1. Introduction to Object-Oriented Programming
Scratch the Cat

2. Object Oriented Programming
Writing computer programs
Based on Objects Instead of Actions
Based on Data Instead of Logic

3. Traditional Programming
Input Data
Process the Data using Logic
Output the Data
Example: Add 2 Numbers
Get Number 1
Get Number 2
Process the Data
Add Number 1 + Number 2
The Answer is: _______

4. Object Oriented Programming
Focus is on the Object we want to Manipulate/Change
Person
Animal – (Scratch the Cat)
Building
Buttons / Icons
Identify What Objects You Need
How do the Objects Relate to Each Other

5. Objects All Objects have 3 Parts Properties Methods Events
What the Object looks like
Methods
Things the Objects can do
Events
What activities (by the user or by the computer) that the object responds to

6. Our Programming Example
Dog vs. Cat
The cat and dog will walk up to one another and say hi (bark and meow)
Object Oriented Programming
What objects will we need? (Objects and Properties)
What do the objects need to do? (Methods)
When do they need to do these things? (Events)

7. Let’s Get Programming Start Scratch the Cat
Double Click on the Scratch Icon

8. Scratch Interface
Properties
Methods and Events
Display
Code
Objects

9. Objects What Objects do You Need? Cat – Already Exists (Scratch) Cat
Dog
Cat – Already Exists (Scratch)
Set Properties for Scratch

10. Properties – Edit Cat Properties
2) Click Costumes
3) Edit Costume1
1) Click on Scratch

11. Smaller
Rotate
Flip
Bigger
Tools
Colors

12. 2) Use Mouse to Erase Scratch
Edit Cat Properties
Change the Animal
Erase the Current Cat
2) Use Mouse to Erase Scratch
1) Click Eraser

13. Edit Cat Properties
1) Import
2) Click Animal Folder
3) OK

14. Pick New Animal
Scroll
1) Pick a Cat
2) OK

15. Edit Properties
Change the Size
Make Smaller
?????

16. Add Sound Properties
Sounds

17. Add Sound Scratch already has One Sound Add Another Sound MEOW Import
Animal  Cat  OK
2) Animal
3) Cat

18. Add A 2nd Object - DOG
Add New Sprite (Object)
Add New Sprite

19. Add a 2nd Object - Dog
Look Familiar – What Should We Do??

20. Edit the Object Properties
Resize – Make Smaller
Rotate if Needed So Dog will Face the Cat

21. Add Sound New Objects will not have any existing sounds Add a Sound
1) Import
New Objects will not have any existing sounds
Add a Sound
Import
Animal  Dog1  OK
2) Animal
3) Dog1

22. Add Events Events are the Actions Each Object will Respond to
Can be Computer Generated or User Generated
Cannot Respond to EVERY action
1st Event in this Program
Starting the Program
When the User Clicks the Green Flag

23. Events Responses to Actions 1st Event – Starting the Program
2) Make sure Scripts is Clicked
Responses to Actions
Can be Computer Generated or User Generated
We need to tell the Objects (Cat and Dog) what Actions to Respond to
They cannot respond to everything
1st Event – Starting the Program
When the Green Flag is Clicked
1) Make sure CAT is Clicked
Move Objects around using Mouse to look like this.

24. Event – When Flag Clicked
1) Control
Flag Event is in the Control Section
Using Mouse – Drag
to the Script area
2) Drag Flag to Scripts Area

25. Methods – When Flag Clicked
What do we want the Cat to do when the Flag is Clicked
Say Meow
Make a Meowing Sound
These are Methods
Add Method to Code Window (Attach to Event)

26. 2) Drag “Say “Hello” to Code
Method – Say Meow
Find Method
LOOKS section
Drag “Say HELLO for 2 Secs” to Scripts area
We don’t want Cat to Say “Hello”
Boxes in Methods are things we can change
1) LOOKS
2) Drag “Say “Hello” to Code

27. Method – Say Meow Click on “Hello” box Click on “2” box
Type “MEOW”
Click on “2” box
Type “1”
Test – Click the Green Flag (Code a Little – Test a LOT)

28. Add Next Method – Play Sound
Add Sound
SOUND Section
Drag “Play Sound Cat” to Scripts Area
Change “Cat” to “Meow”
Test
1) Sound
2) Drag to Event

29. Variables
A variable is a placeholder for a number that we don’t know what it is yet, or a number that can change
It has a name
We assign it a value when we are ready
We can change the value when we need to
Let’s include a variable to count how many times the cat meows

30. Make a Variable - Counter
Click the “VARIABLE” section
Click on the “Make a Variable” option
Give the Variable a Name: COUNTER
Make sure “For all Sprites” is clicked OK
1) Variables
2) Make a Variable
3) Give a Name

31. Variable Shows Up
Variable Choices

32. Give Variable a Value We need to set a Value for our Counter Variable
When all variables are created in Scratch they are given a value of Zero
We need to set it to zero so that it resets every time we start over and hit the green flag

33. Give Variable a Value
Drag “Set Counter to 0” method over to the Scripts Area
Put just under
Drag to Code List

34. Update Value
We want to change the value of the counter every time the cat meows
What options do we have to do that??

