the goal to score, I'll kick the ball towards the goal." In 

this example, we "kick the ball towards goal" IF the TEST "I am close enough to score" is 

TRUE. If the test is false, then we don't kick the ball. We can see from this diagram the flow of 

the program; notice that there are two paths from the test statement. One, yes, carries out 

the required task "Kick the ball". The other, no, skips this instruction. 

file://D:\_Contribute_Material\cpt120\chapter\03\if-statements.html 

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IF Statements 

We can also say what to do if the test failed. In the previous example, what would our player 

do if "I am close enough to score?" is false? They need to have another action. Maybe they 

should kick the ball to another player on the same side.. our if statement then becomes 

If I am close enough to the goal to score 

I'll kick the ball towards the goal 

else 

The flow of an if statement 

I'll kick the ball to another player on my side 

Page 2 of 3 

The new if statement takes into account both of the possibilities for our test. If the test is 

TRUE - I'll kick the ball towards the goal. If the test is FALSE - I'll kick the ball to another 

player on my side. The next diagram shows the If - else scenario. If oru test is yes (TRUE) we 

do one action. Whilst if our test is no (FALSE) we do another action. 

We can also use functions in our test to get information on an object. For example, if we need 

to make a decision based on the color of an object, such as telling if a ball is red or green: 


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IF Statements 

if ball.color == red 

cow say "the ball is red" 

else 

cow say "The Ball is green" 



Page 3 of 3 

Up | Site Map 

18/03/2008





Up | Site Map 

Sometimes our scenario might require us to make a decision, based on a previous decision. In the example of 

the football player discussed earlier, the player made a decision of, "If I am close enough to goal to score, I'll 

kick the ball towards goal; else, I'll kick the ball to another player on my side." What happens, though, if the 

goal being kicked to is the opposition's goal, or there is no other player to kick the ball to? 

In this case, we need to include more if statements. "If the goal I'm kicking to is the opposition's goal, I'll turn 

around and kick the ball to a player on my side; else, I'll kick the ball towards goal." 

If I am close enough to the goal to score 

If the goal is my team's goal 

I'll kick the ball towards goal 

else 

else 

I'll turn around and kick the ball to someone on my team 

If there is a team member on my side close enough to click to 

else 

I'll kick the ball to that player 

I'll run and bounce the ball in front of me till I find a player on my side 

As you can see, programs can become complex fairly quickly. The above example is only nested two levels; 

imagine if statements were nested five (or more!) levels. 

Mana and Zombie 

The "Mana and Zombie" example we'll go through now, will further explain the need for nested loops. 

The Scenario 

You will notice in the above example that each section of the if statements are indented. 

This is important, as it gives you an indication of the actions that belong to each section. 

Page 1 of 5 

Mana is a short-sighted child who can see dead people. She can turn left (left arrow key) and move forward 

(space key). If a zombie is within 0.5 meters in front of mana, she says, "I see dead people." 

file://D:\_Contribute_Material\cpt120\chapter\03\nested-if.html 

18/03/2008


Mana and the Zombie. 

We have learnt earlier that we can create new methods, and use these new methods in events. Our solution to 

getting Mana to respond to the required key presses may be: 

The mana.forward and mana.turnleft methods. 

Go ahead and try this scenario. Unfortunately we have a problem with Mana moving; she can walk right 

through the Zombie. 

We can walk Mana right through the Zombie. 

Ok, let's change our mana.forward method so that she can only move to within .25 meter of the Zombie -- 

we'll need an if statement for this. If mana is greater than .25 meters in front of the zombie, she will be able to 

walk. 

We need to build this test. The value required in this case is the distance that the Zombie is in front of Mana; 

and to test if this value is less than .25 meters. 

The modified mana.forward method. 


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We have another problem with our modified method. What happens if Mana is behind the Zombie? We have 

only said that Mana can walk if she is in front of the Zombie, at a distance greater than .25 meters. If Mana 

moves behind the Zombie, she will not be able to move any further. In this case, the if statement would fail 

and the move instruction would be skipped over. 

We need to change the test again. This time we need to take into account if Mana is in front of the Zombie and 

if the distance in front is less than or equal to .25 meters. If both of these tests are true, then we need to stop 

Mana from walking any further forward. For this we need to use a both a and b logic (in World functions). The 

first test, Zombie is in front of Mana, needs to be added to the a area of the logic construct. For the other test, 

we first need a less than or equal to math function (in World functions). We should then drop into this the 

zombie distance in front of mana function. Our modified mana.forward function then becomes 

We could also use nested if statements to do the same task. In this case, if the test Zombie is in front of Mana 

is false, it skips the second if statement completely, and she can walk forward. If Mana is in front of the 

Zombie, the nested if statement is tested. if Mana is within .25 meters in front of the Zombie, she can go no 

closer. 

there are three possibilities with the above example: 

The Zombie is not in front of Mana 

The Zombie is in front of Mana, but at a distance greater than .25 meters 

The Zombie is in front of Mana, at a distance of less than or equal to .25 meters. 

Let's look at each of these in turn: 

The Zombie is not in front of Mana 

The second modification to the mana.forward method. 

The mana.forward method modified with nested if statements. 


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The Zombie is not in front of Mana. 

The program first looks at the test in the first if statement and evaluates, "Is the Zombie in front of Mana?". As 

this is not correct, the result is false. The commands in the else clause of the first if statement are then 

executed. 

The Zombie is in front of Mana but at a distance greater than .25 meters 

The Zombie is in front of Mana. 

Once again, the program looks at the first if statement. This time the Zombie is in front of Mana, so the result 

is true. 

The Zombie is in front of Mana but over 0.25 meters away. 

The program then evaluates the second if statement, "Is the the Zombie less than or equal to 0.25 meters 

away from Mana?". In this case, the distance is greater than 0.25 meters; the result is false. So the commands 

in the second if statement's else clause are executed. Mana is able to move forward. 

The Zombie is in front of Mana at a distance of less that or equal to .25 meters. 


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The Zombie is in front of Mana. 

The program evaluates the first if statement. This time the Zombie is in front of Mana, so the result is true. 

The Zombie is in front of Mana but less than 0.25 meters away. 

The second if is then evaluated; "Is the Zombie in front of Mana, and is the distance less than or equal to .25 

meters?". In this case the result is true; the distance is less than .25 meters. The second if statement is 

processed, with the action of "do nothing". As a consequence, Mana is not able to move any nearer to the 

Zombie. 

If we look at our original scenario, we need to add code to get Mana to say, "I see dead people". She must only 

say it if the Zombie is in front of her, and the distance between her and the Zombie less than 0.5 metres. The 

final Mana.move method then becomes. 

The final Mana.move method 

Note: the "I see dead people" if statement must also be added to the Mana.turnleft method. 



Page 5 of 5 

Up | Site Map 

18/03/2008

Tutorial 3 


C ha p ter 3: IF St atements 


The Scenario 

Up | Site Map 

Mana is a child of slightly spooky appearance, who can move forward (up arrow key), left (left 

arrow key) and right (right arrow key). The initial scene contains Mana and a fire-hydrant. 

Mana obeys the usual laws of physics, in that she can't walk through objects. The aim is to 

allow Mana to walk around this world. 

For the purpose of this exercise, Mana has the following functions: 

Mana distance to object 

Mana distance infront of object 

Mana is infront of object 

Mana's width 

Mana's depth 

She has the following methods: 

Mana move distance 

Mana move towards object 

Mana turn distance 

The world with Mana and the Fire Hydrant 

file://D:\_Contribute_Material\cpt120\chapter\03\tutorial.html 

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Tutorial 3 

Distance can be a simple amount (e.g. 0.1 metre) or a formula (e.g. a - b , where a and b are 

then replaced with numbers, functions or other formulae). 

The fire-hydrant has functions and methods of the same type as Mana's (e.g. [fire-hydrant 

distance to Object]) 

The World provides the following functions: 

a = b 

a > b 

a < b 

both a and b (meaning test a is true and test b is true) 

either a or b, or both 

not a 

The dummy items a and b are then replaced by functions or formulae. (e.g. [ (4 - 2) < Mana 

distance to fire-hydrant ] ). You can create events saying what method to run when a key is 

pressed (e.g. [ when space is typed do method ] ) 

Questions 

You are encouraged to discuss these questions and share useful links you've found with others 

in the discussion forum. 

The first task is to get Mana to move. We need to get Mana to respond the the three arrow 

keys. 

1. How can this be accomplished? 

2. Do we need to create any extra methods for Mana's movements? 

Problem: Mana will happily walk right through the fire-hydrant. Note that she can only walk 

through the fire-hydrant by moving forward (not by moving sideways etc as the only action 

keys are move forward, and turn). Think about what information you need to prevent walking 

through the fire-hydrant. 

3. What information is required to stop Mana from walking through other objects? 

4. What functions will give us the required information? 

5. Do we need to use an if statement with this information? 

6. If we do need an if statement, suggest how this should be implemented. 

Once you feel there has been sufficient discussion for you to attempt to code this problem, 

please go ahead and write the solution using Alice. 


file://D:\_Contribute_Material\cpt120\chapter\03\tutorial.html 

Page 2 of 2 

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18/03/2008

Practical 3 




Lets have another look at our cow and snake scenario; it got a bit scary for the cow last week so we'll change the 

scenario a bit, and we'll also add some user interaction. 

The Scenario 

Up | Site Map 

One day a cow was out walking in the field, where there is also a snake. (The snake will slither away when the user 

presses the space key, or if the cow jumps up and down.) The cow stops, and swishes its tail a few times. the cow then 

looks down; if it sees the snake, the cow says, "Eek!! A snake!", and jumps up and down to frighten away the snake. 

The snake then slithers away from the cow. If the cow doesn't see the snake, the cow says, "There is no snake here." 

and does not need to frighten away the snake. 

What are the objects in our scenario? 

The Cow and the Snake 

What is the interaction required? 

When the user presses the space bar, the snake slithers away. 



Cow stops 

Cow waves tail 

Cow looks down 

if the Cow sees a Snake 

else 




Cow says, "I see no snake here." 

Page 1 of 5 

We already have most of this completed from last weeks' work. Open up the interaction demo from last week (don't 

forget to save this week's work with a different name!) We'll first change the interaction required. change the 

cow.lowerHead to snake.slide by clicking on the arrow at the end of the event. 


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Practical 3 

Change cow.lowerHead to snake.slide 

Page 2 of 5 

We now need to add back our cow.lowerHead to under the cow.tailSwish method. Next select the if/else statement from 

the bottom of the Editor area, and drop this below cow.lowerHead. For now, select true; we'll change the test to what is 

needed later. The world object has the logic functions that we require. Click on world in the object tree, and functions in 

the details area. 

We need to test if the cow can see the snake. For this, we'll assume that if the distance from the cow to the snake is 

greater than 2 metres, the cow cannot see the snake. In the math functions, we need a < b; select this function and 

drop it over the true section of the if statement. For now, just make it if 2 < 2. Now we need to get our distance 

between the cow and the snake, but we need the end of the snake that's closest to the cow. Now what do we need to 

do is: if the distance is less than 2 metres, move the required methods into the top box in the if statement. 


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Practical 3 

We can now place the actions that the cow and snake do if the result of the test is true (if the cow sees a snake): 

Cow says, "Eek!! A snake!"; 

Cow jumps up and down; and 

Snake slithers away. 

These get placed in the box just below the test section. 

The if statement with the test included. 


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Practical 3 

The actions to carry out if the if test is true. 

Drag and drop the second cow say in the lower box of the if statement. Change what the cow says to what is required. 

Adding the cow say text 


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Practical 3 



Page 5 of 5 

Up | Site Map 

18/03/2008

Chapter 4: Loops 



At the completion of this weeks study students should. 

Know how to repeat actions a specified number of times 

Know how to use if statements within a loop 

Understand infinite loops 




Introduction to Loops 








file://I:\20080325\cpt120\chapter\04\index.html 

Page 1 of 1 

Up | Site Map 

Up | Site Map 

25/03/2008



Chapter 4: Lo ops 

I ntroduction to L oops 

Basic Scones (From My Mum's notes) 

Ingredients 

3 cups self-raising flour 

80g butter, cubed 

1 to 1 1/4 cups milk 

Method 

Preheat oven to 200°C. Lightly dust a flat baking tray with plain flour. 

Sift the self-raising flour into a bowl. 

Using your fingertips, rub the butter into the self-raising flour until the mixture resembles breadcrumbs. 

Make a well in the center of this mixture. Add 1 cup of milk. 

Mix with a fork until the mixture forms a soft dough, add more milk if required. 

