Exploring projects 1

Exploring with MicroWorlds EX

Projects

Exploring With MicroWorlds EX

Projects
by Gary Stager and Susan Einhorn

© Logo Computer Systems Inc., 2003. All rights reserved.
No part of the document contained herein may be reproduced, stored in retrieval systems or transmitted, in any
form or by any means, photocopying, electronic, mechanical, recording or otherwise, without the prior approval
from Logo Computer Systems Inc.
Legal deposit, 1st semester 2003
ISBN 2-89371-528-1
Printed 6-03

MicroWorlds EX is a trademark and

is a registered trademark of Logo Computer Systems Inc.

Table of Contents
Overview...........................................................................3
Section 1: Models and Simulations........................................5
Exploration 1 - Gas Atoms in a Jar .....................................6
Exploration 2 - Environmental Chaos .................................13
Exploration 3 - Of Dice and Math......................................31
Section 2: Videogame Design .............................................49
Game 1 - Snacman .........................................................51
Game 2 – Squish the Turtle: A Web-ready Game ..................67

1

Overview
The MicroWorlds EX Projects Book contains two main sections. The first contains
several projects in which you can investigate ideas in math and science by
creating on-screen simulations and explorations. The second section introduces you
to the world of game-building. Although games are fun and, well, yes, games, they
also involve a great deal of logic and problem-solving. You’ll find that building
games is even better than playing them.
You can do these projects in any order. All the prerequisite skills are introduced in
the MicroWorlds EX tutorials, although a number of those skills are also reviewed in
this book. Before starting this book you should be familiar with:
• MicroWorlds EX objects, such as text boxes and turtles - how to create them
and set them up
• the Painting tools,
• the turtle’s backpack,
• creating animation,
• defining procedures,
• messaging and broadcast,
• randomness (although you learn many more uses for random in this book),
• turtlesown properties, and conditionals.
Simulations and games have many characteristics in common. Randomness plays a
role in almost all the projects included in this book. See if you can find other
ideas and/or techniques that these two types of projects have in common.
Remember, these are starting points for your own personal projects – extend
and/or change them as much as you want. if there’s a technique you’ve forgotten
how to do or that you want to learn, refer to the Techniques panel in the
MicroWorlds EX Command Center.

3

When you work through the book, you’ll notice the following:

N

This icon indicates the description of the overall project plan.

This icon appears when there’s a definition of a MicroWorlds EX primitive or an
explanation of a specific programming technique.

This is a bug box. Explanations found here tell you what to do if something goes
wrong or doesn’t work as you expect.

This icon appears next to a list of ideas that are additional features you may want
to add to your project.

Sometimes the key combinations are different for Windows and Macintosh versions.
In these cases, the Windows key combination appears first, the Macintosh, second;
for example: Right-click/Ctrl-click.

4

Section 1
Models and Simulations
Professionals in all walks of life use computers to create numerical or visual models
of real-world phenomena. These models may be used to illustrate an argument,
make sense of data or predict a result if variables were to change. The computer
offers a low-cost, quick and risk-free (nothing should explode) way of modeling.
The computer’s ability to perform repetitive computational operations quickly,
accurately and without tiring makes it a powerful tool in the sciences and
social sciences. Models in which particular variables may be changed are known
as simulations.
This section presents three examples of the types of models you can construct
with MicroWorlds EX.

5

Exploration 1 Gas Atoms in a Jar

This project uses multiple turtles to represent atoms in a jar. The behavior of the
simulation changes based on the number of atoms you place in the jar. Atoms
move randomly and accelerate whenever they bounce into the walls of the jar or
other atoms. The atom escapes this chaos when it hits the open mouth of the jar.
For the sake of simplicity, new atoms will be created and added to the jar by
copying and pasting an existing atom (turtle). There are more complex ways in
which we could build an interface allowing a user to click a button and have a
new atom introduced into the experiment.
In this project you:
N

• Draw the jar
• Create atoms
• Set the atoms’ rules
• Run the experiment

6

Getting Started
Start by creating a new project. Name the project and remember to save often.
Use the Painting tools to paint a jar that looks like the following. Use thick black
lines for the walls of the jar and pink for the opening at the top of the jar.

Hatch a new turtle
• In the turtle’s Shapes Tab, create a small ball shape and set the turtle to this
shape. The color and size are up to you, but keep it simple.

7

The speed variable
In this experimental model, speed is one of the dependent variables. You need to
have a way to set up the starting speed any time you run your model.
• Each atom should have a property called speed. To create this property,
type the following in the Command Center:

turtlesown “speed
• Type the following in the Command Center:

setspeed 0.1
This sets the initial value for the current turtle-atom’s speed.

Collisions
When the turtle-atom collides with the side of the jar or another turtle, it should
bounce and accelerate a little. That means it needs a bounce procedure.
Since all the atoms will use the exact same procedure, you can write it in the
project Procedures Tab.

to bounce
bk 20 rt random 360
setspeed speed + 0.1
end
Speed reports the current (pre-bounce) value of speed.
This procedure causes the turtle to back up, turn randomly and increase its speed
slightly. Note – use .1 as the increase in the speed so that the simulation doesn’t
come to a boil too quickly!

8

Now, give the turtle its rules. Open the turtle’s backpack and click on the Rules Tab
• Type the following in the OnClick instruction field:

fd speed
Set it to Forever mode. This rule lets you start the atom by clicking on it
• Type the following in the OnColor instruction field for black:

bounce
Leave it set to Once mode. This rule tells the turtle to bounce and accelerate
when it touches the wall’s of the jar.
• Type the following in the OnTouching instruction field:

bounce
Leave it set to Once mode. This rule tells the turtle to bounce and accelerate
when it collides with another turtle.
• While in this tab, type the following in the OnColor instruction field for pink:

clickoff
Leave it set to Once mode. This rule tells the turtle to stop when it touches the
pink mouth of the jar.

A color that has a dot in its middle has instructions associated with it.

9

Testing the experiment
• Put the atom in the jar
• Click on the atom to set it in motion
• Observe its behavior
Does the model work the way you had planned? If yes, it’s time to add more atoms.

Create more atoms
In the Command Center, type:

clone turtle-name
Use the name of your atom-turtle in place of turtle-name
(for example, clone “t1).
To re-start the experiment, type the following in the Command Center:

everyone [clickon]

10

Reset your experiment automatically
In order to set your experiment back to its initial state again, it’s a good idea to
write a reset procedure. Since this sets the whole project back to the initial state,
and not just one turtle, define the procedure in the project Procedures Tab.

to reset
everyone [setspeed 0.1 setpos [0 0]]
end
Note that when you run the reset procedure, the turtles do not stay at position
[0 0]. That’s because they all arrive at the same point and then collide with each
other. They react to the collision at the [0 0] position by moving away. You may
want to create a reset button as well as part of your user interface.

Debugging - Escaping atoms
Sometimes an atom escapes the jar. This happens because speed, the input to
forward, is larger than the thickness of the walls of the jar. If this happens, you
could make the walls of the jar thicker or modify the bounce procedure to limit the
speed increase:

to bounce
if speed < 1 [setspeed speed + .1]
bk 20 rt random 360
end
This means that if speed is less than 1, the speed will be increased a little.
This way the atom won’t “jump” over the wall.

