Tutorial: Part Two of Methods
By Deborah Nelson
Duke University, July 16, 2008
Under the direction of Professor Susan Rodger
As you follow the instructions below, click on an image to enlarge it.
Then use the'go back one page' arrow in your web browser to come back to
We will now complete the world that you started in part one of the tutorial entitled "Methods."
If you have not yet done Part One, you must go through that tutorial first.
Loading the World
For this tutorial, you can use your completed version from part one of the Methods tutorial.
That world was entitled methodStart.a2w.
Or you can download the version of the world with the solution from part one here.
Remember to save it in a directory that you can find again, and then start Alice and open the
world. NOTE: You cannot double-click the file to open it; Windows will not know what to
use, and even if you select Alice from a list of programs, the loading will fail.
Part 1: Parameters
- Now that the kangaroo and the turtle have raced, let's make a method for the kangaroo to
hop back to the turtle and challenge him to a race again.
- Click on the kangaroo, create new method, and name it 'challenge'. Drag a 'do in
order' into your method.
- Then, click on kangaroo in the object area, drag it into the method and select
the method 'turn to face' - select the turtle - the entire turtle.
- When you finish dragging all of the instructions into your method, it should look like
- If we save the kangaroo and put it in a world where there is no turtle, what will happen?
Alice will crash because the method kangaroo.challenge refers to a non-existent object.
A class-level method should not have any references to other characters or world-level methods.
- In other words, instead of refering to the turtle in the first instruction of this method,
we're going to use a parameter. A parameter is a place holder.
- For example, in another world, you may want your kangaroo to be able
to challenge a turtle or a bunny or a penguin. We could write three different methods:
one for the kangaroo to challenge the turtle, another for the kangaroo to challenge the bunny
and a separate one for the kangaroo to challenge the penguin.
- In this case, a parameter is going to be a placeholder for an object that the kangaroo
will challenge - such as a bunny, a penguin or a turtle.
- Then we only write one method, but it works for bunny, penguin, and turtle
- Click on 'create new parameter' and name it 'obj'. Before you click okay, make sure
you select the type 'object'
- Now, you can see that the 'obj' parameter has appeared beside the name of the
method. I've highlighted it with a red box. Drag 'obj' into the method to replace 'turtle'.
- To test your code, drag kangaroo.challenge into world.myfirstmethod underneath the
world.race method that is already there.
- When you drag kangaroo.challenge into the method, once you release your mouse
you will have to select turtle, then entire turtle as the obj to replace parameter.
See the two images below for an illustration:
Selecting the turtle as the parameter argument:
Testing kangaroo.challenge on another object
- To reinforce your understanding of parameters, let's call the method on another object.
Add the tortoise (from the Animal folder) to your world by clicking on the Add objects button.
Below, I've highlighted in red where I placed the tortoise.
- Drag kangaroo.challenge into your world.myfirstmethod and select
the tortoise as the parameter.
Play your world. Now after the race, the kangaroo challenges the turtle and then the
Depending on where you placed the tortoise in your world, you may notice that having the
kangaroo hop twice toward him does not look very good. Once you know how to use the built
in function 'distance to' you can improve the appearance of this method. For now, don't worry
Let's finish making the rest of our world. In world.myfirstmethod, delete the second call
to kangaroo.challenge for the tortoise. If you want, you can delete the entire tortoise from
Part 2: Properties
- Finally, we want to write a method to make the turtle go into his shell. Click on turtle
in the object tree. Click on the method tab and create a new method named 'hide' (If you
use the world given to you as a starter world, hide will have already been created, but there
is no code in it).
- We are going to make all of the turtle's body parts invisible at the same time, except for
his shell. To do this, first drag a 'do together' into the 'hide' method.
- Then, click on the + beside turtle in the object tree. Click on the 'backRightLeg'.
- In the details area: Click on the properties tab. Click on 'isShowing' and drag it into
the 'do together.'
- Set the value to false. Click on the 'more'
and set duration to 0.1
- Do the same thing for each of the body parts by clicking on each of these in the object
tree- backLeftLeg, frontLeftLeg, frontRightLeg, tail and head - and dragging the isShowing
property of each into the turtle.hide method.
- Your code should look like this:
- Now drag the turtle.hide method into your world.myfirstmethod underneath
- If you want, you can have the kangaroo say something at the end.
- Here is what my final code looks like for world.myfirstmethod:
Press play to watch your entire simulation.
- If you want to write a method in which an object interacts with another
character, you can either write a world-level method or write a class-level method
- A class-level method with parameters is a good choice if you want to be able to
save your object out so that it can perform your new method in different worlds.
- Keep in mind that parameters are not only used in class-level methods. For example,
if you have five characters in your world and you want them to all flip together, you can
write one world level method with an object parameter that flips. Then in a do together,
call the method for each of the objects in your world.
- You can change certain properties of an object while you're writing a method