Turn onto a lightly floured surface. Knead gently until the dough is smooth. 

Push the dough into a 2cm-thick round. 

Using a 5cm round cutter, cut out as many scones as you can. 

Press the dough together and cut out the remaining scones. 

Place scones onto prepared baking tray, 1cm apart. Sprinkle tops with a little plain flour. 

Bake for 20 to 25 minutes or until golden and well risen. 

Transfer to a wire rack. 

Serve warm with jam and cream. 

Up | Site Map 

You could be forgiven for thinking that this course on programming has suddenly turned to cookery, but have another 

look; many times in this recipe we are repeating the same actions. 

For example, where we need to "mix with a fork until the mixture forms a soft dough", or "knead gently until smooth". 

In these steps, we are repeating something until a condition is met. In programming, this is called looping. Loops are 

used when we need to repeat one or many actions a number of times. The first example could be written as: 

loop: 

mix with a fork 

while the mixture has not formed a soft dough. 

In this example the loop is started and we start to mix the mixture with a fork. After a while, we then test the mixture 

and see if it has formed into a soft dough. If this test is true (it is not a soft dough), we then go back to the start of the 

loop. In this case seeing that we have only one instruction "inside the loop", we continue to mix with a fork until the 

mixture has formed a soft dough. 

As with our previous topic on if statements, we use loops countless times in our everyday lives. We go to a petrol 

station to fill up the car with petrol, and we add petrol to the tank while it is not full. Likewise with programming, if 

statements and loops are the most common program control structures. 

Let's Have a Look at Loops in Alice 

Page 1 of 6 

In the Wild West folder is a Sheriff; let's make him dance 5 times. We could write a method to make him raise and 

lower his left leg, the method for the right leg would be similar. 

file://I:\20080325\cpt120\chapter\04\loops-intro.html 

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The method to make him step with his left leg 

Now we need to make him do this five times. We could place the stepLeft and stepRight methods in a do in order box 

and then repeat this five times. 

The stepLeft - StepRight methods repeated five times 

What would it be like if we needed to make the Sheriff dance fifty times, or even five hundred times? The program 

would become huge and unmanageable. 

There is a type of loop that we can use to accomplish this, for cases such as these where we know the number of times 

an action needs to be repeated. In Alice this is referred to as a counted loop. Each time through the loop, a counter is 

incremented until the count matches the required number of repetitions. 

In many programming languages this type of loop is called a for loop. If we have a look at the complicated version, we 

can see that there is a counter called index that starts from zero. the counter gets incremented (in this case by 1) each 

time the loop is executed. The final value of the counter will then be up to but not including 5. As a result, the counter 

will start at 0 and end at 4, a total of five times through the loop. 

The program flow in this type of loop is: 

The various sections of a counted loop 

firstly the counter is initialized to the start value; 

then a test is made to see if the counter is equal to the 'stop before' value 

If this test is false, the code within the loop is executed. The counter is then 

incremented and the test is done again. 

Once the result of the test is true the loop ends. 


Page 2 of 6 

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Note: If the test returns true at the first iteration of the loop, no part of the loop's insides are 

executed. 

Loops Containing IF Sta t ements 

The two views of the Sheriff loop 

Page 3 of 6 

The Scenario: One day a cow was out walking in the field. Suddenly it sees a snake in the grass and waves its tail in 

fright. The cow then says, "Eek!! A snake!", and then walks around the snake at a distance of 2 meters. 

The actions in this scenario are more complicated than the previous cow and snake examples. We cannot use the cow 

walktowards method, as we actually have to walk past the snake. 

We must first get the cow walking towards the snake. After this, we must use a loop to get the cow walking a short 

distance, and then check if the distance between the snake and the cow is within the required range. If this is the case, 

the outer loop is interrupted and the actions within the if statement are executed. 


25/03/2008


Using an if statement within the loop to interrupt the program flow 

Note: I have also used another loop within the outer loop. Once the actions within the if statement have completed, the 

program flow returns to the first loop. 

We could further modify the previous method by using a distanceTo function to enable our "smart cow" to know the 

number of times it must walk towards the snake. The program would then use this value in the loop, and allow the cow 

to walk that many times towards the snake. 

The distanceTo function used to get the distance between the cow and the snake 

Page 4 of 6 

As we have seen in the complicated version, this type of loop uses a counter to count between the start value and the 

end value. We can view this counter as the program runs, by placing a print statement (available below the editor area) 

within the loop: 


25/03/2008


Infinite Loops 

A print method used within a loop 

In most cases, we want a loop to eventually finish; a loop that never ends would be a problem. That being said, there 

may be times where an infinite loop can be used to our advantage. 

Take the example of creating a continually busy highway: we could have a car move along the highway. Once it's at the 

end, make it invisible and move it back to the starting position. Make it visible again, and move it along the highway. 

If we placed these actions inside an infinite loop, it would appear as if many cars were traveling along the highway, 

even if each car looks the same as the one before. 

Using an infinite loop to simulate a busy highway 


Page 5 of 6 

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25/03/2008




Page 6 of 6 

25/03/2008

Tutorial 4 


Chapter 4: L o o ps 

Tutor i al E xercises 

Aim: To investigate loops. 

Questions 

You are encouraged to discuss these questions and share useful links you've found with others in the discussion 

forum 

Up | Site Map 

1. Write a loop to make a ballerina perform a jumping pirouette (she jumps a metre high in the air, and makes a 

complete turn while in the air) 5 times. 

2. Write a loop to make a penguin walk in a (equilateral) triangular path. Then change it to walk in a hexagonal path. 

3. Sometimes the meaning of what a programmer wrote isn't what s/he intended. We then say the program has a 

logic error. Consider the following loop - if the programmer intended the loop to run 3 times, does it have a logic 

error? How many times will the loop run? 

Is there a logic error in this loop? 

4. Consider the following loop. Describe what the Sheriff will do. 

What will the sheriff do in this example? 

5. Design a program using loops where two skaters perform a routine. They start at diagonal corners of a virtual 

square and skate around the square. Use as few move commands as you can. 

file://I:\20080325\cpt120\chapter\04\tutorial.html 


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Up | Site Map 

25/03/2008

Practical 4 




The Scenario 

Up | Site Map 

Six penguins are practicing a new dance routine. They start lined up in two rows of three; 

each penguin is two meters away from the penguin alongside or in front/behind of them. The 

penguins perform a synchronized walk, moving in a square (the two rows walking in different 

directions). Their dance lasts for five complete squares. There is also an igloo, which has a 

dance sign that constantly flashes. 

Let's stop and think about this program before rushing in to writing the code. Firstly we need 

to discover what objects are needed for the story board. 

What objects in our scenario ? 

Six penguins 

An Igloo 

A Dance sign 

Download the initial PenguinDance file here penguinDance.a2w 

The initial penguinDance screen 

Page 1 of 4 

We are told in the scenario that the Igloo has a sign that flashes continuously. We have used 

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Practical 4 

an infinite loop and the visibility property of an object previously, with the busy highway 

example. This is a similar situation here, in that we need to flash the dance sign. To do this, 

we need to make its visibility property false. This effectively turns the sign off; making it 

visible turns the sign back on. This needs to be done continuously - a good use for an infinite 

loop. The story board up to this point is then: 

// get the sign to flash continuously 

Loop infinity times 

dance set isShowing to false 

dance set isShowing to true 

Now we can work on getting the penguins to do their dance. We are told that the penguins 

move in a square and that the two rows move in different directions. Let's get our penguins to 

do one square first. At each side of the square, all of the penguins walk forward one metre. 

One row of three penguins will then turn left, and the other three will turn right. We then need 

to get our penguins to do this four times to complete the "square dance". Our storyboard for 

one square dance is: 

// get the penguins to do their dance 

// Dance one square 

Loop 4 times 

Do together 

// get the penguins to walk a side of the square 

penguin walk 1 meter forward 

penguin2 walk 1 meter forward 





Do together 

// get the penguins to turn left or right 

penguin turn left .25 revolutions 

penguin2 turn left .25 revolutions 


penguin4 turn right .25 revolutions 



We now only need to combine these two parts of the storyboard using a do together. We also 

need to get the penguins to complete the full dance, by placing our 'square dance' inside 

another loop that is set to repeat five times. The final storyboard is then: 

Do Together 

// get the sign to flash continuously 

Loop infinity times 

dance set isShowing to false 

dance set isShowing to true 

// get the penguins to do their dance 

Loop 5 times 

// Dance one square 


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Practical 4 

Loop 4 times 

Do together 

// get the penguins to walk a side of the square 

penguin walk 1 meter forward 






Do together 

// get the penguins to turn left or right 

penguin turn left .25 revolutions 






Page 3 of 4 

Remember that when you code this scenario up in Alice, there is a method for each penguin 

called Walking. please use this method with a length of 1. Remember also that each penguin 

is a separate object, so you will need to make each penguin walk and turn individually, as 

shown in the storyboard. 


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Practical 4 

The final penguin dance program 



Page 4 of 4 

Up | Site Map 

25/03/2008

Chapter 5: While Loops 




Understand the difference between for loops and while loops. 

Understand the use of functions in loops 

Understand that While loops continue only while the test is TRUE 

Understand the difference between if statements and while loops 

Understand the danger of creating infinite loops 




While Loops 

Nested Loops 






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Textbook: "Learning to Program with Alice", W. Dann et al., Pearson Prentice Hall 



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While Loops 


Chapter 5: While L oops 

W hile L oops 

While Loops | Up 


Last week we looked at counted loops (aka for loops). These are loops that repeat a specific number of times. This 

week, looks at another type of loop called while loops. In this type of loop, we repeat an action or series of actions for 

however long a particular test expression is true. Once the test expression is false, the loop ends and the program 

continues with its next instruction. 

Think back to the scone recipe from last week, there was a line that said, "mix with a fork until the mixture forms a soft 

dough". This line could have also be written as: 

While the mixture has not formed a soft dough 

beat the mixture with a fork. 

In Alice, there are times when we need to repeat some action while a condition is true. Let's go back to the cow and 

snake scenario from last week: in that example, we used an if statement to test if the cow was getting close enough to 

the snake. 

Using an If statement to test the distance between the cow and the snake 

We could have also used a while loop to move the cow towards the snake. In this case, "Snake distance in front of cow 

> 0.5" would be the condition that must be kept true while the cow is walking towards the snake. At each iteration of 

the while loop, it will perform the distance check and move the cow 0.25 metres closer if necessary. This process of 

checking the condition and issuing the instruction will repeat for as long as the condition is true. When the cow is finally 

close enough, the while condition will become false, and the loop will end. 

The initial scenario, changed to use a while loop to test the distance between the cow and snake 

There can also be situations where the instructions inside a loop are not executed. If the initial scene had the cow 

standing on top of the snake, or the cow in front of the snake, the test "Snake distance in front of cow > .5" would be 

false; in these cases the snake is not in front of the cow. In this instance, the instructions inside the loop would be 

skipped over. 

Confusion Between IF Statements and While Loops 

Page 1 of 5 

If you consider the previous two examples with the cow and snake, both the IF statement and the While loop use a test 

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While Loops 

to see if commands should be executed. The major difference is that an IF statement will only test once; the While loop 

repeats the test and the commands within the loop until the test fails. 

While Loop Example 

Scenario 

IF Statement: "move cow" can only be done (at most) once While Loop: "move cow" might be done many times 

Mana is able to walk freely around her world. Also living in the in the same world is a Zombie that will try and catch her, 

if she gets too close (less than 2 meters in front). Once Mana is within range of the Zombie, Mana will be chased by the 

Zombie until she is caught. 

In the above methods, an if statement is used to first test if Mana is within range of the Zombie. Once the test 'Mana is 

in front of zombie and mana distance in front of zombie < 2' is true, the program flow switches to the zombie.attack 

method. If the test in zombie.forward is false (Mana is not within range of the Zombie), Mana is free to move forward. 

Once program flow has changed to zombie.attack, a test is continually made (in a loop) to make sure that the distance 

between Mana and the Zombie is still greater than 0.1 meters. Once this test is false, i.e. the distance between Mana 

and the Zombie is < .25 metres, the loop ends. The next instruction after the loop is executed" 'zombie say "gotcha"'. 

We can see from these two functions that in zombie.forward, the test and the statements within the if statement are 

executed once only. In contrast, in the zombie.attack method, the actions within the loop are repeated while the loop 

test is true. 

Accidental Infinite Loops 

The zombie.forward and zombie.attack methods from the above scenario 


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While Loops 

Let's have a look at a few Alice examples that do not go according to plan. When using while loops it is important to 

actually have a situation where the test can be false. 