11

Challenges:
• Can you create a way of displaying the “temperature” of the atoms in the jar?
• What happens if the jar is smaller, larger or has a smaller/larger opening? Does
this change the behavior of the atoms?
• Freeze the background of the page in order to preserve the jar with
freezebg. Then set the turtles’ pen colors to any unused colors like yellow
and put their pens down and observe the paths of the atoms. After a certain
amount of time, clean up the trails by using the command clean, in the
Command Center. You may want to add a clean button to make the process easier.
• What if one atom is bigger than others?
• Feel free to adjust the initial speed of the atoms if you are impatient.
• When you experiment with several atoms, something strange starts to happen.
The atoms stuck on the jar’s mouth are blocking the way and the other atoms
are bouncing off them (collision detection) instead of stopping. Change the
instruction for the color pink, so that when an atom hits pink, it is moved to
the side of the jar, for example: clickoff setpos [300 0] (use your own numbers)
• Mimic a nuclear reaction by “spitting the atoms” when they hit at high speed. If
the speed is greater than a certain number, clone the atom and reset its speed:

clone who setspeed 0.1
• Create different bounce procedures (such as atombounce and wallbounce) for
collision detection and color detection. For example, splitting the atoms or
speed increase could occur only on collision detection while hitting a wall
would only change the heading.
• Import the Stopwatch turtle (in the Turtles folder inside the MicroWorlds EX
folder) and calculate the duration of your experiment with 2 atoms, 3 atoms,
and so on.

12

Exploration 2 Environmental Chaos

In this modeling project, you explore predator-prey relationships. Turtles play three
roles in this simulation – predators, prey and food.
Feel free to enhance and improve on the project ideas suggested in this chapter.
Be creative!
N

• Design a background
• Create the environment’s inhabitants – food, prey, and predators
• Program the background
• Add interaction rules
• Run the simulation

13

Getting Started
Start by creating a new project. Name the project and remember to save often.
In this simulation, there exists:
• food. The food is eaten by
• small fish (the prey). The small fish are eaten by
• larger fish – the predators.
The predators also eat algae – they eat anything, so watch out!

Designing the background
The predators and prey in this simulation all live in and on the banks of a lake.
So, first, set up the environment.
• Fill the screen with brown. Use the Painting tools to draw a large blue lake.
• When you are happy with your lake, type the following in the Command Center:

freezebg
This freezes the background image so you can restore your original image again,
in case you draw something on it that you later want to erase.

14

• Save your project!

Defining the species
Since some turtles will be food, some prey (small fish), and some predators, it would
be a good idea to assign a species property for all turtles to know who is who.
In the Command Center, type:

turtlesown “species

Create the food source
Think of the food as algae. Algae growth is represented by hidden turtles who
move through the lake and at certain intervals stamp their shape on the screen,
leaving food for the other creatures to eat.
Begin by creating one turtle in the role of food. Only after you have one turtle
with all of the desired properties should you then copy/paste 3 or 4 duplicates.

15

Hatch a new turtle and open its backpack. First give it an algae-like shape:
• Click on the Shapes Tab.
• Create a new algae-like shape. Use the spray can and dark green paint to make
a food blob like the one illustrated below

• Click on the turtle to give it the shape.
Now, identify what it is:
• Click on the State Tab. Under the words I Have, double-click on species.
Set the species to food

16

The food needs to “grow” at a specific rate. To have it “grow” on the lake, stamp
the turtle’s shape. Then the turtle moves in some random way to a new location
and stamps again.
• Click on the Procedures Tab
• Type the following procedure:

to grow
stamp
rt random 360
fd random 20
wait 100
end
Next:
• Click on the Rules Tab
• Set the instruction for OnClick to:

grow
• Set it to Forever mode.
This means that food will be produced every 10 seconds.

Controlling the rate of growth a foodrate slider
You can also control the rate at which the turtle stamps its shape by using a
foodrate slider.
• Create a new slider
• Name the slider foodrate

17

• Set the minimum to 1 and the maximum to 20 and set the current value to a
number between 1 and 20.
This slider represents the rate at which new food grows in the lake.
These values were chosen arbitrarily and can be changed later.
Now edit the grow procedure to read:

to grow
stamp
rt random 360
fd random 20
wait 100 / foodrate
end
Important! Important! Important! Do not type a new procedure.
Edit the procedure you already have.
If foodrate = 20, new food appears every 1/2 second. The food “grows” in a
slightly random pattern.
Test your turtle by clicking on it. Does it multiply too quickly or not quickly
enough? If so, adjust the foodrate slider.
Once you’re happy with the turtle:
• Select the food turtle
• Copy the turtle and paste one or two duplicates
• Drag the food turtles you create into different areas of the lake

18

Create the vegetarians
Now it’s time to create the fish that eat the algae. Hatch a new turtle.
• In the Shapes Tab in this new turtle’s backpack, create a friendly fish or
fantasy shape for the turtle to wear, or drag clipart of a fish from the
Painting/Clipart palette.
• Name that shape fish
• Open the State Tab of the turtle’s backpack. Under “I have,” double-click on the
word species. Set the species to “prey. This will differentiate the fish who
eat others and those who are eaten.

19

Moving
The prey-turtle does two things – it moves and it eats.
In order to control the turtle’s speed, create a prey_speed slider. Set the minimum
to 1 and decide on a reasonable maximum speed (10 sounds reasonable). You can
always adjust this limit later.
• Give the turtle the following OnClick instruction in the Rules Tab of its
backpack:

fd prey_speed wait 1
• Set the mode to Forever.
Test your fish. Adjust the prey_speed slider so that the fish is not swimming
too quickly.

20

Eating
How does the prey-turtle eat the food? It stamps over it. Create a new shape for
the prey-turtle. It will use this shape to “eat” food in the lake whenever the
prey-turtle moves over the food. When the prey-turtle detects the color of the
food, it will briefly change shape and stamp over the food with the color of water.
Voila! The food disappears. You can even program the prey-turtle to get bigger
as it eats.
• Create a second shape in the prey-turtle’s Shapes Tab. This shape should be a
round figure in the same color as the lake water. Name that shape, eatstamp
• Type the following procedure in the Procedures Tab of this turtle’s backpack:

to eatfood
setsh “eatstamp
stamp
setsh “fish
if size > 80 [burp]
if size < 20 [hungry]
if size < 100 [setsize size + 1]
end
• In the Rules Tab, set the OnColor instruction for dark green (or whatever color
family you used for your food-turtle) to:

eatfood
Burp and hungry are recorded sounds. Record a sound for each, but keep
in mind that these sound effects should be very short. Burp should remind the
user how gluttonous the fish is. Hungry should indicate that the fish is still
hungry. Make the icons for both sounds invisible.
Test your prey-turtle and your food-turtles. Are they growing, moving, and eating
as you planned?

21

Respecting boundaries
By this time, you probably noticed that the fish are swimming all over the place –
even out of the lake! To control this, program the color brown so that the
prey-turtle bounces when it gets on land..
• Write the following procedures in the project Procedures Tab, since it will apply
to all the fish, both prey and predators:

to bounce
bk 20
rt –30 + random 61
if not species = “food [shrink]
end
to shrink
if size > 5 [setsize size - 1]
end
The bounce procedure causes any non-food turtle (the value for species is not
food) hitting the brown exterior of the lake to back up and then turn either left or
right by up to 30 degrees. Do you understand what the –30 + random 61 is doing
in that instruction? (Note that rt –30 is the same as lt 30.)
If the turtle hitting brown is not food, it is reduced in size by 1 each time it
bumps into the wall. This helps make the predator or prey weaker over time.

