Transcript Session 3 (SS3)

Summer Computing Workshop
PROGRAMMING WITH SCRATCH
Session 3
Conditional Branching
 Conditional branching is used to alter the normal flow of execution
depending on the value of a Boolean expression
 The use of conditional branching allows programs to become much more
flexible
 The primary block associated with this behavior is the if block
 The triangle shaped area is called the predicate or conditional part of the
structure. This is what determines if the consequent (internal) blocks are
executed
 The triangle shaped blocks in Scratch always represent a true or false value.
This “typing” is not as definite in Scratch as in other languages, but it is
there nonetheless and certainly prepares you for the much more strict
typing of languages you will encounter in the future.
Conditional Branching – If Block
 Let’s take a look at an example
 Starting at the top, the sprite will move 100 steps in whatever direction it’s
pointing
 Then the computer asks itself the question “Does 2 equal 3?” Obviously it
does not so the green comparison block evaluates to false
 Since the predicate is false, the turn block is skipped
 The normal flow of execution is then resumed and the second move block
is encountered
 As a result, this script causes the sprite to move a total of 200 steps
Conditional Branching – If Block
 What if we change the numbers so the comparison returns true?
 Since 2 does equal 2, the comparison returns true and the turn block is
executed
 Then, as before, the normal flow of execution resumes and the remaining
blocks beneath the if block are executed
 Therefore, this snippet of code moves the sprite 100 steps, takes a 90
degree right turn, and then travels 100 more steps
Conditional Branching – If/Else Block
 The if block isn’t the only block capable of branching based on a condition
in Scratch. The if/else block is equally, and sometimes even more, useful.
 The structure is very similar to the if block with the only difference being
the addition of the “else” or alternate area
 The top part of the if/else block behaves the same way the if block does,
but while the blocks in the consequent area are executed if the condition is
true, the blocks in the alternate (else) area are executed only if the
condition is false
 By definition, it is impossible for blocks contained in both the consequent
and alternate areas to be executed. Once the decision is made, it is strictly
one or the other
Conditional Branching – If/Else Block
 Let’s look at an example of if/else block behavior
 The sprite moves 50 steps then reaches the if/else predicate
 The comparison evaluates to false since 5 is NOT less than 2
 Due to the false value of the condition, the turn 45 block is skipped and the
turn 135 block is executed instead
 To make the sprite movements more visible we can use the pen tool to
draw a line as the sprite moves
The Pen Tool
 To make the sprite movements more visible we can use the pen tool to
draw a line as the sprite moves
 Click the pen category in the upper area of the blocks palette
 Drag the
script
block to the scripts area, but don’t link it to the existing
 Click the
block, you should see a colored rectangle appear near the
lock symbol in the sprite info area
 Now, whenever the sprite moves, it’s path will be marked by a line. The size
and color of this line can be altered if you want, but the default settings will
work for now
 After a bit of drawing, the stage can become quite colorful. When that
happens, the
block can be used to erase the lines
Conditional Branching – User Input
 In the previous examples, the condition was unable to change. The
comparison would evaluate to either true or false and stay that way forever
because the relation of numbers never changes. Let’s modify the previous
bit of code so the condition changes based on input from the user
 With this small change, the true flexibility of the if/else structure begins to
emerge
 Now, every time the script is ran, a different branch can be taken. The
number of choices is limited to two in this case, but through the power of
nesting, this limitation is no longer an issue
Conditional Branching – Nesting
 What if the question asked had more than 2 valid answers? A single if/else
block is unable to handle a third result, and even if it could, there is no way
to select it since all Boolean expressions give a true or false answer
 This problem is addressed with a technique know as nesting. By placing
if/else blocks inside of other if/else blocks, powerful decision trees can be
created to handle any number of situations
On Your Own
 Create a program that asks the user which direction the sprite should
move, then prompt for a distance to move in that direction
 Once you have completed the above project, modify it so the user is asked
the questions twice in a row instead of just once (get direction, ask
distance, move, ask direction, ask distance, move)
Session 3 Questions
1. How might the flow of execution be affected by a conditional branch?
2. What part of an if/else control structure determines which sequence of
blocks is executed?
3. The two different types of input are single-character input and ______
input.
4. Asking for a name is an example of what kind of input?
5. The expression in the predicate (conditional) part of an if/else structure
evaluates to a _____ or _____ value.
6. What part of the structure would execute if the predicate value was false?