The Scenario 

Mana is facing a crate, she moves forward while she is greater than or equal to 0.25 metres away from the crate 

Consider what would happen if Mana was metres 2.1 meters from the crate: 

Mana is 2.1 metre away from the crate. The loop test is true, and she moves forward 1 metre 


Mana is 0.1 metre away from the crate. The loop test is false, and the loop ends. 

However, consider what would happen if Mana was initially 1.5 meters from the crate: 



(Mana walks through the crate. The crate is now .5 metres behind her, which is greater than 0.25 metres) 


Mana is 1.5 metre away from the crate. The loop test is tru, and she moves forward 1 metre 


and so on. The loop will never end. 

Another Scenario 

Mana is being chased by a Zombie, but she is able to outrun any Zombie. For every 0.5 metres the Zombie moves, 

Mana moves 1 metre. If the Zombie gets to within 0.25 meters of Mana, she will be caught. 

In this case the distance between Mana and the Zombie can never get to within .25 meters. For each move the distance 

between them is increasing by .5 meters. If we take the start distance as .5 meters 

Mana is 0.5 metres away from the Zombie. The loop test is true. She moves forward 1 metre. The zombie moves 

0.5 metres. 


0.5 metres. 


0.5 metres. 


0.5 metres. 


0.5 metres. 

From this behaviour, we can see that the distance between them will never get to less than 0.25 metres, and the loop 

will never end. 

Using an Object's Properties in a Loop 

Mana moves forward while she is greater than .25 meters from the crate 

An accidental infinite loop 

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We can also use an object's properties in the loop test. In this instance the isShowing property of the fan's off bottom is 


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While Loops 

tested. This is to make it such that while the off button is showing, the blades will turn. Once the user clicks the off 

button, the off button is made invisible and the loop test returns false. The loop then ends, and the fan's blades stop 

turning. 

Ra ndomness 

The ability to use randomness in our programs can, at times, be used to good effect. In the world object, we have 

provided for us two functions. One called choose true, which returns true a defined percentage of times. The other 

function, random number, allows us to generate a number within a upper and lower range. 

Scenario 

Testing the properties of an object in a loop 

A Cow and (possibly) a Snake are in a field. 50% of the time the program is run, the snake is not there. If the Snake is 

in the field, the Cow walks towards it, stops just before it reaches the Snake, looks down and says, "Eek! A snake!". If 

the Snake is not there the Cow just keeps walking. 

To obtain the 50% randomness, we'll use the choose true method, and make the visibility property of the snake true or 

false depending on the returned value. The resultant code would look like: 

Using the 'choose true' method to make the snake visible or invisible 

Page 4 of 5 

Let's change the scenario a bit now. If the snake is not there, the cow turns a random revolution (and either left or 

right), and then walks off. The random value can be obtained from the random number function. By setting the range 

of this function from -0.25 to 0.25 revolutions, a turn left method can then be used to obtain either a left or a right turn 


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While Loops 

for the cow. 

Using a 'random number' function to generate left or right turn for the cow 



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Nested Loops 



Neste d Loops 

Nested Loops | Up 


In last weeks material, you were introduced to a programming structure known as looping. With this in mind, 

let's have another look at the Dancing Sheriff. You will recall that we wrote two methods, called stepLeft and 

stepRight. We then used these in a loop to get the sheriff to dance five times: 

The next step in this scenario: The Sheriff has learnt the next part to his dance routine; this time he does two 

steps with his left leg, then two steps with his right leg, and repeats this set of actions five times. 

We could write a loop that repeats the step routine the required number of times: 

Nested Loops 

The Sheriff Dance loop from last weeks material 

The stepLeft, stepRight methods repeated in a loop 

Page 1 of 4 

Another method we could use to solve this problem, called nested loops, is to place one or more loops inside 

another loop. With this type of looping, for each time through the outer loop, the inner loop(s) will repeat. In the 

Sheriff Dance we have already got a loop that repeats the Sheriff stepping five times. 

Within this outer loop two inner loops are required, each repeating two times. In one inner loop the stepLeft 

method is listed, the other contains the stepRight method. In this way, each stepLeft/stepRight method is 

repeated two times and these two 'inner loops' are then repeated five times in the 'outer loop'. 

file://Z:\contribute\cpt120\chapter\05\nested-loops.html 

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Nested Loops 

Investigating a Nested Loop 

In this nested loop: 

the outer loop will repeat 3 times (the value of index will start at 0 and go to 2) 

Each time the outer loop repeats, the inner loop will repeat two times (the value of index_#2 will start at 0 

and go to 1). 

This table provides an indication of the values in each of the loop indexes, at each step. 

Deeply Nested Loops 

Using nested loops to get the sheriff to dance 

A nested loop 

index index_#2 

Another scenario: This time the Sheriff has learnt another part of the dance routine. He does the previous 

routine (stepLeft, stepLeft, stepRight, stepRight) and repeats this four times; but now, after each repetition, he 

makes a quarter turn to the left. 

All we need to do this time, is add the add the method to turn the Sheriff after the loop. 

0 

. 0 

. 1 

1 

. 0 

. 1 

2 

. 0 

. 1 


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Nested Loops 

We could make the Sheriff repeat this dance a number of times, by adding yet another loop. 

Analysing a Nested Loop 

The Sheriff makes a quarter turn after each loop 

Repeating the Sheriff's dance by adding another outer loop 

We can test this for ourselves, with a loop structured like so: 

Nested loops to a depth of three 

If we examine the index variables of this loop at each stage, the table of values looks like: 

index index_#2 index_#3 

0 

. 0 

. . 0 

. . 1 

. 1 

. . 0 

. . 1 

1 

. 0 

. . 0 


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Nested Loops 

In this nested loop: 

the outer loop will repeat 2 times (the value of index will start at 0 and go to 1). 

Each time through the outer loop, the middle loop will repeat two times (the value of index_#2 will start at 

0 and go to 1). 

Each time through the middle loop, the inner loop will repeat two times (The value of index_#3 will start at 

0 and go to 1). 

. . 1 

. 1 

. . 0 

. . 1 



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Tutorial 5 



T utorial Exercises 

Aim: To investigate while loops, nested loops and method parameters. 

Questions 

Tutorial Exercises | Up 


You are encouraged to discuss these questions and share useful links you've found with others in the 

discussion forum. 

Q1. What is going to be the result of running the following while loop? What other loop is this 

equivalent to this in behaviour? 

Q2. Consider the following loop. What will the value of index be each time through the loop? The 

loop will operate 4 times. The first time, index will be 0. The second time, index will be 1, and so on. 

What path will the penguin take? 

Q3. There is an outer loop (4 times) and an inner loop (3 times). It might help to think of the loops 

as wheels. We have a wheel inside a wheel. For every 1 turn of the outer wheel, the inner wheel will 

turn 3 times. 

The expanded version of the loops are shown below: 

file://Z:\contribute\cpt120\chapter\05\tutorial.html 

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Tutorial 5 

Keep track of index and index_2. What path the penguin will take this time? 

Q4. Consider the following loops. What will the penguin do? 



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Practicals 5 




Practical Exercises | Up 


Implement the following scenarios (that include loops and the World.random function) in your 

program: 

Q1 Scenario: A penguin is rapidly running in the shape of a square, ten times. This makes 

the penguin very dizzy. It abruptly staggers 0.25 meters by a random degree to the left or 

right five times, then falls face first into the snow. 

Q2 Scenario: Using the file ComboLock.a2w create four class methods - leftOne, rightOne, 

leftRev and RightRev - that turn the combination lock dial one number left, one number right, 

one revoultion left and one revolution right, respectively. Then create a method called open 

that opens the lock, and another, close, that closes the lock. 

Hint: one position on the dial is 1/40th of a revoultion. 

Finally, use loops and the above methods to implement a combination of 10 left, 41 right, 16 

left, then open the lock. Wait 5 seconds and close it again. 


file://Z:\contribute\cpt120\chapter\05\practical.html 

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Chapter 6: Method Parameters and Modelling 


Chapter 6: Method Parameters and Model l ing 

At the completion of this weeks study, students will have: 

Knowledge of what method parameters are, and what they may contain 

The ability to create methods with input parameters 

An awareness of what generic methods are, and how to create them. 

Knowledge of how to use modelling to design a program 




Method Parameters 

Modelling 





Method Parameters and Modelling | Up 

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Chapter 6: Method Parameters 


Chapter 6: Method Parameters a nd Mode lling 

Me t h od Parameters 

Method Parameters | Up 


Up to now, you have used many methods to do various actions. With the Cow and Snake scenarios 

you have used move, turn and other methods in your programs. These methods allow us to pass 

information (or objects) to them. 

Input parameters tell a method information such as (for a move method) which object to move, how 

far to move it, or what percentage of a circle to turn an object and in which direction. The move 

object has a number of parameters passed to it, including the object to move, the direction to move 

and how far to move. 

With each of these parameters, a different value or object can be specified. This allows us to carry 

out different actions on the same object, or use a different object with the same method. In the case 

of the move method, we can tell to move the snake one time, and at another time use it for another 

object. 

When we create a method, it may be necessary to pass various pieces of information to it. We can do 

this by defining the required method parameters, along with the required type of each parameter. For 

instance, if we need to pass in an object, the parameter must be defined as an object type. Likewise, 

if we are passing in a number, the corresponding parameter must be befined as a numeric type. 

Defining Method Parameters 

The input parameters to a move method 

With the following scenario, we are able to define method parameters that allow the method to be 

used with any object. 

A Zombie is chasing the Mad Scientist across a field; in the same field is a poor cow. The Zombie 

wants to chase everyone and everything from "his" field. 

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To accomplish this task in the way we know so far, we would need two methods, 

zombie.ChaseScientist, and zombie.ChaseCow. The storyboards for these methods might be: 

zombie.ChaseScientist 

Zombie turn to face Scientist 

while Zombie distance to Scientist > 1 meter 

do in order: 

zombie.ChaseCow 

Zombie move forward 1 meter 

Zombie says, "Get out of my field!!" 

Zombie turn to face Cow 

while Zombie distance to Cow > 1 meter 

do in order: 



In the two storyboards, the only difference is the object that the Zombie is chasing. If we look back 

to the beginning of this week's material, we saw that parameters can enable us to pass information 

into a method. In the move method, we have seen that the first parameter of this method is the 

object we wish to move. Instead of creating two methods [ChaseCow and ChaseScientist], we can 

create a single method, where we can pass the object to chase as a parameter. The storyboard might 

now look like: 

zombie.chaseSomething 

Parameter: Victim 

Zombie turn to face Victim 

The Zombie, with "his" world 

while Zombie distance to Victim > 1 meter 


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do in order 



To use a method using parameters, first create a method as you have done previously in other 

scenarios. Because it is the Zombie who is doing the chasing, the new method will need to be created 

in the Zombie's method window. In the top right corner of the new method is a create new parameter 

button. Click on this button; you be will then asked for what type of parameter you wish to pass into 

the method, and a name for the parameter. 

Click on 'create a new parameter' and select object from the menu 

For this method, we need to pass in an object (the one who is to be chased). Select object type in the 

Create New Parameter window. Once you have done this, each time you include a method (e.g. turn 

to face), the name of the parameter you have previously defined is available for you to select. 

The new parameter is available for you to select 

Page 3 of 5 

Instead of selecting either the Professor or the Cow in the turn to face and distance to methods, this 

time we should select the input parameter we have previously defined (Victim) from the list of 


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objects. Once we have included all the steps from the zombie.ChaseSomething storyboard, The final, 

generic, ChaseSomething method is below: 

In our previous scenario it was always the Zombie that was doing the chasing, and the distance 

moved and what they said was always the same. To make the ChaseSomething method truly generic 

we can add more parameters to be able to change who is doing the chasing, how far they move and 

what they say. In this case, we would need to include three more parameters: 

Who is chasing? (this is an object) 

How far to move? (this is a number) 

What do they say? (this is a string) 

With this new generic method we do not have a specific object that is doing the scenario, we must 

include it in the world's method list. After adding the three parameters, the new scenario for our 

generic ChaseSomething method is then: 

world.ChaseSomething 

Parameters: WhoIsChasing, Victim, HowFar, WhatIsSaid 

WhoIsChasing turn to face Victim 

while whoIsChasing distance to Victim > 1 meter 

do in order: 

The generic chaseSomething method 

WhoIsChasing move forward HowFar 

WhoIsChasing says WhatIsSaid 

Page 4 of 5 

We need to add the three extra parameters and change the existing ChaseSomething method to 

include them. Our final 'generic' ChaseSomething is shown below. When we use this new method the 

program will ask in turn for what information or objects you wish to pass to the ChaseSomething 

method. 