22

Right-click/Ctrl-click on the brown area on the screen.
Select Edit brown from the menu.

A dialog box opens. In the turtle instruction field, type:

bounce

Leave it set to Once.
Even it you’re satisfied with how your prey-turtle responds in your simulation, do
not duplicate it yet. Wait until you’ve completed all the programming for the
predator-turtle.

23

Create the meat eaters
Now it’s time to create predator turtles that consume the prey-turtles and algae, too.
Hatch a new turtle. Open the turtle’s backpack.
• In the predator-turtle’s Shapes Tab, create one or more shapes depicting a
menacing creature. Also, copy the eatstamp shape from the prey-turtle and
paste it in the Shapes Tab of the predator-turtle (or create a new eatstamp
shape.)
• In the State Tab, under “I have”, double-click on species and set the
species property value to “predator
• In the Rules Tab, set the predator-turtle’s OnClick instruction to:

fd predator_speed wait 1
Set the mode to Forever.
• Create a new slider, predator_speed, with a minimum of 1 and a
reasonable maximum speed.

Keeping track of bites
The predator behaves very much like the prey accept it can eat the prey. When the
predator touches another turtle, it bites it. The predator grows a little bit when it
eats algae and a lot when it bites a “fish” (a prey-turtle). In this simulation, if a
prey-turtle is bitten more than four times, it dies.
In order to make sure the bites have the proper effect on the prey-turtle, the
prey-turtle needs a way to keep track of how many times it has been bitten.
A good way to do this is to create a special property only for the prey-turtle.
Here are two ways to do this:

24

1) Click on the prey-turtle to make it the current turtle (if you started the
prey-turtle’s animation by mistake, you can stop it by clicking on the Stop All
button). In the Command Center, type:

giveturtle “bites
setbites 0
Look at the State Tab of the prey-turtle; you see the variable and its value.
2) Another way to make the prey-turtle the current turtle is to address it by its
name. For example, if its name is t2, you can type:

t2, giveturtle “bites
setbites 0
(Remember to use the name of your prey-turtle, if it is different. Also, you must
use the comma after the name.)

Giveturtle is just like turtlesown, except it assigns the property to
the current turtle only.

Eating – Part 2, Algae
Open the predator-turtle’s backpack and click on the Procedures Tab. The following
procedure is similar to the eatfood procedure in the prey-turtle’s backpack. Type:

to eatfood
setsh “eatstamp
stamp
setsh “dragon (use the name you gave to your menacing shape)
if size > 80 [burp]
if size < 20 [hungry]
if size < 100 [setsize size + 1]
end

25

In the predator-turtle’s Rules Tab:
• Set the OnColor instruction to:

eatfood

Eating – Part 3, Biting Prey
When the predator touches another turtle, there are several things it must do.
It first must determine the species of the turtle being touched. If it’s prey, then
the predator-turtle bites it. Other turtles (for example, other predator-turtles)
are ignored. It then bites the turtle, triggering an action, a question and
consequences, depending on the results of the question.
The action – the prey-turtle’s bites increase by 1.
The question – how many total bites does the prey-turtle have?
The consequence – if the prey-turtle has 4 bites, it dies.
In order to carry out these three actions, the predator-turtle runs the bite
procedure (Note: Some of the lines in the following procedures are long.
Enlarging the backpack may help if you want to format the long lines).

to bite :victim
ask :victim [setbites bites + 1]
if (ask :victim [bites]) = 4
[announce [You’re toast - and yummy, too]
ask :victim [setbites 0 clickoff ht]]
end
Ask temporarily “talks to” a turtle other than the current turtle, telling it to run
a list of instructions.

26

Ask is used to ask the turtle specified (ask’s first input) to run whatever is in the
second input. If it’s a command, ask acts like a command. For example:

ask :victim [setbites bites + 1]
Setbites is a command, so ask acts as a command.
If the second input is a reporter, ask acts as a reporter.

ask :victim [bites]
Bites is a reporter, so ask acts as a reporter.
The second input must be in square brackets.

Bite is a procedure that takes an input. Just like primitives can take an input
(for example, forward), procedures that you define can also have inputs. The name
of the input is written on the title line when defining bite. You can choose any
name for the input (it could be victim, you, x, or any other word), but it must
always be preceded by a colon. You can then use the input name (preceded by the
colon) wherever you would want to use the value of that input when defining the
procedure.
The title line of bite tells MicroWorlds that the procedure bite has a single
input named victim. Then, in the body of the procedure, :victim is used
as input to one or more commands, in this case, the primitive ask. When the
procedure runs, whatever turtle name is used as input to bite will also be used
as input to ask.

27

Eating – Part 4, Eating Prey
Finally, in order to eat, the predator-turtle runs the bitefish procedure. The bite
procedure is a sub-procedure in bitefish. The predator-turtle checks what kind of
turtle it is touching and, if it is a prey-turtle, it runs the bite procedure.
The predator-turtle also gets bigger.

to bitefish
if (ask touchedturtle [species]) = “prey
[bite touchedturtle
if size < 40
[setsize size + 2]]
end
As you can see, the input to bite is the reporter touchedturtle, which
reports the name of the turtle being touched. The name of the turtle reported by
touchedturtle is used everywhere :victim appears in the bite procedure.
The parentheses help MicroWorlds EX interpret the if statement correctly.
Everything inside the parentheses is evaluated before being transmitted to
the “equal sign”.
In the predator-turtle’s Rules Tab:
• Set the OnTouch instruction to:

bitefish
Test all your components. If you are satisfied with them, duplicate the
predator-turtle and the prey-turtle several times.

28

Add a reset button
As always, having a reset procedure and button comes in very handy if you want to
restart your simulation.
Since this is designed to reset the whole project, type the following procedure in
the project Procedures Tab:

to reset
setprey_speed 1
setpredator_speed 1
everyone [setsize 40]
everyone [st clickon]
end
• Make a button with the instruction reset. Leave it in Once mode.

Run your simulation
• Click the reset button to set up the simulation
• Adjust the speed of the food, predator and prey to see what happens.

29

Challenges:
• Can you program the predator-turtles so that they are animated in more than
one direction? Create shapes facing in the opposite direction and use setrotate.
• Can you program the predator’s bitefish procedure so that it actually
changes the shape of the prey when the prey is bitten? Perhaps a chunk could
be taken out of the prey.
• Can you make the predator eat less, the fatter it gets?
• How might you add toxic waste to the lake?

30

Exploration 3 Of Dice and Math

The theory of probabilities is nothing more that good sense
confirmed by calculation – Laplace, 1796.
If you roll one die, each number (1, 2, 3, 4, 5, and 6) has an equal chance to be
rolled, but if you roll two dice at once, each possible total for a roll (2, 3, 4, 5, 6,
7, 8, 9, 10, 11, 12) does not have the same chance of appearing. You have an
equal chance of throwing a 2 as you do of throwing a 12 (1 chance out of 36),
but what is the probability that you’d throw a 9 or a 7? What happens when you
roll three dice?
You can use MicroWorlds EX to test your hypotheses. One of the benefits of using a
computer to help you is that you can repeat a simulated throw of the dice a huge
number of times quickly and without the computer getting tired.
N

• Design a die
• Roll the dice
• Create a record of all the throws
• Graph your results

31

Getting started
Create a new project.
Start by creating one die. When it is complete, clone it.
The die is really a turtle that is set to different shapes to show different sides of
a die. Right-click/Ctrl-click on the turtle and select Open Backpack.
• Click on the Shapes Tab.
• In the first space, draw a frame shape. Then draw a dot or circle in the center
of the shape to make the 1 side of the die.
• Click OK to save the shape
• Copy the shape and paste it into the second space on the Shapes Tab.
Repeat this process so that the same shape appears in shapes 1-6
• Modify each of the six shapes to give each of them the correct number of dots.
Be sure that the die shapes are in the correct numerical order
(the 1-side is shape #1, the 2-side is shape #2, etc.)

