Functions and Expressions

Objects in 3D Space

Objects in Alice have a centre point.

That centre point is the intersection of three axes:

• Green axis: up/down
• Red axis: right/left
• Blue axis: forward/backward

For people, this centre point is often positioned at the bottom of the model:

Distances between objects are measured between their centre points:

Exercise: Shrine Time

With that in mind, consider the following scene:

What if we wanted the person to move over to the shrine, but not go inside?

Consider this carefully: remember our discussion earlier this year about algorithms.

Instructions must be precise – computers are not able to “guess” at what we want them to do.

Discussion: An initial plan

After some thought, you may written something like the following:

• Make the person walk over to the shrine.

If your plan looks like this – it is not detailed enough! 😅

Remember, objects in a 3D space like Alice have distances between them measured from their centre points:

Discussion: A more detailed plan

A revised plan might look like this:

• point person in the direction of the shrine
• move person to "edge of shrine"

Find edge of shrine by:

• finding distance from person to shrine
  • subtracting half the width of the shrine

Let's program that together now.

You can download the starter world here.

In completing this task, we made use of two new ideas: functions and expressions.

Functions

In Alice, a function is used to get information we need about:

• the properties of objects
  • e.g.: Is the snowwoman's face red?
• the relationship between one object and another
  • e.g.: What is the distance between the mummy and the pyramid?
• a current condition
  • e.g.: What key (on the computer keyboard) was just pressed?

When a function is used to ask a question or perform a computation, an answer is returned.

This answer is called a value.

The type of the value depends on what the function does for us.

In the example we completed earlier, we expected to get a numeric value because the function retrieved a distance.

That distance could be a whole number, such as 3 meters.

It could also be a fractional value, such as 1.2 meters.

Other value types include:

• Boolean
  • true, false
• String
  • "Oh, Yeah!"
• Object
  • snowman, helicopter, et cetera
• Position in the world
  • (0, 0, 0) – the center of an Alice world

Expressions

An expression is a math or logical operation on numbers or other types of values.

Alice provides math operators for common mathematical expressions:

• addition  +
• subtraction −
• multiplication ⨉
• division   ÷

We can combine functions and expressions to do calculations that are responsive to the location of objects in a scene.

Return to Alice now and move the shrine to a new position, something like what is shown in the animation below:

Run the program again. Did the person still go to the edge of the shrine?

This works because we used a procedure to turn toward the shrine – whereever it happens to be in the world:

Additionally, the person always moves the correct distance because we used a function within an expression to find that distance:

Try moving the shrine a few more times.

Every time you run the program, you should find the person moves to the edge of the shrine, but doesn't go inside.

Exercise: Lost and Found

Oh dear! You have lost your AirPods case... again. 😭

Download this Lost and Found world and open it in Alice.

Run the world a few times. What do you notice about the position of the objects?

Now, add a person to the world.

Make your person:

• turn toward one of the objects
• move right up to the object, but not overlap with it
• turn back toward the middle of the room
• return to the middle of the room

Repeat this process for each object in the room.