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The final 'generic' method 

It is also possible to use the method we have just created as an event method: 

The ChaseSomething method used in an event 



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Chapter 6: Modeling 


Chapter 6: Method Parameters and Model l ing 

Modelling 

Modelling | Up 


We have seen earlier that objects have properties. For example, our poor Cow has many 

properties including its shape, size and colour. The Cow also has parameters for its position, 

the direction it is facing and whether it is visible. To enable Alice to display a visual 

representation of a cow, all of the information that is required to model a cow is stored. Alice 

makes some of this information available to us in the properties tab. In this tab, we have 

methods that allow us to change the colour of an object, or an objects viability. Alice uses this 

information to model a world and the objects in it (for example, the position and colour of an 

object). In Alice, this model is presented to us visually. 

Part of the skill of programming is to use your imagination to see how a model can be built 

from stored information. When we are given a scenario, we can extract from it the objects we 

require. Then extract the storyboard and the sequence of events necessary to play the 

scenario out. 

Looking Back 

Think carefully about this: everything in the Alice world is 

simulated/modeled from stored information. All Alice really 'sees' about the 

modeled world is the stored information (and the code telling it how to use 

that information to, for example, display an object, and so on.) 

Take, for instance, the Robot-Alien scenario from week 1. We found that the objects in this 

scenario were the Robot-manned craft, a Robot, Rocks, an Alien and the Moon Landscape. We 

then wrote the storyboard for this scenario: 

Alien peeks out from behind rock 

The Robot is surprised 

Robot looks around 

Robot sees the alien 

Robot moves towards Alien 

Alien is frightened and hides. 

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We are presented here with a simple scenario and storyboard. If we use our imagination on 

this scenario, questions like, "How big is the alien?", "What colour are the rocks?", and "How 

does a robot move?", need to be answered and added to the stored information in the 

program. We need to think beyond the scenario, to gather information about the objects in 

the world. This will enable a detailed model of each object in the world, and for the world in 

general to be assembled. 

Once we have this model, we can then start to write code to manipulate it. For instance, in the 

previous scenario one of the lines from the storyboard is, "The robot is surprised". How does a 

robot act when they are surprised? Does their expression change? If so, we need to write code 

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that will make the robot look surprised. In this line then, we need to 'model' a surprised robot. 

Up to now we have been using a visual tool to display and manipulate this stored information. 

Not all programs can (or should) be represented visually. As a consequence of this, you need 

to move away from the visual representations that Alice provides. It's necessary to start 

thinking of what's required to model a scenario, at a more abstract level. 

For instance, what objects (and properties of each) are required to model the following 

scenarios: 

A bank account? 

A theatre booking office? 

A student enrollment program? 

An air traffic control system? 

Modelling 

Let's consider a (simple!) Air Traffic Control system; what objects are required? 

Aircraft 

Runways 

Flight Paths 

What properties do these objects have? 

Aircraft 

Runway 

Registration number 

Flight coordinates 

Scheduled arrival time 

Number 

Length 

Flight Path 

Height 

And for the Student Enrolment program, what objects are required? 

Students 

Subjects 

Courses 

What properties do these objects have? 

Student 

Subject 

Student number 

Name 

Address 

Enrolled course list 

Name 


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Course 

Number 

Lecturer 

Room 

Name 

Number 

Subjects 

Course Leader 

The next stage in modeling a problem is to determine what methods we need to manipulate 

the objects and object parameters. For instance, with our student enrolment system, we most 

likely need methods to generate a unique student number, input the student name and 

address, input the course the student is enrolled in (among many others). 

How Does Modelling Tie In With Programming? 

Programming involves: 

1. Determining which aspects of the task we need to model. 

In the previous two scenarios we first looked at, what items were required to model the 

task? In the Air traffic control system the items may include the aircraft, runways and 

the planes coordinates 

2. Determining what information we require to build the model. 

For each item that forms part of our model we now need to model the required qualities 

of that part. 

3. Determining what code we need to manipulate the information and implement 

our model. 

In the Air traffic control scenario, there may be a requirement to continually update the 

coordinates of each aircraft in the current air space. As a programmer, we must be able 

to design and write code to implement each task in the program. 

4. Implementing and testing the model. 

It is important that at each stage in the program's implementation, that the code be 

tested to make sure there are no bugs, or errors that would cause a problem. 

(Especially with the air traffic control scenario!) 



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Practical 6 


C ha p ter 6: Method Parameters and Modelling 

Pr actical Exercises 



You are encouraged to discuss these questions and share useful information you've found with 

others in the discussion forum. 

Q1. Write a method comboLock.comboturn that turns a combo lock's dial a number of positions 

specified by a parameter. A single position is 1/40 of a revolution. 

Q2. Write a method, leap, that makes a Kangaroo leap a specified height for a specified 

distance. 

Q3. Write a method, levitate, that allows Mana to practice her telekinetic power for levitating 

objects. The method should be called when an object is clicked on (using the event "when 

mouse is clicked on anything do levitate..."). When the method is called, Mana turns to face 

the object and levitates it five times the object's height. The object then falls back to the 

ground. 

Q4. Write a method, patternskate, that makes an ice-skater skate in a specified pattern 

(triangle, square, pentagon, etc.) The pattern is specified by the number of sides the pattern 

has. If less than three sides are specified, the method should only print out an error message. 

The method should also allow you to specify the length (in meters) of a pattern's side. 

Q5. Extend your method from the previous question, so that the ice-skater now does a series 

of complete spins while completing the pattern. For every meter she covers, she does a 

complete spin. 

Q6. You have been asked to write a program that will control the time tabling for your states 

capital city's train system. What information will you need to effectively model this program? 



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Chapter 7: Property and Method Variables 


Chapter 7: Property and M ethod Varia bles 


Have an understanding of the use of Property variables in a program 

Have an understanding of the use of method variables in a program 




Property Variables 

Method Variables 





Property and Method Variables | Up 






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Chapter 7: Property Variables 


Chapter 7: P roperty and Method Variab le s 

Pro perty or Obje ct Variables 

Property Variables | Up 


In last weeks material, we saw that Alice uses stored information to model a world and the objects within; for example, 

to display an object at a certain position, or to define the colour of an object. We saw that Alice uses this stored 

information to model an object and display it in a visual environment. Alice allows us to add further information to 

objects, in the form of additional properties. After adding an additional property to an object, the programmer is also 

required to add code to specify how the extra property is used. 

Scenario: Mana has just discovered that she has an identical twin sister Hana. But they need to have a way for each 

sister to remember their own name when they introduce themselves to other people. 

Mana and her twin Hana, but which is which? 

One way we can solve this problem is to define a new property variable for each of the sisters. A property variable 

allows an object to store unique information about itself. For this property variable, we need to store the sisters name 

as a string. To create a property variable, click on the object in the object window, then the properties tab in the object 

details window. Click on the button at the top of this window labeled create new variable. 

Create a new property for this object. 

The program can now access each sisters name, as shown below. 

file://Z:\contribute\cpt120\chapter\07\property-variables.html 

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Let's have a look at property variables in another scenario. 

A program to access the property variables. 

Page 2 of 6 

Scenario: In a fairground is a carousel that is operated by pulling a lever forward. This is to be done in the program by 

clicking on it with the mouse. 


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The carousel operated by a lever. 

If you wish to follow this scenario on your computer, please download this file: carousel.a2w (right click and use save 

as) 

Note: do not use the carousel from alice in this scenario, as the carousel object in this file has been modified. 

In this scenario, we need to be able to move the lever forward to be able to start the carousel. We can do this by 

creating a method called MoveLever. Then this method can be used in a 'when the mouse is clicked on an object' event 

to start the carousel when the lever is moved forward. 

The method called MoveLever and the moveLever event. 

Page 3 of 6 

The MoveLever method, as we have it, now has a problem: the first time we click on the level, the handle turns 

forward. The next time we click on the lever, the handle moves forward again till finally the lever has turned enough to 

be under the ground. Obviously, we only want the lever to turn forward on the first click, then back to its original 

position on the next click. To do this, we need to know which direction to move the lever; this depends on its current 

position. If the lever is in the forward position, we need to move it backwards; and it should move forwards if in the 

backwards position. 

We learnt last week that objects can store information about themselves; for example their colour, size and visibility. 

This stored information about an object can be changed, for instance the colour of an object can be changed, or the 

visibility property of an object can be changed from true to false. We call these properties variables. We can change or 

vary the value of a variable. In the previous example, we created a myName property of type string for the sisters' 

names. This time, we need a property that will tell us if leverForward is true or false. To store true/false, the variable 


14/04/2008


must be of type boolean. 

Create a property variable of type Boolean 

We now have a variable that can remember if the position of the lever is forward (leverForward = true) or backward 

(leverForward = false). 

The variable to remember the position of the lever 

We can now use this property variable to test if the lever is in the forward or backward position. By placing the variable 

in the test of an if statement (remember: the if test returns a true/false result). The test in our if statement is for 'if 

lever.leverForward' will give us a true or false value. If the if test returns false (meaning the lever is in the backward 

position), the lever is then turned forward, the value of leverForward is changed to true and the carousel animation is 

started. Once the animation has finished we can click on the lever a second time. This time, as levelForward is set to 

true, the if test returns true the lever is then moved backwards and the value of leverForward is reset to false. 

The leverForward variable used in an if statement 

Page 4 of 6 

It may be helpful to think of variables as "boxes" that can hold values of a particular type. In the first example, we 

created a box of type string that could hold the sisters name. In the second example, we had a box of type boolean that 

held a true/false value as to whether the lever was in the forward or backward position. 


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It may be helpful to think of a variable as a box 

There are times where we need to count how many times a certain action has occurred. To do this, we can use a 

property variable of type number. This can be set to a starting value, and incremented each time a certain action or 

event has occurred, as we will see in the following example. 

Scenario: In a fairground is an old carousel that is operated by pulling a lever forward. The carousel is so old it needs to 

be given a 10 minute (10 second) rest after every 5 uses. 

For this scenario, we need to be able to count the number of times the carousel has been used. To do this, we need to 

add a numeric property variable to the lever object. We can then increment this newly created carouselRun variable 

each time the lever is moved to the forward position. This variable will store the number of times the carousel has been 

used. 

Adding a property variable to act as a counter 

Page 5 of 6 

Once the value in the carouselRun variable is greater than 5, the program pauses for the required time and the lever 

cannot be operated till after a delay. Note: I have also included a number of print statements to display the value of the 

carousel counter. 


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The carouselRun counter used to count the number of times the lever is moved forward 



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Chapter 7: Method Variables 


Chapter 7: Property and Method V a r iable s 

M ethod V a riables 

Method Variables | Up 


There may be times when we need to remember some information while code is running within a method. For 

example; the results of a calculation, or intermediate calculations that can be combined to form the final result. 

Sometimes we may need to use the result of a calculation a number of times in the same method. In each of 

these situations, we can use a method variable to store the result and reuse it at a later point in the program. We 

can also access the same method variable many times within a method, this will avoid having to calculate the 

same result each time we require the result of the same calculation. 

A method variable only exists while the code is running within the method in which it has 

been defined. Once the code has finished in a method, all values stored within a method 

variable are lost. 

Think of method variables as "throw away" variables. 

Scenario: Clara the jumping cow is learning the high jump for the Cow Olympics. But she needs someone to 

estimate the height before she will attempt the jump. 

The initial set up for the cow highjump 

Page 1 of 5 

In this scenario we need to get the user's input for the estimated height. We need a method variable to store this 

value, prior to using it in the program. 

file://Z:\contribute\cpt120\chapter\07\method-variables.html 

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Creating a number method variable to store the estimated height 

Because we have stored the user input for estimated height in a method variable, we are now able to access this 

value multiple times within the method. Remember though, once the program has finished, the method variable 

ceases to exist, its stored value is lost, and the data must be reinput the next time this method is called. Below is 

the final cowJump method: 

The cowJump method 

We can also use a method variable to store a random number. Then we are able to use this stored value 

anywhere in in the current method that a numeric variable can legally be used. 


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Note: in the above example, we have had to use multiple nested if statements to test each possibility of the 

random number. Then once the program has found a test that evaluates true, the commands for that particular if 

statement are executed. All other commands are skipped over. 

Tips and Tricks 

Creating and using a method variable to store a random number 

There may be times in a program that you need to swap two variables. A method variable can be used to provide 

temporary storage in this situation. Let's see how this works in a short program. 