32

Randomly changing shape
Randomness is an important mathematical concept and is used in nearly every
kind of game.
Try this:

setsh random 2
Did the turtle change shape? Try this several times.
You’ll notice that sometimes the turtle is shaped like one of the die shapes and
sometimes like a turtle. You may also notice that you never get a 2. That’s because
random 2 reports a number (from 0 to one less than its input, 2) to setsh.
So, it reports either 0 or 1. Setsh 0 sets the turtle to the turtle shape.
In order to not get the turtle shape, you need a way of getting random to report 1
through 6. Try typing this in the Command Center:

setsh 1 + random 6
This instruction reports a randomly selected number from 1 to 6. It adds one to
the number random chooses, which can be a number from 0 through 5.
Move the cursor back to this line and press Enter/Return again to see if the
number changes. Try:

repeat 10 [setsh 1 + random 6 wait 2]
Does the die look more like a real die rolling now? You may want to change the
input to wait to adjust the timing.

33

Rolling the die
Create a roll procedure to simulate a die roll. Open the turtle’s backpack, if it is
not already open.
Click on the turtle’s Procedures Tab and type:

to roll
repeat 1 + random 20 [setsh 1 + random 6]
end
As you can see, random can even be used to determine how many times the
shape should change when the die is rolled. This adds to the illusion that the die
is rolling.
Type:

roll
show shape
3 Your number is probably different.
Try it a few times.
Once you’re satisfied with your die, create a second and third die. You can do this
using the Copy/Paste keys or menu items or you can use the command:

clone “t1
You should now have three dice. In the Command Center, type:

everyone [roll]
Did all the dice change?

34

Displaying Your Information
In MicroWorlds EX, there are many ways to display and use information. This project
uses two possible presentation methods. There are also many ways to get MicroWorlds
EX to automatically keep track of all the totals. One method is used in the project; a
second is shown in the Extreme Challenges section at the end of the project.

Text box records
Create a text box for each possible total using one, two or three dice. Name each
text box accordingly. For example, the first text box shows how many times 1 is
rolled, so name the text box Got1 (You cannot use a number alone as the name of
the text box, although names can have numbers in it.) Create 18 boxes and your
setup will work with one, two or three dice.
Right-click/Ctrl-click on the text box and select Edit. Set the text box to single
line (turning on the Grid helps). The box does not have to be large. It only needs
to be large enough to show a 4-digit number. Once satisfied with your first box,
copy it, once for each possible total, and edit each box. Then, type the number 0
in each box.
Don’t write the word zero, use the number.
When you are finished you should have the following:

Text boxes act like variables. They each have a name connected to a value
(the contents) that can change.

35

Anytime you create a text box, two dynamic primitives are also created.
They’re available in the project as long as the text box exists. The first is the name
of the text box; it reports the text box’s contents. The second is the word set- and
the name of the text box; it sets the “value” (the contents) of the text box.
For example, if you entered a 0 in the text box named three, type:

show got3
0
setgot3 1 + got3
show got3
1
If you did not enter a 0, you get an error message. The + sign needs to have two
values to add together.

Rolling and Storing Your Results
Try this:

everyone [roll]
Each of the dice should change. Now type:

show t1’s [shape]
5

Use the name of your turtle.
Your number will be different.

Using an apostrophe with the name of a turtle gets the turtle to do whatever is in
the square brackets that follow. (It is exactly like the ask primitive, which you
may have used already.) If the square brackets contain a reporter, such as shape,
this information is reported. In this case, it’s reported to show.
In order to get the sum of the dice, you need to add together the numbers rolled –
which are also the shape numbers – for the three dice. If each turtle reports its
shape, you can then add them together.

36

Write a procedure in the project Procedures Tab to roll all three dice and
calculate the sum.
Remember to use the parentheses. They help MicroWorlds EX interpret the
statement correctly. Everything inside the parentheses is evaluated before
being transmitted to the “+” sign.

to roll3
everyone [roll]
make “sum (t1’s [shape]) + (t2’s [shape]) +
(t3’s [shape])
end
Make creates a variable. Make’s first input (sum), is the name of the variable.
The second input is the value of the variable. In this case, the value equals the
sum of all the turtles’ shape numbers.
(A variable is like a box. The value of the variable is like the contents of the box.
The same box can sometimes hold books and other times hold shoes. On the
outside, though, it’s still the same box. In roll3, sometimes the box – :sum –
holds the number 8, sometimes 12, for example.)
Try the procedure:

roll3
Next, type;

show :sum
9
You probably will get a different number
The “dots” ( : ) before the word sum means that you don’t want the word sum
printed, but you want the value of the variable named sum (its contents –
whatever it’s representing).
Try it again.

roll3
show :sum
9
The value of the variable (:sum) changes with each roll.

37

Keeping Track
Finally, you need to keep track of how many times each sum is rolled. Each time
a number is rolled, you need to add one (+ 1) to the corresponding text box.
In the project Procedure Tab, write a procedure to roll and store a number in the
variable, and then add one to the correct corresponding box. (Note: You will never
get a 1 or a 2 with three dice, but including them in the procedure means you can
use this same procedure when rolling one or two dice, too.)

to keeptrack
if :sum = 1 [setgot1 got1 + 1]
if :sum = 10 [setgot10 got10 +
end

1]
1]
1]
1]
1]
1]
1]
1]
1]

Make sure all the number boxes are set to 0.
Now try the roll3 and keeptrack procedures a few times.
Do you notice the values in the boxes changing?

38

Repeatedly Repeating
One task at which the computer is great is repeating an experiment thousands
of times without getting tired. The computer can roll the dice as many times as
you want.
Reset all the boxes to 0.
Type:

repeat 100 [roll3 keeptrack]
Do you get the results you expected?
It would be interesting to try repeating the keeptrack procedure different numbers
of times to see if you get the same results each time.
Using a slider is one way to easily change the input to repeat. Create a slider and call
the slider something like Times. Set the minimum to 1 and the maximum to 5000.

Now, reset all the text boxes, set the slider, and try your experiment:

repeat times [roll3 keeptrack]
Create a button with that instruction to run the experiment. The button’s label
should tell about what it does. For example: Experiment 3 dice
Set the mode to Once.
Reset all your boxes and try your experiment several times.

39

Resetting Everything
It would be easier to reset all the boxes with one command, rather than one-byone. In the project Procedures Tab, write a procedure that resets all the text
boxes to 0.

to reset
setgot1 0
setgot2 0
setgot3 0
setgot4 0
.
.
.
.
end

You fill in this part

Make sure you reset all the boxes. Create a Reset button to run this procedure.
Your project may look like this:

40

Testing Your Hypothesis
Run your experiment several times. Are the results the same as what you
predicted? Do the results change the more you roll the dice?

