Summer Computing Workshop
PROGRAMMING WITH SCRATCH
Introduction

Boolean Expressions – In programming, a Boolean expression is an
expression that is either true or false. In Scratch, any block shaped like
an elongated diamond is a Boolean expression. The three types of
advanced Boolean operators we will discuss are not operator, and
operator, and or operator.

Messages – Message passing is a very useful tool in Scratch. It is a way
that sprites can specifically start a script by broadcasting a message,
either a script belonging to itself or a script belonging to another sprite
can be executed when it receives that message. This is the only way
Scratch allows one sprite to run scripts in the other. Interesting enough,
like stated before, it can also be used to run scripts from the same sprite
that broadcasted the message.

Arrays – In programming, arrays are used to store multiple data of the
same type in order to easily and quickly find and use the data later in the
program. In Scratch, since there are no defined types, arrays are groups
of variables that can contain both words and numbers.
Session 6
Boolean Review
 Some basic Boolean expressions we discussed in Session
3 are:
 The above operators return true or false and can be used
in structures we covered earlier such as
 There are three other commonly used Boolean operators
that can be combined to create extremely flexible
expressions.
Boolean Expressions

Now we will discuss other useful Boolean expressions and some examples of
each.

This operator is used when two conditions need to be met in order
to execute specified blocks. If the first condition is true and the second
condition is true, then this operator returns true. If either of the conditions
are false, the expression returns false. For example:

Since the 3 < 4 evaluates to true and 8 = 8 evaluates to true, then the
predicate part of the if block evaluates to true and the sprite says “Hello!”
Boolean Expressions

In this example, 5 isn’t less than 4 so it evaluates to false. Even though one
condition is true (8 = 8), using the and operator requires both conditions to
return true

This operator is very similar to the and operator except it only
requires one condition to be met.

Even though in this example 5 = 1 evaluates false, all the block needs in
order to execute is one condition to evaluate true. The full expression
evaluates true.
Boolean Expressions

This is another useful operator that changes true to false and false to
true, for example

In the above example, because 5 is not equal to 3, and the not block was
used, the expression evaluates to true. The not operator in the next example
will show the if block not executing.

Because the expression 5 = 5 is true, using the not operator causes the
predicate part of the if block to evaluate to false. The say block will not be
executed. These Boolean operators can be used in the conditional part of a
repeat until control structure but most commonly are found in the predicate
part of an if control structure.
Message Passing

There are three blocks that are used in message passing. They are found in the
control blocks category. Two of these blocks are used to broadcast messages
and the other is used to receive messages.

One of the control blocks used for broadcasting messages is
. By
clicking the down arrow, we can create a new message to be broadcasted.

Another of the control blocks used for broadcasting messages is the
block. This is very similar to the broadcast block except the
sprite that is broadcasting the message waits until the scripts it triggered have
finished running. It then continues with the rest of the current script.

The final control block we will discuss is actually necessary in order to properly
use the previously discussed control blocks. The
allows a sprite to
receive a specific message and respond accordingly.
Broadcasting/Receiving

Our example will require the Scratch cat and the bat. We will also introduce
costume changing but not in great detail. We will build a script moving the
bat towards the cat (changing costumes during the movement to make it
more realistic). When the two sprites touch, the bat will broadcast a
message and the cat will receive the message and execute the script written
for it.

In order to utilize both bat costumes, click the costumes tab in the middle
scripts area. If the other bat isn’t imported do this now. Now we should
have two bat costumes that belong to one bat sprite. Now we will construct
the example (see example 6.1 on the Constructed Examples page at the end
of this section).

The same thing can be done with many sprites. Not all sprites have multiple
images of themselves so some may not have any other costume choices.
Message Passing

For our next example, we will show the difference between the broadcast
and broadcast and wait blocks.

We will be using the same example except we will take the broadcast block
out and replace it with the broadcast and wait block.

What is the difference? Now, go into single stepping mode and notice what
is being executed.

This could be very useful if we wanted a Sprite to broadcast a message, then
continue with it’s script after the receiving Sprite is finished with it’s script.

Passing messages is a way two sprites can communicate. Its a way to start
another Sprite’s script inside of a script.
Broadcasting/Receiving

Now we will discuss the only block in Scratch that allows another sprite or
itself to receive messages that the previous sprite sent.

For this to work, the first sprite has to broadcast a message using the
broadcast block in its scripts. The second sprite (which has different scripts)
needs to have a when I receive block. This is the only block in Scratch that
can actually receive messages, so the two previous ways of broadcasting
messages are useless unless we have a sprite receiving the message
broadcasted.

It is possible, however, to have the same sprite broadcast, receive its own
broadcasted message, and execute scripts accordingly.

Ok, let’s construct this short example (see example 6.2 on the Constructed
Examples page at the end of this section)
Project 6.2
 Create a short animated story (similar to the
project in session 2) that uses broadcasting
and receiving to queue cinematic events (you
needn’t be too detailed).
Arrays

Arrays are a very useful programming tool. As stated previously, Arrays can
store multiple values that you may need later in your program. Creating an array
is, in a way, like creating multiple variables. For example, if we were to have an
array of length 5. That means we could have 5 different values held within the
array. That doesn’t mean we can’t have 4 values within the array with one blank
spot for a future value.

Each value in the array is indexed by a number. In programming, indices begin at
zero so the first number is indexed with the number 0 and the second number is
indexed with the number 1. In Scratch however, the creators make it more nonprogrammer friendly and start indexing at 1. Indexing is the way we keep track
of exactly where a specific value is in the array.

The length of an array in programming is specified by the programmer. The
Scratch creators make things more convenient for us by giving arrays the ability
to dynamically change in size. Unlike arrays in other languages, arrays in Scratch
have no definite size.
Creating an Array in Scratch

To create an array in Scratch, click on the section where we created
variables in session 5. Then create a new list named groceryList. This will be
our first example.

We add items to the list by using the

We delete items from the list by using the
block. We do
this by specifying the index of the item we want to delete. If the block
above were executed, the first item of the list would be deleted because of
the index 1. All other items are then shifted down by one; 2 becomes 1, 3
becomes 2, etc.

We can use the
block to actually use an item from the list.
Here are some other blocks in which it can be used.
block.
Grocery List Example

This example only requires one sprite. We will continue asking the user to
enter an item to add to the list until they enter done. After the user enters
done, have the sprite say each item and delete that item. We will need a
variable to hold our index value. Let’s construct this example.
Project 6.3
 Modify Project 5.1 so that all the values
entered by the user are stored in a list. When
the user enters “done” the program goes
through the array and calculates the average,
then displays it along with the number of
values entered by the user.
Constructed Examples
Example 6.1 Bat script
Example 6.1 Cat script
Constructed Examples
Example 6.2 cat script
Example 6.2 bat script
Session 6 Questions
1.
Which six Boolean operators do we cover?
2.
What is message passing used for in Scratch?
3.
Where are Boolean expressions most commonly used?
4.
You have a program that has an if/else structure, and two variables, X and Y. The
only time you want the alternate block(s) to execute is when both variables equal
10. What Boolean expression could you use in this situation?
5.
In your own words, explain the purpose and structure of an array.
6.
Explain the difference between the broadcast and broadcast and wait blocks.