Swapping two numbers (frame every 5 seconds) 

In the above animation the variables num1 and num2 are first set to the values 10 and 20 respectively. The next 

step is to copy the value from num1 into the temp variable. Having done this, we can now overwrite the value in 

num1 with the value in num2 (we have the value that is in num1 stored in the temp variable so it won't be lost). 

The last step is to copy the value from the temp variable into the num2 variable. 

Method Variables and Property Variables: An Important Difference 

Page 3 of 5 

In the following example we have created a method called myTest. In this method we have created a method 

variable called testV which is initialized to the value 1. We then change the value in this variable to 2. 


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If we print out the value in the method variable, the value of 2 is printed. Once we have finished the code in 

myTest, the program returns to world.my first method. If we then print out the value in testV the result is 1. 

Although method myTest changed the value in testV from 1 to 2, all changes to testV vanished after the method 

myTest has finished running. This differs from object property variables in that the value in a object property 

variable exists for as long as the object exists. 

Passing Method Variables as Parameters 

Method myTest including a method variable 

The code in method world.my first method 

The code in method world.my first method 

We have seen earlier that we can pass information into methods as input parameters. 

Creating a new parameter 

Page 4 of 5 

These parameters can be objects, or some other simple information (such as numbers or strings). In the following 

example, we have created a variable in world.my first method called num and initialized it to the value of 10. We 

have also created a method called testMethod1 that has a numeric input parameter. In this example we have 

used the method variable num as the input parameter to world.testMethod1. note again the values in the 


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printout, the value of data that gets set to 20 is lost once testMethod1 has ended. 

The two methods and the print out from the previous example 



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Practical 7 


Chapter 7: Property and M ethod Varia bles 




Q1. A horse is in a field containing a crate and a fence. Write two methods jump and runat. 

Method jump takes a number parameter, the height to jump. The jump occurs over 2 meters. 

Method runat makes the horse run at an object (given as a parameter) - the horse runs in 

steps of 1 meter until it is 1.5 meters from the object - then it will jump the object. 

Q2. You've seen in the notes how world method ask user for a number can be used to obtain 

a number from the user, and store it in a variable. Design a method that will get a valid PIN 

number from the user. Valid PIN numbers are numbers in the range 1111 to 9999 inclusive. 

You can assume the user will enter an integer. If the user enters an invalid PIN (e.g. 98), they 

should receive an error message and be told to enter another number. This should be 

repeated until the user enters a valid number. 

Q3. Design a program that will randomly generate a number between 1 and 9 inclusive. The 

program will then play a guessing game with the user until they guess the number. The user 

is asked to enter a number from 1..9. You can assume the user will enter an integer. The 

program should ensure the user number is between 1 and 9 inclusive, giving the user an error 

message until they get it right. The program then tells the user if the guess was too high or 

too low. If the guess was high or low, the user is asked to guess again. The game ends when 

the user guesses the number. 

Q4. Using the file Kangaroo Jump construct a program to instruct the kangaroo to jump from 

the ground to the first crate then onto the second crate. Each jump will require user input for 

height and length of the jump. Your solution must use method parameters and variables. This 

avi movie shows the program running. KangaJump 

Q5. Modify the program in question 4 so that when the kangaroo jumps off the second crate 

after a delay of 5 seconds after landing on it he returns to the original position (the invisible 

cone might be of use in this.) 



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Chapter 8: Functions - Returning a Value 


Chapter 8: Functions - Returning a Value 


Functions - Returning a Value | Up 

Prev | 1 p | Next 

Understand that a function may be used to information about an object or the result of a 

calculation 

Be aware that functions may also have input parameters 

Know how to create and use a function in a program 

Be aware that functions must return information 




Writing and Using Functions 









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Chapter 8: Writing and using Functions 


Chapter 8: F unctions - Re t ur ni n g a Value 

W rit ing and U sing Functions 

Writing and Using Functions | Up 


Functions allow us to obtain information about an object, or the world an object is in. This may perform 

simple functions, such as to return information about the properties of an object. We can also use a function 

to obtain the result of a calculation or to provide us with a true/false choice. We have used functions many 

times in the past weeks, When asking the user for a number, we use the 'ask user for a number' function to 

return a value. We have used 'distance in front of' to obtain a numeric value of the distance between two 

objects. We have also used the math functions from 'world' to test if two values are equal or if one value is 

greater than another. 

Creating New Functions 

Examples of functions with an object and 'world.math' functions 

Alice allows us to create new functions both in an objects function area, and in the worlds function area. In 

an object's details window, within the function tab there is a button labeled 'create new function'. Clicking on 

this button will open a window that allows the details of the new function to be entered. Let's have a look at 

this in practice. 

The create new function button 

Page 1 of 4 

Scenario. A mad scientist is being chased by a zombie; if the zombie gets to within 1 meter of the scientist, 

the scientist will be caught. We need to know if the scientist is in danger of being caught (if the zombie is 

within 3 meters). 

We can translate this problem into a function that will give us a true/false indication of whether the scientist 

is in danger. As we have seen earlier, click on the 'create new function' button and enter the name for the 

new function and the return type. In this case select boolean (as we require a true/false result) and we'll call 

file://Z:\contribute\cpt120\chapter\08\writing-functions.html 

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this function 'is in danger'. 

The create new function dialogue box 

In previous weeks we have used the 'distance to' method to find out how far one object is from another. In 

this instance we need to compare this value with the value inside which the scientist IS in danger. This will 

give us a true/false resul, based on the distance between the scientist and the zombie. 

A function MUST return a value. This can be a number, a boolean (true/false) value, an 

object or other miscellaneous type (including string) 

In the above example we are only able to discover if the scientist is in danger from the zombie. What if 

multiple monsters were chasing the scientist? Fortunately, functions allow us to pass parameters to them. In 

this case we could add a parameter (for a monster) that we can use to check with. 

The monster who is chasing the scientist passed to the function as a parameter 

Page 2 of 4 

Within the function, we can use most control structures (including loops and if statements) that are available 

to us in other areas of code. We are also able to include variables within functions. The following function 

uses a control structure (a loop) and a variable to obtain a valid number in the range 1 to 10. We can see 

the loop test will result in true (continuing the loop) if the number is less than 0, or the number is greater 

than 10. This means that the function will not finish until a number within the proper range is entered. 


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Functions can contain variables, loops etc. 

Functions can also contain more than one return statement. The following example converts between a 

numeric value and its string equivalent for numbers between 1 and 5. Once the test in any of the if 

statements returns true, the function will immediately return the corresponding text value. No lines of code 

will be executed beyond that point - a function cannot return more than one value. 

We can also use multiple return statements in a function. 

Page 3 of 4 

We have used functions in earlier exercises to return a value. Functions that you create are used in the same 


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manner. We are able to use these functions anywhere that a value is required. In this example, we use the 

'numToString' function to provide a countdown prior to the helicopter taking off. The full program is in 

copterTakeOff.a2w (right click and select 'save as') 

In this example, we have a new function that calculates the length of the hypotenuse (diagonal) of a rightangled 

triangle from the two sides passed in as parameters. The return value from this function is then 

displayed within the main program. The full program is contained in Hypotenuse.a2w (right click and select 

'save as') 

Important Things to Remember 

Using the numToString function in a program 

The function to calculate the length of the hypotenuse 

The return statement in a function can only return one piece of information. 

If you require to return more than one piece of information from a function, this cannot be done using return 

values. Use a method with an object type as its parameter, then change the objects property variables 

within the method before returning it. 

Generally, it is considered poor programming for a function to alter any object's property. The only effect a 

function should have on the world around it is to return a value. This value can be numeric, boolean 

(true/false) or any number of other types of information. 



Page 4 of 4 


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Practical 8 


C ha p ter 8: Functi ons - Returning a Val ue 

Pr actical Exercises 

Aim: To investigate functions. 



Q1. A Kangaroo needs to know how high to jump, in order to clear various objects. When 

presented with an object, the Kangaroo wishes to jump 1.5 times the height of the object. 

Design a function with an object parameter (called item). The function will determine how high 

the Kangaroo must jump in order to clear the item. 

Q2. In question 2 of last weeks Practical you were asked to "design a method that will get a 

valid PIN number from the user. Valid PIN numbers are numbers in the range 1111 to 9999 

inclusive. You can assume the user will enter an integer. If the user enters an invalid PIN (e.g. 

98), they should receive an error message and be told to enter another number. This should be 

repeated until the user enters a valid number." Alter your solution so it is now implemented by 

a function. The function returns the valid PIN number to World.myfirstmethod, where it is then 

printed. 

Q3. A hungry rabbit is asking for the user's help in selecting objects to eat. The user nominates 

an item for the rabbit to eat by clicking on the item. Design a function that will return whether 

the rabbit should eat the item or not (the item is passed to the function - you only need to 

design the function, not the entire program). Edible objects are daisy, purpleFlower, redFlower, 

and flower. 

Q4. A magic square is a n *n sized grid of integers, where each row, column and the two 

diagonals all add up to the same number. For example, the following is a magic square: 

2 7 6 

9 5 1 

4 3 8 

Write a Boolean (i.e. one that returns true or false) function to determine if a 3 * 3 grid is a 

magic square. The grid is made of number variables (numpos0, numpos1..numpos8) - you can 

assume these variables are World properties. 



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Chapter 9: Comparing Alice to Java 


Chapter 9: Comparing Alic e to Java 


Be aware of many programming languages 

Understand the differences between Alice and Java environments 

Be able to get input from the user using Java 

Be able to use if statements within a java program 




Other Programming Languages 

Alice and Java 

Reading Input 





There are no additional materials available at this time. 



Page 1 of 1 

Comparing Alice to Java | Up 

Prev | 1 2 3 t p | Next 


28/04/2008

Chapter 9: Alice and Java 


Chapter 9: Comp a ring Alice to Java 

Alice and Java 

Alice and Java | Up 


Up to now you have been using Alice to learn the underlying concepts of computer programming. These 

fundamental programming building blocks (variables, if statements and loops) can be applied to almost any 

other programming language. In the next few weeks we will apply these concepts to a commercial 

programming language known as Java. With Alice, you have been able to see the visual result of your code; 

for instance, an object moving, or the visibility of an object changing. When you are programming Java this 

is not the case; you will have to use your imagination to see how variables and code produce the final 

program. 

With Alice you have been using 'drag and drop' statements to build a program with very little chance of 

getting syntax errors. This is not the case with Java, as the program code is written in an editor. As a result 

it is very easy to create illegal statements (syntax error). 

Objects in Alice and Java 

Both Alice and Java are object oriented languages. In Java there are the familiar concepts you have seen in 

Alice; methods, parameters, variables, and so on. In Alice we saw that all information regarding an object is 

stored in that particular object. We can think of this as a blueprint or plan of that object. 

The builder of a house works from a blueprint 

Page 1 of 6 

A builder of a house works from a blueprint that provides information about exactly how it will look. The 

blueprint gives details of sizes, materials, colours and other 'parameters' that go together to form the final 

'object' (the house). In Alice and Java this 'blueprint' is called a class. 

file://Z:\contribute\cpt120\chapter\09\alice-and-java.html 

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All the programs you have written in Alice have had at least one object; the world object. This is the same 

with Java programs, they must also have a starting object (called a class). Within the class, the program 

enables us to develop the blueprint for that class by defining object properties and methods to manipulate 

them. We can name this starting object anything we want (not just world as it is in Alice). 

Let's take the example of the standard "Hello World" program. We first need a class, which we could call 

HelloWriter. 

The first line allows the program to use the java methods it requires. In this case we are only printing to the 

screen, so the only java methods we require are the input/output methods. When saving this file it must be 

given the same name as the class - HelloWriter.java. 

MyFirstMethod Equivalent 

An object's blueprint in Alice and Java is called a class 

import java.io.*; 

// Tells Java where to find details about how to print to the screen 

public class HelloWriter 

{ 

// Methods for HelloWriter are described here 

} 

The // at the start of a line tells Java that it is a comment, and should be ignored by 

the computer. 

In previous weeks when you have run programs written in Alice they have usually started running at 

world.myFirstMethod. As with Alice, Java programs must have a starting point. In Java, this is called main(). 


public class HelloWriter 

{ 

public static void main(String[] args) 

{ 

System.out.println("Hello World"); 

} 

} 


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Object Property Variables 

In Alice we have added property variables to objects to enable us to store additional information about an 

object. Java allows us to create property variables within a class. As with Alice, these property variables can 

be of any variable type that is available to us, for example string or integer. 