Graphing Your Results
Although all your results are visible on the screen as numbers, you may be better
able to understand the results if you graph them. Create a bar graph to show
which sums are rolled most frequently.
Add a new page to your project. Draw an x and y axis on the screen
(turning the Grid on will help you with the page layout).

Freeze your background:

freezebg

41

Use a turtle to represent each possible sum. The turtle will draw a line based on
the number of times the sum is rolled.
Start by creating one turtle and placing it on the x-axis.
Place it 20 steps from the y-axis.

Give the turtle a new property - a homebase.

giveturtle “homebase
Open the turtle’s backpack. In the turtle’s State Tab:
• Put the turtle’s pen down
• Then check the turtle’s current position (xcor and ycor).
• Double-click on the word homebase under the heading “I have”.
Set homebase to a list containing the turtle’s current xcor and
ycor positions.
In the turtle’s Procedures Tab, type the following procedure:

to drawline
fd 2 * got1
setpos homebase
end
The turtle puts its pen down and goes forward two times whatever value is in the
got1 text box on the other page. It then returns to its homebase.

42

Try your turtle. Type a number in the got1 text box and type this in the
Command Center:

setpos homebase
drawline
Did your turtle draw a line in the correct place? To erase this line, use:

clean
If you’re happy with your turtle, copy and paste a new turtle. Edit the turtle in the
following ways:
1. Change the homebase. Double-click on the value of homebase in the
new turtle’s State Tab and change the xcor so that the next turtle is 20 steps
to the right.
2. Change the drawline procedure so that it uses the value reported by the
text box named got2:

to drawline
fd 2 * got2
setpos homebase
end

43

Repeat this process, creating one turtle for each of the possible sums. You should
have 18 evenly spaced turtles, each with a slightly different drawline procedure.

Test the turtles:

everyone [setpensize 10]
everyone [drawline]
Each turtle uses its own drawline procedure – the one that’s in its backpack.
Add two buttons, one to draw the graph (using the above instruction) and one to
clean the graph. You may also want to label the x and y axis, and use the Painting
tools to give a different colors to all the turtles.
Your screen may look like this:

You can also hide the turtles:

everyone [ht]

44

Run your experiment and then click the Graph button to record your results.

Challenges:
• Run the experiment with 1 or 2 dice. All you need is an extra button that rolls
the right number of dice. You also need to create a roll1 or roll2 procedure.
(You have too many text boxes for this experiment, but it doesn’t matter.)
• The numbers you get on page1 of your experiment are called raw scores. If your
times slider is set too low, the graph will be too flat. If it is set too high, the
bars may exceed the length of the y axis. Convert raw scores to percentages to
get values between 0 and 100 no matter the number of times you throw the dice.
Write a procedure in the project Procedures Tab to convert the scores
to percentages.
It could look like this:

to convert
setgot1 int (100 *
setgot2 int (100 *
setgot3 int (100 *
setgot4 int (100 *
.
.
You fill in this part.
.
end

got1
got2
got3
got4

/
/
/
/

times)
times)
times)
times)

45

Add a Convert to Percentages button on page1.
When graphing the results as percentages, you will probably find that the bars
on the graph are too short. Do you know how to change this?
• Change your dice. Try giving one die two sides with the same number of dots.
You will need to adapt your roll procedure, since the shape number will not be
the same as the number rolled. Here’s one way you can do it. In this example,
shape two - the side with 2 dots – is changed to a shape with 4 dots.
Your procedure could be:

to roll
repeat 1 + random 20 [setsh 1 + random 6]
if shape = 2 [setsh 4]
end
Do you understand why this works? If 2 is rolled, the shape of this turtle is
reported as 4.
How does this change the outcome of your experiment?
• Add a fourth die. Change your set up and procedures accordingly.

Extreme Challenges
• Before your friends try your experiment, ask them “Which numbers will be
rolled the most before trying the experiment?” After the experiment is run, your
friends can compare their guess to the actual results.
One way to ask this question is to create a dialog box that contains the question
plus multiple answer choices.
In the Dialogs menu, select New Dialog Box. The Dialog Editor opens.
You can add text boxes and choosers in your dialog box.

46

1- Add a text box in which you can write your question. If you want, make the
text box transparent.
2- Next, add a Round button set. In the round button set dialog, give the
Round button set a meaningful name, such as challenge. In the field next to
Label, type one of the answer choices. Click Add. Repeat these last two steps
for each of the choices. It may look like:

Keep the default value set to (empty). This means none of the choices will
be selected when the dialog box is first displayed.
3- Arrange the text box and the Round button set close to the OK and Cancel
buttons in the bottom right hand corner of the dialog box. These buttons are
automatically added to your text box. Drag the little black squares on the
edge of the editor screen to reduce the size of the dialog box so that there
isn’t too much blank space.
4- Click OK (the one at the top of the Editor) when you’re finished.
To test your dialog box, type:

dialog1

(use the name of your dialog box)

A dialog box should open and your question should appear.
Select an answer and click OK.

47

Type:

show getlabel “challenge
(this depends on which answer choice you selected).
6, 7, 12
Getlabel gets the contents of the label of the selected choice in
challenge.
5- Add a text box on the graphing page that displays the guess
(for example, the text box could be named guess).
6- Add a button to your first screen to display the Dialog box.
The instruction could read:

dialog1 setguess getlabel “challenge
Getlabel “challenge reports its information to setguess,
which sets the contents of the guess text box.

• Try writing a very short procedure to record all the rolls in text boxes.
A one-line procedure could look like:

to keeptrack
run list (word “set “got :sum) 1 + run
(word “got :sum)
end
Run executes whatever instruction is in the word or list that is its input.
In this case, you must use a list as input, since the instruction has several
components. The first component is the word created by set, got, and the number
rolled (the value of sum) – for example, if you roll 12, the word would be
setgot12. The next component is the outcome of 1 + whatever is reported by the
word created with the instruction word “got :sum. You must use run
before word “got :sum so that a number is reported, not the word.
See if you can write a short procedure to graph the information using only
one turtle.

48

Section 2
Videogame Design
Videogames are great to play, but even more fun to build. With MicroWorlds EX its
easy to design your own games and share them with friends. These games can
even be played on the World Wide Web via the free MicroWorlds EX Web Player.
You will build two simple projects based on popular videogames, FroggerTM and
Pacman. These are excellent games to build in MicroWorlds EX because of their
simplicity and room for improvement. You should be able to enhance the games
and customize them to suit your own goals.
These two videogame projects offer an opportunity to introduce a number of more
sophisticated (but not too difficult) programming concepts.

What’s in a game?
Videogames all include the following elements. You will explore each of them
during the construction of the projects.
• Logic
What are the rules of the game? How does a successful player think while
playing the game? What sorts of strategies are most effective? Does the player
play against herself? The computer? Another human player? If the computer
plays a human contestant, how will you teach the computer to play the game?
• User Interface
What buttons does the user have to click? Which keys do they have to press?
Are there instructions available for new players? Does the screen layout make
sense to the user?

49

• Motion
Videogames differ primarily from other types of game in that things move in
videogames. These games are often tests of eye-hand coordination more than
they are of logic and strategy.
• Collision detection
Crash! Boom! Pow! Videogames are fun because things crash into other things:
cars into walls; vehicles into other vehicles; projectiles into people, buildings
or vehicles. You earn points for hitting things and lose the game when certain
things touch you. The MicroWorlds EX environment can be programmed to
interact with your moving objects and for the moving objects to interact with
each other.
• Scoring (data-keeping)
There are all sorts of ways to keep track of important data and success levels –
game level, points, collected treasure, etc… Your games will probably include
some form of scoring.

