Scratch 2: Control and Sensing

Control: Making Choices

Open the Control group of script blocks.

when (green flag) clicked is used to start a script by clicking on the green flag in the upper right corner of the Scratch window.
The wait block makes the script pause (so we can get ourselves into position)
The if block lets the script perform an action provided a condition is true. The "action" can actually be a sequence of actions. If the condition is not true, it will do nothing.
The if/else block lets the script choose between two actions. If a condition is true, it will perform the first action; otherwise, the second.

Example 1A

Try this:

A script for the cat to meow if it's close to the mouse

Example 1B

Let's make our sprite chase the (computer) mouse, if it's not close enough. This is almost the same script as before, only we are changing the if block to an if/else block and inserting a second action.

A script for the cat to meow or move closer to the mouse

Okay, now our sprite will do something, no matter what. Still, it does not seem to chase the mouse very hard. Let's make it more persistent.

More Control: Repetition

The repeat block lets us repeat an action a set number of times. (Say "hello" 4 times")
The forever block makes the script repeat an action forever. Okay, really, not forever, but until you make the script quit — for example, by clicking on the red stop sign beside the green flag. Or you could turn your computer off.
These kinds of control block are called loops because, after executing an action, they go back and do it again.

Example 1C

This is like Example 1B, except we are wrapping the if/else block in a forever control block.

A script for the cat to chase the mouse

Example 2

Use a beach ball sprite for this one.

A script for a bouncing ball

This is a bit boring, as the ball bounces back and forth along the same path all the time. Let's spice it up, by adding some randomness to the ball's direction. Add a second script for the same sprite (keep the first one there):

A script for some random variation of a bouncing ball

First, try out the second script all alone by clicking on the script. The effect is very subtle.

Now let's click on the green flag. With two scripts in the panel for the same sprite, both scripts will start running together. The first script manages the basic back-and-forth bouncing act. Only now, every 2.5 seconds, the second script will cause the sprite's direction to deviate a little.

Having two scripts run together is called concurrency. It's a powerful way of controlling behavior, but, in many cases, difficult to use because of complicated interactions between the scripts. (Our example is quite simple.)

Using the Pen

We can use our sprite to draw a picture using the "pen."

Open the Pen group:

clear erases the picture
pen down lowers the pen, so that moving it will draw a line
pen up raises the pen, so that we can move without drawing
There are blocks for changing the pen's color, shade, and size.
stamp causes the pen to leave an imprint of the sprite at the current location.

Note, also, that in the Looks group we can control the visibility of our sprite with show and hide.

Try a few pen and motion commands to draw a line and turn a corner.

Example 3

Now, can we draw a rectangle?

This would do it:

A script to draw a square

But there's a much better way:

An improved script to draw a square

Okay, now let's get a little more rambunctious. Here's a little script that will, repeatedly, wait for a mouse click, draw a square at the mouse location, and then have the sprite retreat to a corner.

A script to draw squares repeatedly

Input and output

Our sprite can ask a question and read a typed-in response, and can "say" something based on the response.

Example 4

A script to greet by name

In the Sensing group we see that answer is something associated with the ask block.
- It's a variable. More on variables in the next lesson.
Checking it makes the answer visible on stage.
The answer can also be dragged into a suitable hole in a block of a script.
join puts two strings (sequences of characters) together.

Summary

Previously, we learned about primitive actions, such as the motion commands, and sequences of actions.

Today, we've added to our control repertoire:

The ability to select between an action and no action, or between two actions, using if and if/else
The ability to repeat an action, either forever, or for a fixed number of times (repeat).
Sequence, selection, and repetition are the three structural building blocks of programs. Any program that could ever be written, can be written using these three control structures. I.e., these are the only three ways in which actions are combined to make programs, or scripts.
Repetition is particularly powerful, because we can instruct the computer to do the same thing many times. Writing programs is usually pretty difficult — for example, it is typically easier to add 10 numbers yourself than to instruct the computer to add 10 numbers.
If it weren't for the power of repetition, most programs would not be worth writing.

In addition, we've learned to make graphical output with the pen, and to use the ask and say blocks for text input and output.

Revision log:
- Version 1.1, 2010 April 17. Added script snapshots.
- Version 1.0, 2010 April 12. Added text input/output.
- Version 0.9, 2010 April 10. Initial draft.
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