In the previous example, a class called NumberWriter has been defined which includes a property variable 

called age. 

In the above program the line static int age requests a variable of type integer called age. The line 

System.out.print() prints the required text to the screen. Note: the two uses of System.out.print() 

(prints without the new line character at the end of the line), and System.out.println() (includes a new 

line at the end of the line). 

Method Variables 

The equivalent program in Alice 

Alice Java 

Class name world HelloWriter 

Starting point myFirstMethod main() 


public class NumberWriter 

{ 

//property variables go here 

static int age; 


{ 

age = 21; 

System.out.print("Property variable age is "); 

System.out.println(age); 

} 

} 


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public class VarDemo 

} 


{ 

// a method variable inside the main method 

int num; 

num = 2; 

System.out.println(num); 

} 

} 

In week 7 we saw that method variables can be used in methods as a temporary storage (remember: their 

value is lost once the method has finished running). In the above programs a method variable called num has 

been defiined and set to 2; the variable is then printed to the screen. 

IF statements in Alice and Java 

Instruction in Alice instruction in java Meaning 

if(num > 1) if the value in variable num is greater than 1 

if(num >= 1) if the value in variable num is greater than or equal to 1 

if(num < 1) if the value in variable num is less than 1 

if(num


When the program requires an if..else statement, two sets of braces must be used, one around each block of 

code. 

Alice and Java AND test 

Both AND tests and OR tests are slightly different in Alice and in Java, as the following two examples 

demonstrate. 

AND 

if (numA > numB) 

{ 

System.out.println("In this method"); 

System.out.println("numA is greater than numB"); 

} 

An if..else statement in Alice 

if(numA > numB) 

{ 


System.out.println("numA is greater than numB"); 

} 

else 

{ 


System.out.println("numA is less than or equal to numB"); 

} 

By placing a block of code inside braces, we are telling Java that a particular 'block' of 

code belongs to either the true or false part of the if statement. 

if(numA >= 1 && numA


Note the two ampersands (&&): this is the code for AND in Java 

OR 

{ 

} 

System.out.println("in this method"); 

System.out.println("the value of numA is between 1 and 10"); 

An OR test in Alice 

if(numA < 1 || numA > 10 ) 

{ 


System.out.println("The value of numA is NOT between 1 and 10"); 

} 

In Java, the code for OR is '||' (two pipe characters; shift-backslash on most keyboards) 



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Chapter 9: Reading Input 


Chapter 9: Compari ng Alice t o J ava 

Readin g in p ut 

Reading Input | Up 


In this subject you are not expected to write methods to get input from the user. A file called 

ConsoleReader.java has been provided for that purpose. Download the file here ConsoleReader.java 

(Right click and save as) keep this file in a safe place, you will use it in many of the java programs. 

Console reader contains 2 methods, readLine() which reads a line of text and the method we will be 

using readInt() which reads an integer (a number) from the keyboard. If you need to get user input 

within a program you will need to place ConsoleReader.java in the same directory as your java program 

file. You will also need to have created a ConsoleReader object in your program to handle input from 

the keyboard. (more on that later) 

In week 7 we had an example of "Clara the jumping cow" where the user was asked for the height of 

the bar. let's have a look at that again but this time we'll just print the value the user has input. 

ConsoleReader console = new ConsoleReader(); 

Here we are asking for a ConsoleReader object which is contained in the ConsoleReader.java file. Once 

we have created the ConsoleReader object we are then able to use the methods within the object. This 

is very similar to Alice, in that we first had to place an object in the world before we were able to use 

the methods contained in that object. 

console.readInt() 

Reading user input in Alice 


public class GetHeight 

{ 

public static void main (String[] args) 

{ 

int height; 


System.out.print("How high is the bar? : "); 

height = console.readInt(); 

System.out.println("The value you entered was " + height); 

} 

} 

file://Z:\contribute\cpt120\chapter\09\reading-input.html 

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Chapter 9: Reading Input 

Here we are using a method contained in the ConsoleReader object. readInt(), reads an number from 

the user and in this case stores it in variable height. 

You will need to include ConsoleReader.java in the same directory as your Java 

program, in order for this to work correctly. 


file://Z:\contribute\cpt120\chapter\09\reading-input.html 

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Tutorial 9 


Chapte r 9: Com p aring Alice to Java 


Installing Eclipse and the Java Runtime Environment (JRE) 

Downloading and installing the Java Runtime Environment 

Tutorial Exercises | Up 


The following instructions are for Windows. For any other operating system please consult the system manual 

for directions of how to install software. 

For the rest of the course we will be also writing programs in Java. For this you will need to download the Java 

runtime environment. This is available from the sun download page at 

http://www.java.com/en/download/manual.jsp and click on the appropriate version for your operating 

system. 

Click on the appropriate JRE for your operating system 

Page 1 of 5 

A box will ask you if you want to run or save the file. Select save. Another box will popup and ask you where 

to save the file. Select a location where you will be able to find the file later and click on OK. 


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Tutorial 9 

Find the file you have previously downloaded, double click on it and follow the installation instructions. 

Downloading and installing Eclipse 

Click on save 

Once you have downloaded and installed the JRE you can download and install Eclipse. This is available for 

download from http://www.eclipse.org/downloads/, click on the appropriate version for your operating 

system. Remember to select 'Eclipse IDE for Java Developers'. 

The Eclipse download page 

Page 2 of 5 

A page will open asking you where you want to download eclipse from (usually the selected site is the closest 

to your location) click on the selected download location. 


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Tutorial 9 

A box will then popup and ask you if you want to run or save the file. Select Save. Another box will popup and 

ask you where to save the file. Select a location where you will be able to find the file later and click on OK. 

For students using any other operating system than Windows, please refer to the system manuals for how to 

install software. 

Extracting the Eclipse Archive 

Select the location to download from 

Click on save 

Page 3 of 5 

The copy of Eclipse that you have downloaded comes as a zip file. Students that are using windows XP or later 

have a zip extraction program as part of Windows, and the following procedure should be close to what you 

will need to follow. For students that do not have a zip extractor built in to Windows either download 

FreeByteZip (right click and select save) or go to their download page at http://www.freebyte.com/fbzip/. 

Start FBZip by double clicking on the file. You will be asked to 'Choose one of the following actions' select - 

Open an existing archive. 


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Tutorial 9 

Find the copy of eclipse you have previously downloaded, click on it and select open. 

Click on the extract icon. 

Select open an existing archive 

Find the copy of Eclipse, select it and click on open 


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Tutorial 9 

Click on the extract icon 

A popup will ask where you want the files to be extracted to, select C drive and make sure that 'Extract all 

files' and 'Use full path of files' is selected. Click on ok to extract the files from the Eclipse file. 

Extract Eclipse from the zip file 

You will now need to create a shortcut to the program. Open my computer and locate the directory with the 

files you have just extracted (if you have followed this setup they will be in C:\eclipse) double click on the 

Eclipse directory to open it. in this directory will be a file called eclipse.exe (This file will have a round purplish 

icon). Right click on the file and select 'create shortcut'. Click and drag the shortcut to the desktop. 



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Practical 9 


Cha pt er 9: Compa rin g Al ice to Java 

Pr ac ti ca l Ex er cises 

Starting Eclipse 



Double click on the Eclipse icon you previously created. The first screen you see will ask you to create or select a workspace. Click on OK. This will 

create a workspace in MyDocuments for your work. 

Create a workspace for your Java programs 

Once you have created your workspace the Eclipse IDE will startup and, if this is the first time you have used eclipse a welcome screen will be 

displayed. 

The Eclipse welcome screen 

If the welcome screen is displayed, click on the Workbench icon on the right of the screen. 

Page 1 of 7 

Once the workbench screen is displayed you will be presented with the default workbench (shown below). Before you can use this screen you will 

need to have created a new project. 


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Practical 9 

Creating a New Project in Eclipse 

A project is an area where you can group programs together. For instance - All Java programs you write in this subject can be grouped under the 

same project. Firstly click on file on the menu bar at the top of the screen. select new and then select project. 

You will need to create a new Java project, this is under the Java tab. 

The default Workbench screen 

Select, file - new - project 


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Practical 9 

Select Java project 

You will then be taken to the Java project wizard. For this subject you will only need to give the project a name (I suggest CPT120_exercises). 

Once you have done that click on the 'Finish' button. 

The Java project wizard 

Page 3 of 7 

Once you have created the new project it will be added to the 'Package explorer' window on the workbench. You will then be able to add new 

classes to the project. 


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Practical 9 

Define a New Class and Add it to the Explorer Window 

The workbench with the new project added to the explorer window 

Creating a new class is similar to the task you have just done. For a new class once again click on file on the menu bar and select new. This time 

select class. 

Select, file - new - class 

Page 4 of 7 

Once you have selected new class you will be taken to the new class wizard. This wizard allows you to customize the new class to your 

requirements. For this subject we need to give the class a name (for your first Java program name the class hello) also select the main method 

stub. Once you have done this click on the finish button. (Ignore the warning about the default package) 


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Practical 9 

Once you have completed this the workbench will look as below. 

Your first Java program "Hello World" 

The create new class Wizard 

the workbench set up with the new project and the new class 

In the previous section you created a new class called 'hello'. Go to the main method block and type in the line System.out.println("Hello 

World !!!") - for now do not put the ';' on the end of the line. 


Page 5 of 7 

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Practical 9 

The "Hello World" program with errors 

Now click on the save icon or click on file - save, note that the syntax checker has highlighted the errors as shown below. You can see a 

description of the errors by placing the mouse over the error flag in the editor window 

An error in the program found by the syntax checker 

Page 6 of 7 

Now go to the line you previously entered and type the ';' on the end and save the file again. If no errors are reported we can then run the 'Hello 

class. Click on the icon with the white arrow on a green background. The output from the program will be displayed in the console area at the 

bottom of the screen. 


28/04/2008

Practical 9 



Page 7 of 7 


28/04/2008

Chapter 10: Java Loops and Methods 




Understand for and while loops in Java 

Understand the use of methods in Java 

Understand the use of method parameters in Java 

Understand the common mistakes that can occur in Java 




Looping in Alice and Java 

Java Examples 







Page 1 of 1 

Up | Site Map 

Up | Site Map 

5/05/2008

Chapter 10: Java For Loops 



Looping in Alice and Java 

The For Loop 

Looping in Alice and Java | Up 


In week 4 you were introduced to loops. We found if we needed to repeat a section of code a 

fixed number of times, we could use a for loop to accomplish this task. In Java, a for loop has 

the same sections as one in Alice. That is we have a initialisation section, the test section and 

the increment. In this example, the same code is displayed in both Alice and Java. 

The output from both of these programs is shown below, the full Java code can be downloaded 

here forLoop.Java (right click and save as) 

The While Loop 

A simple for loop in Alice 

int index; 

for(index = 0; index < 5; index = index + 1) 

{ 

System.out.println("In the loop - The value of index is " + index); 

} 

The output of both of the above programs (shown in Alice) 

Page 1 of 6 

The syntax of while loops in Java is also very similar to those in Alice. The code below shows 

the same while loop implemented in the two languages. And like the previous example, the 

while loop in Java is configured with the same sections as in Alice. 

file://Z:\contribute\cpt120\chapter\10\for-loops.html 

5/05/2008


The output from both of these programs is shown below (Alice output shown). The full Java 

program can be downloaded here WhileLoop.Java (right click and save as) 

Metho ds in Alice and Java 

A while loop in Alice 

numA = 0; 

while ( numA < 5 ) 

{ 

System.out.println("In the loop - The value of numA is " + numA); 

numA = numA + 1; 

} 

The output from the while loop example (Alice output shown) 

In the material for week 2, we looked at object methods. We used methods in Alice to 

manipulate the properties of an object. We say in Alice that methods may have input 

parameters. Let's first compare a method without parameters in both Alice and Java. 

A method without parameters in Alice 


Page 2 of 6 

5/05/2008


import Java.io.*; 

public class SayHi() 

{ 

public static void sayHi() 

{ 

/* \" prints the " character */ 

System.out.println("In method sayHi - \"Hi\""); 

} 

} 


{ 

sayHi(); 

} 

In Java, a method is called in the same way as we would call a method in Alice. We can see in 

the Java main, the text sayHi() has been used to call the method called sayHi(). The code 

for this example can be downloaded here SayHi.Java (right click and save as) 

We can also write methods with input parameter(s) in the following example: 

SayHi_param.Java 

Page 3 of 6 

We pass into the method the number of times the loop is to run. Of note in this example is the 

placement of the increment line. In previous while loops, this line has been at the end of the 

loop. In this example the increment is at the start; this means that in the last iteration of the 

loop, the value of count will be incremented to equal numTimes. 