Motion and color detection
As you saw in the previous projects, MicroWorlds EX can use colors painted on the
screen to detect when a turtle is over that color.
That should illustrate how easy it is to have a piece of the graphical background
influence the behavior of a turtle in MicroWorlds EX.
You can also program the color to react when the mouse is clicked on it by typing
instructions in the mouse instruction field of the color’s edit box.

50

Game 1 Snacman

Our simple Snacman game has three kinds of players – Snacman, ghosts and food
pellets. Each of these players will be represented by turtles. All of the game play
takes place within a maze, so you also need to create the maze that the characters
inhabit.
The goal of Snacman is to munch around a maze, eating all of the food pellets
before a ghost makes you disappear. You have to try and outsmart and outmaneuver the ghosts.
N

• Draw a maze
• Design Snacman, spooky ghosts, and food pellets
• Create navigational controls
• Program interactions with the background (maze)
• Program interactions between Snacman and ghosts and food
• Track and record the score
• Add final touches to the user interface
• Play the game

51

Getting Started
Create a new project. Use the Painting tools to draw a dark blue maze taking up
much of the page.
Be sure:
• to use dark blue as the paint color and make the walls about the width of a
turtle shape.
• that there is enough “white space” between the walls so the turtles can move
through the maze without touching the walls.

Design Snacman
Hatch a new turtle and turn it so it is facing east (right). Open its backpack.
• In the Shapes Tab, design two turtle shapes – rightopen and rightclosed

• In the State Tab, name the turtle Snacman and type rightopen rightclosed into the
Shape field, or select both shapes in the Shapes Tab and then click on the turtle

52

• In the Rules Tab, in the OnClick instruction field, type:

fd 2 wait 1
• Choose the Forever mode.
Click on the turtle. Does Snacman move and munch? If not, check that you
followed the instructions above correctly.

Program the maze walls
There needs to be a way to keep the turtles within the closed maze. Program the
dark blue so that a colliding turtle backs up and turns right or left at random.
• Right-click/Ctrl-click anywhere on the dark blue walls of the maze
• In the instruction field next to Turtle, type

bounce

• Leave the mode to Once and click OK
Now, create a bounce procedure in the project’s Procedures Tab.
Type the following procedure:

to bounce
bk 20
ifelse 1 = random 2 [lt 90] [rt 90]
end
This procedure causes the turtle to back up and then flip a coin to randomly
decide if it should turn left or right, adding a bit of spontaneity to the game.
Ifelse works in a similar way to if, except it has both a true and a false
consequence as the result of the condition being checked – in this case whether a
1 or 0 was picked at random. If a 1 is picked, ifelse runs the instructions in
the first list; if 1 is not picked, it runs the instructions in the second list.

53

Check it out!
Put your Snacman turtle in the maze (center of an aisle is best). Click on it and
watch what happens when it hits a wall
Does it back up and turn at random?

Ghost!
The ghost just wanders around the maze until it collides with Snacman.
Hatch a new turtle and open its backpack:
• In the State Tab name the turtle “Ghost
• In the turtle’s Shapes Tab, create two ghost shapes (you should know how to
do this by now)
• Put both shapes on the turtle or enter their shape names in the Shape field of
the turtle’s State Tab
• In the Rules Tab, give the turtle the following OnClick instruction:

fd 1 wait 1
• Choose Forever
The ghost initially moves slower and in a dumber fashion than Snacman so the
game is easier for the human player. Put the ghost-turtle in the maze, click on it
and see if it behaves properly.

54

Spookier movement
It would be cool to program the ghost to move in a more ghostly way than
Snacman. You can do this by making it turn more randomly when it bounces into
a wall. This also adds an air of unpredictability for the human player.
Right-click/Ctrl-click on the ghost turtle and choose Open Backpack
• Click on the Procedures Tab in the backpack
• Define a bounce procedure for the ghost

to bounce
bk 20
rt random 360
end
Now whenever the ghost hits dark blue on the page, it will back up a bit and turn
randomly. If any other turtle hits the wall, it backs up 20 turtle steps and turns
either right or left by 90 degrees. Each turtle bounces in its own way - runs its
own version of the bounce procedure - when it hits the wall.

Flickering ghost
You can make the ghost flicker while it moves by adding a setopacity
command to the OnClick instruction in the backpack.
Change the OnClick instruction to:

setopacity random 100 fd 1 wait 1
Setopacity sets how opaque (or transparent) a turtle is.
Setopacity 0 makes the turtle completely transparent.

55

Food pellets
The job of the food pellets is to just lie around waiting to be hit by another turtle.
If that turtle is Snacman, the food pellet will be eaten. If it is any other turtle,
nothing happens and the food pellet can go back to sleep.
Hatch a new turtle and open its backpack.
• In the State Tab, name the turtle “food.
• In the turtle’s Shapes Tab, design a food pellet shape for it to wear. It doesn’t
need to animate so one shape is sufficient.
• Put the shape on the turtle

Clone the turtle
You need lots of food pellets to be in the maze, so make copies of the one you
just created. Use either one of the Copy/Paste methods or the clone command.

repeat 10 [clone “food]
All the pellets are stacked one over the other. Drag each of the pellets to the
desired location.
You may change the input to repeat to clone as many pellets as you wish.
Just make sure that a food pellet is not hiding behind another.

56

Add some navigational control
How do you steer Snacman as he moves? One way is to create four clickable turtles
to use as your joystick.
Hatch a turtle and open its backpack.
• Click on the Shapes Tab and create an up arrow shape or get one from the
Painting/Clipart palette.
• Click on the Procedures Tab in the turtle’s backpack and type:

to move.up
broadcast 0
end
• Click on the Rules Tab and in the OnClick instruction field type

move.up
• Create three more arrow-turtles, one for each direction. Type one of the
following procedures in the Procedures Tab of each turtle and set each turtle’s
OnClick instruction accordingly.
For the right arrow turtle, type:

to move.right
broadcast seth 90
end
For the left arrow turtle, type:

to move.left
broadcast 270
end
For the down arrow turtle, type:

to move.down
broadcast 180
end

57

Broadcast is a command that takes an input and yells that input to any
turtle that may be listening. You need to program Snacman so that when the
arrow-turtles yell out, Snacman gets the message and turns in the right direction.
• Arrange the arrow-turtles in the form of a joystick.

Telling Snacman where to go
You can “catch” a broadcasted message and act upon it by using the OnMessage
field in the Rules tab of Snacman’s backpack.
• Open Snacman’s backpack and click on the Rules Tab
• In the OnMessage instruction field, type:

seth message
Now when a number is broadcast by an arrow-turtle, message reports the number
to seth and Snacman automatically turns in the indicated direction.

Eat or be eaten
When Snacman hits another turtle, it either eats (if the other turtle is a food
pellet) or gets zapped (if the other turtle is a ghost). If Snacman gets zapped, the
game is over. Write two procedures, one for being zapped and one for eating.
This is a good time to add some sound effects. Record a short sound Snacman will
make when it eats a pellet. Name the sound yum and hide the icon if you prefer.
(If you don’t have a microphone connected to your computer, compose a one note
melody with the music tool and name it yum.)

