Computer Science 1000
Algorithms II

Multiple Sprites
Scratch allows us to have multiple sprites
on the screen simultaneously
 three ways:

draw your own
 choose from file (most common)
 let Scratch choose

Draw your own
Choose
from file
Let Scratch
choose

Multiple Sprites
note that each sprite has its own code
 to see the code for a particular sprite, click
on its image in the bottom right pane of the
screen


Multiple Sprites

note that double-clicking on one sprite’s
code does not affect the other

Multiple Sprites
what if we do want both sprite’s code to
execute simultaneously?
 Solution: use


when the green flag is clicked, it will cause both
code blocks to start executing simultaneously

Event-Driven Programming


the previous is an example of event-driven
programming
code block(s) occur in response to a user event


computer today are almost exclusively event-driven



in this case, the user clicking the green button caused
both sprites’ code to be executed
mouse clicks
keyboard events
other examples of events in Scratch exist

we will consider another shortly

Loops

it is very common in any computer program
to repeat operations

Loops

rather than duplicate code,
Scratch allows us to specify
code blocks that should be
repeated

from our previous example, it
would be nice to ask Scratch to
repeat these code blocks 4 times,
rather than duplicating the blocks
multiple times

Repeat Loop (Controls)
the repeat loop repeats the code block that
is placed inside of it
 the number of times it is repeated depends
on the value that is in its input

The code in this
loop will be
repeated 10 times.
The code to be
repeated is placed
in here.

Example: Convert our previous code for
moving the sprite in a square, using a
loop to avoid duplicating code

Example: Write a program that asks the
user for a number, and moves the sprite
by that distance. This process should be
repeated three times. Use a loop to
avoid duplicated code.

Loops
suppose you do not know in advance how
many times you wish to loop
 Solution: use a forever block

the forever block will continue to loop
indefinitely, until the user clicks the stop button
in the top right corner
 e.g. convert our previous example, so that the
cat walks continuously around the square until
stop is pressed


Event-Driven Programming

the previous is also an example of code
responding to an event


the wait block completes execution in response
to one second passing
we can also wait for other events to occur
mouse clicks
 keyboard events
 etc


Event-Driven Programming
instead of using a wait 1 sec, block, we can
use a wait until block
 this block takes an event as input, such as
one from our previous slide
 example: convert our previous program, so
that the cat only moves/turns in response to
the mouse button being pressed


Event-Driven Programming
in your assignment, you are asked to
perform an action with a sprite in response
to another sprite touches it
 Hint: use the structure from the previous
example, but instead of a mouse down
block, use the touching block
