Programming with Scratch

1
Programming with Scratch

• Summer Computing Workshop

2
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.

3
Session 6
4
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.

5
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 lt 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!

6
Boolean Expressions

• In this example, 5 isnt 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.

7
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.

8
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.

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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 isnt 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.

10
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 its
  script after the receiving Sprite is finished
  with its script.
• Passing messages is a way two sprites can
  communicate. Its a way to start another
  Sprites script inside of a script.

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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, lets construct this short example (see
  example 6.2 on the Constructed Examples page at
  the end of this section)

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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 neednt
  be too detailed).

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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 doesnt mean we cant 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
  non-programmer 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.

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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
  block.
• 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.

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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. Lets construct this
  example.

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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.

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Constructed Examples
Example 6.1 Bat script
Example 6.1 Cat script
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Constructed Examples
Example 6.2 cat script
Example 6.2 bat script
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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.