58

Open Snacman’s backpack.
• Click on the Rules Tab.
• Type the following procedures into the Procedures Tab of Snacman’s backpack:

to zapped
everyone [clickoff]
announce [Game over!]
end
to eat
launch “yum
ask touchedturtle [ht]
end
The primitive, touchedturtle reports the name of the turtle with which
the current turtle has collided.

Ask lets you talk to another turtle briefly and then go back to talking to the
current turtle.

Ask has two inputs. The first is the name of a turtle (or text box) – in this case,
whatever turtle name is reported by touchedturtle. The second is a list of
instructions, enclosed in square brackets. Ask touchedturtle [ht]
asks the turtle hit by Snacman to hide – to disappear.
Next, create a procedure to check who the Snacman is touching:

to whoareyou
ifelse touchedturtle = “ghost [zapped][eat]
end
The procedure says that if the turtle you’re touching is the ghost, you’ll be zapped.
Otherwise, eat a pellet.

59

In the Rules Tab of Snacman’s backpack, type the following in the OnTouching
instruction field:

whoareyou
When Snacman touches another turtle, it checks who it is and then does the
appropriate action.
Extension: Can you add sound-effects or an animation to the zapped procedure?

Keeping track of strength
Use text boxes to keep track of Snacman’s strength during your game. Each pellet
Snacman eats is worth 1,000 points (Be generous!). When Snacman eats a pellet,
his strength increases. Can you beat your high strength?
• Create a new text box on the screen. Make sure it is not in the maze or over
the navigation buttons.
• Right-click/Ctrl-click on the text box and select Edit. Name the text box, strength.
• Select the single line mode.
• Choose a font, size, color and style for the text box
• Initialize the strength by typing setstrength 0 in the Command
Center. This is a common practice when programming since you later will want
to add a number to the value in the box. You can’t add a number to emptiness.
• Open Snacman’s backpack and click on the Procedures Tab.
• Change the eat procedure by adding a setstrength line.

to eat
launch “yum
ask touchedturtle [ht]
setstrength strength + 1000
end

60

Important! Important! Important! Do not write a new eat procedure!
Change the existing one:
When there is a text box named strength, setstrength changes its contents.
Now, each time Snacman eats, he gets stronger – a lot stronger!

Keep track of your turtles
Arrange the turtle, ghost and food pellets so that they are where you want them to
be at the start of each game. Then teach MicroWorlds EX to remember where they
are so you can easily reset them with each new game. Be sure that the turtle and
ghost are as far away from each other as possible.
Give all the turtles a new property – homebase:
• Type the following instructions in the Command Center:

turtlesown “homebase
everyone [sethomebase pos]
The everyone [sethomebase pos] line tells each turtle to
remember its current position and set it as the property called homebase.
Look in the State Tab of Snacman’s backpack, under the heading I Have, to see
what properties the turtle owns.
• Since this is a procedure for the whole project, write the following procedure in
the project Procedures Tab:

to setup
everyone [setpos homebase st]
end

61

Test the setup procedure by doing the following:
• Move some turtles around the screen
• Type setup in the command center
Did the turtles march back to their proper places?

Putting it all together
You are just about to play your first game of Snacman. There are just a few jobs
left to do.
First, you need to know when the game has been won by the player. The player
wins when he eats all of the food pellets before being zapped by the ghost.
• Count the number of food pellets in your maze
• Multiply that number times 1,000
• Remember that number
• Create a new button with the instruction, play
• Write a play procedure in the project Procedures Tab

to play
setup
setstrength 0
everyone [clickon]
when [strength = ____] [announce [You’re a
champion!] stopall]
end
Put the number you get by multiplying your number of pellets * 1000 in the
blank found in the when instruction.

62

When is a command that takes a conditional list as input and waits until that
condition is true. When it is true, the list of instructions in the second set of
brackets is run. When runs in the background as a process until you stopall or
cancel the process. cancel [setstrength = ____] will do the trick.
Be careful using when. It should be used sparingly as it may clutter up memory or
confuse you while debugging. Click on the Processes Tab to see a when process in
action. You may Right-click/Ctrl-click on it to pause or delete the process.
If you find that either the ghost or Snacman move too quickly or slowly, adjust the
fd or wait commands in the Rules Tab of their backpacks.
No cheating! If you freeze Snacman and the ghost, they will be controllable using
the arrow buttons and commands, but the player will not be able to drag them
around the maze using the mouse. In fact, you should freeze the entire page so
the arrows don’t “move” by accident.
Click on the play button and have fun!

Challenges:
• Keep track of how long it takes for you to win the game.
MicroWorlds EX has a built-in timer. The timer counts in tenths of a second.
Resett sets the clock to 0
Timer reports the current time since the last reset, in tenths of a second.
Start the timer when the game begins, keep track of how many seconds pass and
calculate a score based on how quickly you munch food pellets.

63

Create two text boxes, score and seconds. They should look like the strength text
box you already created. Find a good place to put them on the page.
Edit the play procedure to initialize two new text boxes, score and seconds:

to play
setup
setstrength 0
setscore 0
setseconds 0
everyone [clickon]
when [strength = ____] [announce [You’re a
champion!] stopall]
end
1- Open Snacman’s backpack
2- Click on the Procedures Tab
3- Create the following procedure:

to track.time
resett
forever [wait 10 setseconds timer / 10
setscore strength / seconds]
end
This says to wait one second (ten tenths of a second), display the time
(in seconds) in the seconds text box (timer divided by 10), and set the
scorein the score text box.
Your score will be based on the strength divided by how long it took to eat
that many pellets!

64

• Make your Snacman munch in the proper direction
You can make Snacman munch in the direction he moves, if you create three more
sets of open and closed mouth Snacmans in the Shapes Tab of the Snacman turtle’s
backpack.
1- Design six new turtle shapes with the following names. Be sure to name
them! You can copy, paste and then rotate existing turtle costumes to make
life easier.
upopen - upclosed
leftopen - leftclosed
downopen - downclosed
2- Write the following munch procedure in Snacman’s Procedures Tab:

to munch
if heading = 0 [setsh [upopen upclosed]]
if heading = 90 [setsh [rightopen
rightclosed]]
if heading = 180 [setsh [downopen
downclosed]]
if heading = 270 [setsh [leftopen
leftclosed]]
fd 2 wait 1
end
3- Change the OnClick instruction for Snacman to:

munch
Now your Snacman should munch in all four directions. Feel free to accelerate or slow
down Snacman and the ghost if you think it will make the game more exciting.

65

Extreme Challenges:
• Create a special “power pellet” that either zaps the ghost or makes Snacman
immune to it’s destructive powers for 5 seconds.
• Can you make a “Level” slider that controls the speed of the game? Can you
program the game to automatically reset the pellets and go to the next level
when the current level is completed? You may want to create the second level
on another page.

