Page 180 - Learn To Program With Scratch

Script for the Guard sprite
Guard
Continue to move left and right
until the Player sprite comes close.

Start Chasing the Player Sprite When Player is within
a distance of 100, start Player
chasing the player.

Figure 7-3: A simple example showing the repeat until block in action

try it out 7-1
Chase .sb2 Open the application Chase.sb2 and run it . Use the arrow keys to move the Player
sprite close to the Guard to see the chase in action . How would you change the
test condition to unleash the Guard sprite if the y-position of the Player sprite goes
outside a certain range (for example, –50 to 50)? Implement this change to check
your solution .

Building a forever if Block
Infinite loops are useful in a lot of programming situations. In the previ-
ous chapters, for example, you used the forever block to play background
music, and you animated sprites by changing their costumes continuously.
The forever block is an unconditional infinite loop because it doesn’t have a
test condition that controls the execution of the commands inside it.
You can easily change that, however, by nesting an if block inside a
forever block to create a conditional infinite loop, as shown in Figure 7-4.
The test condition of the if block is tested at the beginning of every itera-
tion, and its commands only execute when the test condition is true. Note
that since the forever block is supposed to execute forever, you can’t snap
command blocks after it.

False
If the test condition is false,
then check again.
Test Condition
True Command 1 If the test condition is true,
... execute these commands. Then
check the condition again.
Command N

Notice that there is no bump at
the bottom, so you can’t snap
any blocks here.
Figure 7-4: You can create a forever/if loop by combining a forever
block with an if block .

158 Chapter 7
www.it-ebooks.info