INTRODUCTION TO THE
SCRATCH PROGRAMMING
ENVIRONMENT
What is Scratch?
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Developed by the “life-long kindergarten” group at
MIT
Simple, “code-free” style that lets you write
programs by visually combining ‘blocks’
Built-in graphics and sound
Share Projects online
Made for beginners, but ‘expert-level’ programs
are possible
Main Scratch Window
Start (optional) and
Stop Buttons
Block Types
(more to follow)
Scripts tab: add
commands for
Sprite1’s actions
Performance Area =
Stage (background) +
Sprites (characters)
Pick a Sprite to program
Script Inventory (block palette):
drag new commands from here
to the scripts tab
Block Shapes
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Trigger:
Statement
Ending Statement
Boolean (true/false) value
Numeric or Text value
Block Types
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Blocks are sorted into categories:
 Motion:
move a sprite
 Looks: change costume/color
 Sound: play instrument, song
 Pen: draw lines on stage
 Control: have sprite make ‘decisions’
 Sensing: See/hear/touch objects
 Operators: mathematics and text
 Variables: give sprites memory
Motion Blocks
Looks Blocks
Sounds Blocks
Pen Blocks
Control Blocks
Sensing Blocks
Operator Blocks
Variable Blocks
A Simple Manual Program
Click on the block
to make the sprite
walk 10 steps
The GO Button
This trigger block will
make any attached
script run when the
green flag is clicked.
This is a good way to
start multiple scripts at
the same time.
Add some control
Question: How many
steps will Sprite1
take?
About Loops
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Repeat loop: simplest type
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Whatever is inside, do that
same thing as many times as it
says in the top of the block
Equivalent (but tedious!)
solution:
Copy/paste ‘move 10 steps’
30 times
 Not feasible if you want to
repeat something 1000 times
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We’ll learn about the other
two loop types later:
Forever
 Repeat until
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Stuck!
Our Sprite walked as far as he’s allowed and is now stuck!
Here are some possible solutions:
•Click on the sprite and move him back to the center (MANUAL strategy)
•Some sprites can walk off and disappear!
•Add a script that walks backward 300 steps (UNDO strategy)
•Use Motion blocks to ‘transport’ Sprite1 to a fixed location (RESET strategy)
Motion Explained
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The stage is set on an X-Y plane
‘Move 10 steps’ moves about 10
steps on the grid
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But you can turn to move in
different directions
‘Go to x: 0 y: 0’ puts Sprite1 in the
center of screen
Lower-right shows coordinates of
mouse pointer
Xposition, Yposition: current
location of a sprite
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Y
Can be used for boolean testing, or
for drawing shapes in certain
locations
X
What’s Going On?
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Scratch tends to ‘go with the flow’
Whatever script you run, Scratch will run it as best as it can
 After the script runs, the effects remain
 Can lead to unintended outcomes
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This is actually kind of annoying
Testing must always have the same initial situation
 Games always start in level 1
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We’re used to having a ‘reset’ button when our
programs develop problems
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Returns us to the initial error-free configuration
Scratch doesn’t have one, but we can make one
Green-Flag Reset
Reset Script: return to whatever initial
situation you want
•Sprite Positions (motion)
•Pen Markings
•Variable Values (more later)
Green-Flag Reset:
•Upside: Green flag acts as both
‘start’ and ‘reset’ button
•Downsides:
•Other green-flag scripts will
start running right away
•Human must force reset
Keyboard Reset
Keyboard Reset: run reset script when a certain
key is pressed (can choose any key)
•Upside: Good when testing non-green flag
scripts (so the other green flag scripts don’t
run)
•Downside: A human is still in charge of resetting
Subroutines
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Subroutine:
A script that can be run by itself to some useful effect
 Can be executed independently by the computer or initiated
by a human
 Re-use Property: the more a subroutine can/will be re-used,
the more valuable it is
 When combined with abstraction, subroutines make
programs feel simpler
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Real-world examples:
Cooking: “Separate the yolks from the whites”
 Driving: “Take Exit 9”
 Others?
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Reset Subroutine
Subroutines have a:
•Definition: The actual subroutine script, starting with a
‘When I receive <name>’ trigger block
•Call: Subroutines do nothing unless called upon to
execute, via a ‘broadcast <name> (and wait)’ command
•Calls can be attached to human input (above) or
computer control (next slide)
Computer Controlled Subroutine
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Modified walking script:
 Repeating
100 times,
 Move 10 steps
 If I reach the edge of
the stage:
 Call
 After
the reset subroutine
I’ve done the
above 100 times, call
the reset subroutine one
more time
Summary
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Scratch: Easy to get started programming, no
experience required
Different block types: allow us to modify a Sprite’s
behavior in different ways
Script: One or more blocks fastened together
 Complex
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combinations lead to complex behavior!
Loops: Abstraction for repetitive processes
Subroutines: Allow fast re-use of a script by humans
or the computer