66

Game 2 –
Squish the Turtle:
A Web-ready Game
In “Squish the Turtle” the player needs to get a turtle across heavy traffic without
being squished. Does this remind you of any video games from the past?
In this project you should make the page smaller so that the game can be played
via the World Wide Web. Smaller projects take less time to load via the web and
tend to work better online.
N

• Draw the background scene
• Create your turtle, Fred
• Create navigational controls
• Create traffic
• Add collision detection
• Program interactions with the background
• Add traffic signals
• Share your game with others

67

Getting Started
Create a new project.
• In the File menu, choose New Project Size-MicroWorlds Small

• Paint a background image like the following:

68

Add the Turtle
Hatch a turtle. Drag the turtle to a position centered in the green area slightly below
the road. Open the turtle’s backpack and, in the State Tab, name the turtle Fred

Create a slider
Next, create a slider to control Fred’s speed.
• Create a slider
• Name the slider speed with a minimum of 1 and a maximum of 40
In Fred’s backpack:
• Click on the Rules Tab. Type the following OnClick instruction:

fd speed / 10
This causes Fred to not go too fast by dividing the speed by 5.
Remember to put a space on each side of the division sign.
Set it to Forever mode.

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Navigating
Use buttons to navigate Fred across the street. Clicking on the buttons cause Fred
to turn while still moving forward. Once Fred crosses the street, he returns back to
the green field and the game gets faster. The player who is able to get across the
street with the fastest speed is the champion.
• Write the following procedures in the project Procedures Tab:

to north
fred, seth 0
end
to east
fred, seth 90
end
to west
fred, seth 270
end
to south
fred, seth 180
end
• Create four buttons, each with one of the following labels and instructions:
north or south or east or west
• Choose Once and click OK
• Drag the buttons in the shape of joystick controls to the right-hand side of
the playing field.

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Create a traffic jam
Hatch a turtle.
• Turn it either to the right or left.
• Create or drag vehicle shapes into the project Shapes Tab so the shapes may
be shared by multiple turtles. Put a vehicle shape on the turtle.
• Open the vehicle-turtle’s backpack. Click on the Rules Tab. In the OnClick
instruction field, type:

fd speed / 10 or
fd speed / 8 to make slower and faster vehicles.
Clone the vehicle by selecting, copying and pasting duplicates all over the roadway
or use the clone command. Be sure to turn the vehicles in the correct direction.
Use the Shape editor to create flipped images if needed. If the vehicles are too big,
use the magnifiers or the command setsize to make them smaller. Feel free to have
different vehicles use different shapes (cars, trucks, motorcycles, etc…).
Move the vehicles around to see if the ones on this side of the street are in front
(hiding others). If they aren’t, right-click on the turtle and choose In Front or
In Back. (You may have noticed that, in fact, new turtles are created in front of
the older ones.)

71

What Happens When Fred “Meets” a Vehicle
If Fred is hit by a vehicle-turtle, Fred is squished.
Open Fred’s backpack
• Click the Procedures Tab and type:

to check
announce pick
[[I’m squished!]
[Game Over, Man!]
[Ooh, I hate when that happens!]
[That really hurts!]
[Ouch, ooooch, eeetch!]]
stopall
end
In this procedure, the pick command randomly picks an item from the big list
that contains several shorter lists. The announce command then displays what
is picked. The long instruction can be typed all on one line, but it is easier to read
if you format it on several lines.
• Click Fred’s Rules Tab
• Type the following in the OnTouching field:

check

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Setting up
Time to set up the game. First, put Fred at the position where you want him to start.
Give Fred a new property – homebase. To give Fred this variable, type in the
Command Center:

fred, giveturtle “homebase
To give the variable a value, type:

sethomebase pos
Giveturtle is just like turtlesown except giveturtle assigns
the new value or property only to the current turtle. Sethomebase pos
sets Fred’s homebase value to his current position.
The setup and play procedures below should be typed in the project’s Procedures
Tab since they are used by the project and not just one turtle.

to setup
fred, setpos homebase
seth 0
setspeed 1
end
Since there is a slider named speed, setspeed 1 sets the value of that
slider to 1.
Play causes everything to reset before asking everyone to go.

to play
setup
everyone [clickon]
end

73

Add a play button.

Next level
Now program Fred so that when he reaches the sky he goes back to his homebase
and picks up the pace for a more challenging game.
Open Fred’s backpack.
• Click on the Rules Tab
• Select the sky blue color (or whatever color you used for the sky) in the
OnColor menu. Type the following OnColor instruction. Leave the mode set
to Once.

setpos homebase setspeed speed + 1
In fact, the speed slider is used to control the speed of the objects, but it also
indicates the “level” of the game, or your score if you will.

74

The traffic light
Adding a turtle that looks like a traffic light can add another level of control for a
player.
• Hatch a new turtle, name it light and drag it to an open space (off the road).
• In the Shapes Tab of light’s backpack, design two shapes, one of a traffic light
with a green light illuminated and one of a traffic light with the red light
illuminated.
• Name the shapes, greenlight and redlight, respectively
• Type the following procedure in light’s Procedures Tab:

to switch
ifelse shape = “greenlight
[setsh “redlight broadcast “red]
[setsh “greenlight broadcast “green]
end
• Set the OnClick instruction to switch
• Write the following procedure in the project Procedures Tab

to checklight
if message = “red [clickoff]
if message = “green [clickon]
end
• Open the backpack of each vehicle turtle. Click on the Rules Tab and, in the
OnMessage instruction field, type:

checklight
The checklight procedure needs to check each possible message that can be
broadcast. In this case, there are only two, red and green. You could use either
ifelse or if. If there are exactly two messages being checked, it is better to
use ifelse.

75

Now, the vehicles will start and stop as the traffic light is clicked.
Because the play procedure simulates a click on every turtle, you should add
light, setsh “greenlight to the procedure. Without this, the light
will turn red when you start the game, if it was already green.

to play
setup
everyone [clickon]
light, setsh “greenlight
end

Finishing Touches
• If the turtle is pointed down, it could end up going through the bottom of the
page. It would then reappear at the top, reaching the sky color without crossing
the road! To avoid this, add a thick line at the bottom of the page and program
whatever color you use for the line to reset the turtle’s heading to 0.
• Freeze the page to keep the player from dragging things around
during the game.

76

Share with a friend!
Ask some friends to play your games and make suggestions for improved game
play. Then, use MicroWorlds EX to publish your projects on the World Wide Web so
anyone, anywhere can enjoy them. The following instructions will enable you to
put your “Squish the Turtle” game on the web for millions to play and enjoy!
Note: If you do not have experience with posting files on the World Wide Web, ask
a friend or your school’s network guru.
Choose Create HTML Template from the File menu. When creating an HTML
template, MicroWorlds EX asks you to specify a location for saving the mwx and
html files. These files must be saved in the same folder. On most operating
systems, the dialog box allows you to browse and create a new folder for saving
the files in. In some others (Windows 98), you must create the folder yourself,
prior to this operation.
When saving a project as an HTML template, MicroWorlds EX creates two files with
the same name and different extensions (for example.: MyProject.mwx and
MyProject.html). Both these files and any media files must be saved in the same
folder, whether you run them directly on your computer or on the web.
Double click on the html file to view your project in your Internet Browser. You
will need to be sure that you have the latest MicroWorlds Web Player installed on
your machine. You can download this for free from http://www.microworlds.com.
You may also wish to make sure that your browser has enough RAM to run the
project. On a Mac, set the amount of memory allocated for your browser for as
much as you can spare, typically > 35 mb.

77

Challenges:
• Illustrate your screen to make it more attractive
• Design some new vehicle shapes
• Replace the navigation buttons with turtles. Design turtle shapes to look like
the appropriate arrows.
• Figure out a way to get some vehicles to travel at half or double the speed
• Create a crash complete with sound effects and a realistic or
humorous animation

78

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