5/05/2008


An Alice method with an input parameter 

public static void printHello(int(numTimes) 

{ 

int count; 

} 

count == 0; 

while ( count < numTimes ) 

{ 

count = count + 1; 

if( count == numTimes ) 

{ 

System.out.println("The while loop has finished"); 

} 

else 

{ 

System.out.println("In method printHello - \"Hello\" " + count); 

} 

} 

Notice in the above Java example, the brackets around the while loop this 

means, "do all this within these brackets." Notice also the indentation of 

the brackets and code, this makes the code much easier to read and allows 

us to know exactly which statements belong to each part of the loop or if 

statement. 


Page 4 of 6 

5/05/2008


The output of this method is shown below, from Alice. The full Java code for this method is 

downloadable here SayHi_Param.Java. Note that because the increment is at the start of the 

method, the first print will output the value 1. 

Alice and Java Functions 

The output from sayHi_param.Java (from Alice) 

Next we will compare functions. We saw in week 2 that functions return a value to the 

program that it was called from. In Alice we used functions to get information about an object 

which might have been the objects height or the distance from another object. 

In Java we can use functions to do many tasks for us. for instance as we will see below, to 

calculate the area of a square. The following function is shown in both Alice and Java code. 

The square area function in Alice 

public static int squareArea(int sideLength) 

{ 

int sideSquared; 


Page 5 of 6 

5/05/2008


} 

sideSquared = sideLength * sideLength; 

return(sideSquared); 

The output of this method is shown below (from Alice). The full Java code for this method is 

downloadable here SquareArea.Java 

Co mmon Mistakes 

A common mistake in Java is to place the semicolon at the end of the beginning of an for loop, 

for example 

for ( count = 0; count < 5; count = count + 1 ); 

{ 

System.out.println("count = " + count); 

} 

The semicolon indicates the end of a statement. As a result of this the print startment is not 

included within the for loop. This mistake can also happen with while loops. 



Page 6 of 6 


5/05/2008

Chapter 10: Java Examples 


Chapter 10: Java L oops 

Ex a mples 

Java Examples | Up 


Scenario: A customer of yours at Square Deal Motors wants a program that will print a square block of stars at the top 

of each page. Each block must be 5 stars wide and 5 stars high. 

We could solve this by writing a method to print a line of stars the required number wide. The code for this could be: 

(download here print5stars.java) 

public class print5stars 

{ 

public static void printStars() 

{ 

System.out.println("******"); 

} 

} 

public static void main(String[] args) { 

printStars(); 

} 

Another way we could print a line of stars is to print one star within a loop. Because we know how many stars we need 

to print a for loop is most appropriate for this task. (Download here print5stars_loop.java ) 

public class print5stars_loop 

{ 

public static void print5StarsLoop() 

{ 

int count; 

for(count = 0; count < 5; count = count + 1) 

{ 

/* prints the line of stars - Note print, not println */ 

System.out.print("*"); 

} 

/* prints a end of line character */ 

System.out.println(); 

} 

} 


{ 

print5StarsLoop(); 

} 

Now that we have printed one line of stars, we need to repeat this line the required number of times. We can 

accomplish this by putting the previous for loop inside another for loop. Note that in this case we need two loop 

variables; one for the inner loop (this variable counts the number of stars printed per line), and one for the outer loop 

(which counts the number of lines printed). (Download the code here printBox.java ) 

public class printBox 

{ 

public static void print_box() 

{ 

int count_1; /* loop variable for the outer loop */ 

int count_2; /* loop variable for the inner loop */ 

for ( count_1 = 0; count_1 < 5; count_1 = count_1 + 1) 

{ 

for ( count_2 = 0; count_2 < 5; count_2 = count_2 + 1) 

{ 

/* prints the line of stars - Note print, not println */ 


} 

/* prints a end of line character */ 


file://Z:\contribute\cpt120\chapter\10\examples.html 

Page 1 of 5 

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} 

} 

Adding Configurability 

Scenario: the customer of yours at square deal motors likes the program. But this time they want to be able to print 

boxes of different sizes. 

We can do this by adding a parameter that sets the width and height of the box to the print_box() method. (download 

printBox_Param.java here) 

Adding Text Input 

} 

public static void main(String[] args) { 

print_box(); 

} 

public class printBox_Param 

{ 

public static void print_box_param(int size) 

{ 

int count_1; 

int count_2; 

for ( count_1 = 0; count_1 < size; count_1 = count_1 + 1) 

{ 

for ( count_2 = 0; count_2 < size; count_2 = count_2 + 1) 

{ 


} 


} 

} 

} 


{ 

print_box_param(8); 

} 

Scenario: Your customer changes their mind again. This time they want to input the size of the box they require. As we 

already have a size parameter in the printBox() method, we can use the ConsoleReader to read in the required size 

from the user. ( printBox_param_GetSize.java ) 

public class printBox_param_GetSize 

{ 

public static void print_box_param_get_size(int size) 

{ 

int count_1; 

int count_2; 

for ( count_1 = 1; count_1


} 

} 

boxSize = console.readInt(); 

print_box_param_get_size(boxSize); 

Adding Special Cases 

Scenario: The customer likes everything so far, but wants to see what a hollow square would look like. 

To only print the stars on the edge rows and columns of the box we must find what the values of count_1 and count_2 

are when these stars are printed. For the rows of stars, the loop starts with a value of 1 and ends when the value of 

count_1 reaches the value in size. So we can say if the value of count_1 is equal to either 1 , or size, then print a row 

of stars. (Note: we must use if( count_1 == 1 || count_1 == size), we cannot just say if(count_1 == 1 || 

size) ). 

Likewise, when we are printing a row of stars the loop starts at 1 and ends when the value of count_2 equals size. The 

final condition when a star would be printed on an edge of the square is then: 

if( count_1 == 1 || count_1 == size || count_2 == 1 || count_2 == size) 

When this is true, we print a "*", else we print a space. (Download HollowBox.java here ) 

public class HollowBox 

{ 

public static void print_box_param_get_size(int size) 

{ 

int count_1; 

int count_2; 

for ( count_1 = 1; count_1


The input must be in the range 0 to 100. Let's first make sure the program will only let the user enter marks within the 

valid range of values. (This is known as validating the input data.) 

We can use a function for this that takes two input parameters; a minimum and a maximum result. In this case, we'll 

use 0 for the minimum and 100 for the maximum. Also note in the program that the input is placed inside a loop that 

does not exit until the user has entered a valid number; also of note, if the user does enter an invalid number, a 

message to tell them of this is displayed and the program then asks for further input. ( ValidNumber.java ) 

public class ValidNumber 

{ 

public static int Get_ValidNumber(int minimum, int maximum) 

{ 

int number; 


} 

/* we need to set number to a value that will be true for the loop test */ 

/* in this case minimum - 1 is less than minimum and gives us a true from the test */ 

number = minimum - 1; 

while ( number < minimum || number > maximum) 

{ 

System.out.print("Input the students result : "); 

number = console.readInt(); 

if( number < minimum || number > maximum) 

{ 

System.out.println("That result is not in the valid range"); 

} 

} 

return(number); 

} 


{ 

int result; 

} 

result = Get_ValidNumber(0, 100); 

System.out.println("The students result you entered was " + result); 

Now that we are sure that the user can only enter valid marks, we can proceed to write code to display the students 

grade. For this we will need to use a number of if and if..else statements to allow the corresponding grade to be 

displayed. ( StudentGrade.java ) 

public class StudentGrade 

{ 

public static int Get_ValidNumber(int minimum, int maximum) 

{ 

int number; 


/* we need to set number to a value that will be true for the loop test */ 

/* in this case minimum - 1 is less than minimum and gives us a true from the test */ 

number = minimum - 1; 

while ( number < minimum || number > maximum) 

{ 

System.out.print("Input the students result : "); 

number = console.readInt(); 

if( number < minimum || number > maximum) 

{ 

System.out.println("That result is not in the valid range"); 

} 

} 

return(number); 

} 


{ 

int result; 

result = Get_ValidNumber(0, 100); 

System.out.println("The students result you entered was " + result); 

System.out.print("This corresponds to a grade of "); 

/* if result is less than or equalto 100 and result greater than or equal to 80 */ 


Page 4 of 5 

5/05/2008


} 

} 

if((result = 80)) 

{ 

System.out.println("High Distinction"); 

} 

else if((result < 80) && (result >= 70)) 

{ 

System.out.println("Distinction"); 

} 


{ 

System.out.println("Credit"); 

} 


{ 

System.out.println("Pass"); 

} 


{ 

System.out.println("Fail"); 

} 



Page 5 of 5 


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Practical 10 




Aim: To investigate Java IF statements. 

public class tax 

{ 


{ 

int num; 


System.out.print("Please enter an integer : "); 

num = console.readInt(); 

System.out.println("The number you entered was : " + num); 

} 

} 



Using the above java program as a template complete the following tasks (you can download 

the code here tax.java you will also need ConsoleReader.java this must be included in the 

same directory as your source code) 

Q1 A customer has asked you to write a tax program. The program should accept an integer 

value (the amount of earnings) and calculate how much income is subject to tax. 

Incomes of less than or equal to $6000 attract no tax. Incomes over $6000 pay tax only on 

the amount over the tax-free threshold. For example, $7000 only attracts tax on $1000. 

Two sample runs of the program are shown below (user input is shown underlined ): 

run 1: 

Please enter your earnings : 4000 

Your income was : 4000 

You pay no tax 

run 2: 

Please enter your earnings : 100000 

Your income was : 100000 

You pay tax on 94000 

Write an algorithm (i.e. storyboard) for producing the above program. Don't forget to 

consider what variables are required. 

Try converting the algorithm into a Java program. 

Page 1 of 2 

Q2 Alter the program so it now asks for the user's age, and only performs the tax calculation 

if the user is between (inclusive) 18 and 65. 

Q3 Write an algorithm (i.e. storyboard) for reading 2 integers, A and B. The algorithm prints 


5/05/2008

Practical 10 

out whether A equals B, or is less than or greater than B. Convert your algorithm into a Java 

program. 



Page 2 of 2 


5/05/2008

Chapter 11: Arrays 




Understand the basics of single dimension arrays 




Arrays in Alice and Java 







Page 1 of 1 

Arrays | Up 



14/05/2008

Chapter 11: Arrays in Alice and Java 


Chapte r 11 : Arrays 

Arr ays i n Alice a nd Ja va 

Arrays in Alice and Java | Up 


Up until now, all the variables we have been using refer to a single thing (or piece of information); for example, one integer, or one 

object. We can change the value in the variable, but at any one time a variable can only hold one value. There may be times when we 

need to refer to groups of the same type, such as a set of numbers, or a collection of objects. If we needed to remember the results of 

a group of students we could have a number variable for each student. 

eg. StudentResults01, StudentResults02, StudentResults03, (and so on) 

This is manageable if the class was small, but what about if there were hundreds (or more) students in the class... 

StudentResults1000? (!) This kind of technique would end up completely unmanageable and impractical. Alice, and most other 

programming languages, have a structured variable called an array. These can be used to store a group of any particular variable type. 

When we are using this group variable (array) we give it a single name; in this case we'll call it StudentResults and define that there 

are 10 students in the class. 

Retrieving Array Values 

When we are accessing variables in an array, we use the array's index. (In this example, we'll refer to students by their index.) The 

indices start at 0 (the first element of the array), and the last is 9 (the final element). 

For example, if we had the results for 3 of our students (student number 3 has a mark of 80, student number 5 has a mark of 55 and 

student number 8 has a mark of 45) we could say "set StudentResults index 3 to 80, StudentResults index 5 to 55, and set 

StudentResults index 8 to 14" 

Arrays in Summary 

An array allows us to conveniently group together a series of variables. 

To access a element within an array we use the array's name and the index of the element you need. 

Arrays have one restriction. All the elements must be of the same type. eg. All numbers, all strings or all objects. The variable 

type within an array cannot be mixed. 

Arr ays i n Alice 

An array's indices always start at 0. 

A collection (array) of 10 number variables 

The indices in the student_results array 

Using the index number to access an element in the array 

Page 1 of 6 

Alice provides us with an ArrayVisualisation tool. This is located in the 'visualizations' objects in the object gallery. Note: the Array 

Visualisation tool can only be used with objects. 

file://Z:\contribute\cpt120\chapter\11\arrays.html 

14/05/2008


The Array Visualisation object 

To use this tool, we first need to add to the world, the objects we wish to visualise. Once we have done thi,s we can add the array 

visualisation tool into the world. Once this is done, the visualisation object will prompt to add objects to the array. 