35. Update Value
Drag “Change Counter by 1” method over to the Scripts Area
Underneath other instructions

36. Repeating Actions How many times to we want the cat to meow?
As long as we run the program?
We do not want to have to add more instructions for each time
Programmers use something called a LOOP to repeat
Let’s have our cat meow 15 times

37. Adding a Loop Click on “CONTROL” section Find the Loop instructions
Drag the instructions to Scripts Area
Need to put above the cat meowing to repeat this action
2) Put Here
1) Repeat

38. Adding a Loop Change the “10” value to “15” to loop 15 times Test It
Watch Counter
We could use the “Forever” loop to loop as long as the program is running
Change to 15

39. Events - Let the Dog Respond
The dog has to know that the cat has meowed
Cat has to cause a computer generated event
Dog can respond by barking
Broadcast an Event
And wait for a response

40. 1) Drag Broadcast and Wait
Create an Event
Drag “Broadcast” to the Scripts Area
Click on the Down Arrow
“NEW”
Name the Event “CatMeow”
1) Drag Broadcast and Wait
2) Arrow - NEW

41. Have Dog Respond Click on the Dog Sprite
No Scripts yet for the Dog Object

42. Have Dog Respond Dog must “listen” for the CatMeow event
Drag “When I receive CatMeow”
to the Scripts Area
When the Cat Meow and the event “fires” the Dog will hear it
What should the dog do???
Bark

43. Dog Bark “LOOKS” Section Drag “Say Hello for 2 Secs” to Scripts Area
Change “Hello” to “Woof”
Change 2 secs to 1 sec
Test It
1) Drag to Scripts Area
2) Change to Woof and 1 Sec

44. Dog Bark “SOUND” Section Drag “Play Sound” to Scripts Area
Click on Down Arrow
Select “Dog1” sound
Test It
1) Play Sound
2) Dog1 Sound

45. Dog Chases Cat Dog moves toward the Cat Cat moves away from the Cat
Move 20 steps
Cat moves away from the Cat
Move 10 steps
Add Code for Each Object to Move

46. Dog Chases Cat Make sure the Dog Object is Selected “MOTION” Section
“Move 10 Steps”
Change to 20 Steps
1) Dog
2) Move 10 Steps
3) Change to 20

47. Cat Runs from Dog Click Cat Object Motion Test It “Move 10 Steps”
Put Under “Broadcast”
Test It
1) Pick Cat
2) Move 10 Steps
3) Put into Scripts

48. Clear Method Return the Objects to their Starting Place to Run Again
Responds to an Event
Move Dog to Location (-125, -30)
Move Cat to Starting Location (135, -15)
Uses Pixel numbers – (0,0) is the middle of the screen

49. Clear Method
2) CONTROL
1) Cat
3) When I Receive Event

50. Create the Event Drag to the Scripts Area Click on the “CatMeow” Box
NOT part of other instructions
Click on the “CatMeow” Box
Click “NEW” to create a new Event
Name Event “CLEAR”

51. Move the Cat Locations use an X and Y values for Height and Width
“MOTION” Section
Set X to 0
Set Y to 0
Change X to 135
Change Y to -15
1) Set X to 0
2) Set Y to 0
3) Change Values

52. Broadcast the Clear Event
Make the Clear Event Happen
Beginning of Program
1) CONTROL
2) Broadcast “CatMeow”

53. Broadcast the Clear Event
CONTROL Section
Drag to Top – Right below
Click on “CatMeow”
Change to “Clear” Event
1) Below Flag Click

54. 3) When I Receive “CatMeow”
Move the Dog
Uses the Same Event
Still Need to “listen” for it (Already being Broadcast)
2) CONTROL
1) Click Dog Object
3) When I Receive “CatMeow”

55. Move the Dog CONTOL Section Drag to the Scripts Area
Not part of other instructions
Click on the “CatMeow” Box
Click “Clear” event

56. Move the Dog “MOTION” Section Set X to 0 Set Y to 0 Change X to -125
Change Y to -30
1) Set X to 0
2) Set Y to 0
3) Change Values