The array visualisation object will prompt you to add 

objects to the array 

Once you have finished adding the objects to the array visualisation click on 'OK'; the objects you have previously placed in the world 

have now moved to the visualisation object. 

An array of objects using the visualisation tool 

Page 2 of 6 

We can now use the index to access a particular object. For instance, if we needed to make the chicken jump into the air: with the 

chicken at index 1, we would say, "the value at ArrayVisualization[1] move up .5 meter." This would make the object in that element 

(the chicken) move up .5 meter. 


14/05/2008


Accessing Each Element (Traversing) an Array 

Quite often, we need to step through an array and access each element in turn. We have used loops before in Alice to repeat an action. 

In this instance, because we know the number of times we need to repeat the loop, a 'counted loop' is the best choice. We start the 

loop at 0, and continue counting up to, but not including, the number of elements in the array. Because we have three objects in the 

visualisation, the loop will then be "loop index from 0 up to (but not including) 3 incrementing by 1". This will produce the following 

actions. 

Swapping Two Objects in an Array 

In week 7, we looked at swapping two variables about, with the use of a temporary variable to store intermediate data. This technique 

can also be applied to arrays. In the 'visualizations' object list is a variable visualisation tool that allows us to store a single object. 

Using an array of two objects and the variable visualisation tool, we are able to swap -- and watch -- the two objects in the same way 

we did with variables. 

Move the object in ArrayVisualisation [0] to the VariableVisualisation 

Move the object in ArrayVisualisation [1] to ArrayVisualisation [0] 

Move the object in VariableVisualisation to ArrayVisualisation [1], completing the swap 

This is demonstrated in the animation below. 


The value at ArrayVisualization [ 1 ] move up .25 meter 

Using a 'counted loop' to step through the elements of an array 

Page 3 of 6 

14/05/2008


Accessing a Property of an Array Object 

To access a property of an array object (in this example, an object's height), we first need to drop in the height function, such as: 

Once this has been done, we can then drop in the array position that we're interested in. For example, for the second animal in the 

array: 

Iterating Through the Array in Reverse 

There are times when we may need to loop through the array elements in reverse. This can be accomplished by manipulating the loop 

numbers so that when they are used to specify an array element, they will be counting down instead of up. 

For example, if there are three elements in an array, we can use a loop "loop index from 0 up to (but not including) 3 times 

incrementing by 1". We're still not counting down yet, though! To do this, we access element at (2 - index) within the loop. When index 

is 0, 2 - 0 is 2 so we would be accessing element 2. The Alice code for this is shown below. 

Problems With Arrays 

Swapping two objects in an array using a temporary variable 

The first step to access an array object's height 

Accessing the required element in the array 

Looping through an array in reverse 

We must be very careful when using a loop to step through an array, that we don't accidentally run off the end. The loop must only 

access the number of elements that have been created. For example; for a three element array, the limit of the loop must be "up to 

(but not including) 3". This will stop the loop counter before it is trying to access an element that is off the end of the array. The 

example below shows what happens with a three element array when we have set the limit too high. 

The effect of setting the loop limit to high 

This causes a runtime error. (A runtime error is one that will only occur when the program is run.) 


Page 4 of 6 

14/05/2008


Using an Array in an If Statement 

We can use the index value of an array to test an individual element. In the above example, we are using a variable which has been set 

to 2 (the index of the array element we wish to test). We can then use the element variable to test if the height of the animal at that 

index is less than 9. We can also put the if statement in a loop that will test the height of all animals in the array. 

Arr ays i n J av a 

We have seen previously that we can use an array visualisation object in Alice to manipulate the objects within an array. Java also 

allows us to do the same. Within Alice, an array could only contain one type of object. On the other hand, Java allows us to create 

arrays that can contain many different types of objects. 

Note: with Alice, an array can only hold one type of item -- this rule also applies to Java. When we create an array in Java, we first 

need to tell the program what type the items will be; then we need to specify how many items there are. 

We can now add numbers to the array. In Java we use the array index; exactly the same as in Alice. To place the number 411 in the 

second array element we would use: 

Let's have a look at a program in Java that uses an array to add together a series of numbers. 

First, we need to define the array type and name. 

Then we need to specify the number of elements in the array. 

The error message from the previous code 

An array element used in an if statement caption here 

An if statement placed within a loop 

int numGroup[]; /* tells java that the array will contain numbers 

and will be called numGroup */ 

numGroup = new int[3]; /* tells java that the array will have three elements */ 

numGroup[1] = 411; 

As with Alice, arrays in Java start at index 0. 

Two other variables are defined; total (holds the total of the numbers in the array) and index (the loop counter). 

We have seen before how to put numbers into an array; this is done to the three elements of the array. 

The next line total = 0; sets total to a known value before we enter the loop. 

This is important as within the loop we are adding to the value that is already in total (total = total + ...) This is 

saying add a value to total, and place the result back into total. 

Page 5 of 6 

What if total already contained a value before we entered the loop? We would be adding to the value that was previously 

in total, and the final result would be wrong. Within the loop, the loop counter is used to access the value in each array 

element and add this value to the value in total. 


14/05/2008


The Java program can be downloaded here JavaArray.java 

public class JavaArray 

{ 


{ 

int numGroup[]; /* define the array type and name */ 

numGroup = new int[3]; /* define the number of elements in the array */ 

int total; 

int index; 

} 

} 




total = 0; /* total is set to a known start value, because we are adding to it within the loop */ 

for ( index = 0; index < 3; index = index + 1 ) 

{ 

total = total + numGroup[index]; /* using the loop counter to access each element within the array */ 

} 

System.out.println("The total of the numbers in numGroup array is " + total); 



Page 6 of 6 


14/05/2008

Practical 11 




Aim: To investigate arrays in Java. 

Q1. What is the error in the following code fragment? 

int count; 

int data[10]; 

for (count = 0; count

Practical 11 

For example, "Student 0 received a result of 75 this is a Distinction" 



Page 2 of 2 


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Chapter 12: 2D Arrays 


Chapter 12: 2D Arrays 

At the completion of this weeks study students will have 

An understanding of the basics of two dimensional arrays in Java 




2 Dimensional Arrays in Java 







Page 1 of 1 

Up | Site Map 

Up | Site Map 

9/05/2008

Chapter 12: 2D Arrays in Java 


Chapter 12: 2 Di mensional Arrays 

2 Dimensional Arra ys in Java 

Up | Site Map 

In last weeks material we looked at single dimensional arrays. We found that to access a element (room) in a single 

dimensional array we needed to use a single index value. For example StudentResults[5]. We also learnt an important 

rule with all arrays, that is that each element of the array must be of the same type - eg. all ints, or all strings or all 

objects as in Alice. With two dimensional arrays we can think of them as an array with each element being an array in 

itself. In last weeks material the example was of an single dinensional array of 10 studentResults. 

A single dimensional array of 10 students. 

What if our array 'Class room' had not one row of desks but many each row having the same number of 'desks' each 

'desk' having one student. If we had an array 'class room' with 4 'Rows of desks' and each 'row having four desks in it, 

it would look like this : 

A 2 dimensional array of four elements by four elements 

Page 1 of 4 

We have seen before that all elements of an array must be of the same type, let's now assume that the array is to 

store the exam results of each of the students in the 'Class room'. Each element in the array is an integer. 

file://Z:\contribute\cpt120\chapter\12\2d-arrays-java.html 

9/05/2008


As with single dimensional arrays the name of a two dimensional array is the name of the whole array. 

Arrays in Java 

A four by four array of integers 

The name of a two dimensional array is the name of the whole array 

In last weeks examples we used a single index value to access an individual element in the array. With two 

dimensional arrays we need to use two index values one for the row number in the array and another for the column 

number. Say we needed to store the exam results of 16 students but we wanted to store the data simmilar to the 

positions they sat in class. We all ready had the exam results of 3 of the students that we needed to place at the 

position in the array that corresponded to their seating position in class, eg. if the student at desk, row 2 coulmn 1 

had a result of 55 and the student at desk row 0 column 2 had a result of 78 and the student next to them at row 0 

column 3 had the same result. the java code to put the values into the array would be 

public class Array2_1 

{ 


{ 

int StudentResults[][]; // defines a 2 dimensional array of ints called StudentResults 

StudentResults = new int[4][4]; // Sets the size of the array to 4 elements * 4 elements 

} 

} 

StudentResults[2][1] = 55; 



Let's try this again this time inputting the results from the keyboard. In this example there are two loops. the value of 

the outer loop (row) is used to index each row of the array. Likewise the value of the inner loop (column) is used to 

index each column in the array. The first time through the outer loop the value in row will be 0, while the value in row 

remains 0 the inner loop value in column will step from 0 to 3. 

The value in the 

outer (row) loop is 

The value in the inner 

loop (column) is 

The array element we are 

looking at in StudentResults is 

0 0 StudentResults[0][0] 












Page 2 of 4 

9/05/2008








{ 


{ 

int row; 

int column; 

int StudentResults[][]; 

StudentResults = new int[4][4]; 


} 

} 

for (row = 0; row < 4; row = row + 1) // The outer loop indexes each row in the array 

{ 

for ( column = 0; column < 4; column++ ) // inner loop steps through each column 

{ 

System.out.print("Please enter score for student at row " + row); 

System.out.print(" ,column " + column + " : "); 

StudentResults[row][column] = console.readInt(); 

} 

} 

The above example uses nested loops to access each row (this is done by the outer loop) and column (done by the 

inner loop). We can then use the loop values as the index values in the readInt statement, this reads the value from 

the user direct to the StudentResults array at the element indexed by the row and column variables. 

Lets now extend the previous program and print out the results (we'll take it that if a student has a result of zero their 

mark has not been entered and should not be printed). 


{ 


{ 

int row; 

int column; 




Page 3 of 4 

for (row = 0; row < 4; row = row + 1) 

{ 

for ( column = 0; column < 4; column++ ) 

{ 



// we need to enter a valid number here to avoid an error. 


} 

} 


{ 


{ 

if(StudentResults[row][column] > 0) // if a result has been entered 

{ 

System.out.print("The student at row " + row); 

System.out.print(" ,column " + column); 

System.out.println(" recieved a mark of : " + StudentResults[row][column]); 

} 


9/05/2008


} 

} 

} 

} 

We can also use the nested loops to do operations on rows of the array. In this example we get the average mark for 

each row, an also an overall average for the 'Classroom' 


{ 


{ 

int row; 

int column; 

int row_total; 

int overall_total; 




} 

} 

// Input values into the 2 dimensional array 


{ 


{ 



// we need to enter a valid number here to avoid an error. 


} 

} 

System.out.println("Printing the average mark for each row of students : "); 

overall_total = 0; // set overall_total to 0 before starting calculations. 


{ 

System.out.print("Row " + row + " : "); 

row_total = 0; // reset row_total to 0 to start calculations for the new row 


{ 

row_total = row_total + StudentResults[row][column]; 

overall_total = overall_total + StudentResults[row][column]; 

} 

System.out.println(row_total / 4); // divided by the number of students in the row 

} 

System.out.println("The overall average is : " + (overall_total / ( 4 * 4 ))); 



Page 4 of 4 

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9/05/2008

Practical 12 


Chapte r 12 : 2 D Arra ys 

Pra c ti c al E xe rcises 

1. The following code prints out a box of numbers eg. 

1 2 3 

4 5 6 

7 8 9 

public class NumberBox 

{ 


{ 

int row; 

int column; 

int count; 

int NumberBox[][]; 

NumberBox = new int[3][3]; 

} 

} 

// put the numbers into the 2D array 

count = 1; 

for ( row = 0; row < 3; row = row + 1) 

{ 

for ( column = 0; column < 3; column = column + 1) 

{ 

NumberBox[row][column] = count; 

count = count + 1; 

} 

} 

// Print out the reversed box 


{ 


{ 

System.out.print(" " + NumberBox[row][column]); 

} 


} 

// your code goes in here 


{ 


{ 

System.out.print(" " + NumberBox[row][column]); 

} 


} 

your task is to modify the code so that the second time the code is printed the result is swaped from left to right 

3 2 1 

6 5 4 

9 8 7 


Page 1 of 1 

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2. modify the above code so that the second time the array isprinted the output is mirrored left to right and 

top to bottom eg. 

9 8 7 

6 5 4 

3 2 1 


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CPT120 subject material (pdf file) - RMIT University

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