# An intro to Scratch by kbr18539

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• pg 1
```									An intro to Scratch

Camp
Leaving lots of exploration for the future.

ITEC-I5 Scratch Aug 2009       1
What are our goals?

   Learn some computing concepts.
   Use Octave as computing tool.
   Use Scratch as computing and
multimedia tool.
   Have fun with Scratch creating stories,
games, art.
   Learn some computing concepts.

ITEC-I5 Scratch Aug 2009     2
Who computes?
   Scientists
   Engineers
   Social scientists
   Artists
   FBI agents
   Brain surgeons
   Grandkids
ITEC-I5 Scratch Aug 2009   3
Monday concepts (20 min)
What is an algorithm?
An algorithm for an average and min
An algorithm for sorting

ITEC-I5 Scratch Aug 2009   4
Algorithm for computing the
average of two numbers

   Let the first number be A; say A=12
   And the second number be B; say B=15
   To compute the average _____________
   So the average of 12 and 15 = ________

ITEC-I5 Scratch Aug 2009   5
What algorithm for the average
age of two people?
   Input this
   Do precisely this
   Do precisely that
   Output what

12

15

ITEC-I5 Scratch Aug 2009   6
problem: finding min
   Person B is the “ref” watching A.
   Person A has list of numbered cards, but
cannot look at all the numbers.
   A can look at 2 numbers at a time only
-- one card in each hand.
   A must find the min of all cards.
   Exchange roles, with A the “ref” and B “the
player”.
   HOW TO ALWAYS FIND THE MIN?
ITEC-I5 Scratch Aug 2009       7
Problem: team finding the min
   Person A can look at no numbers!
   No numbers can be seen by A, ever!
   Person B can look at only two numbers on
cards given by person A.
   Person B gives back to A first the card with
the smaller number (if numbers are = then it
doesn’t matter which is given first)
   Person A must find the min and show it to
the instructor WITHOUT EVER SEEING IT
   A and B should switch roles and repeat.
ITEC-I5 Scratch Aug 2009        8
Problem: how to sort?

Use set of cards for each team.
How can A sort using B as
comparer?

ITEC-I5 Scratch Aug 2009   9
Sorting two-person game
   This is a 2-person exercise.
   To solve the sorting puzzle, person A must form an
ordered list of cards, which the instructor will check
for order (this is the OUTPUT).
   Person B can only compare two numbers by seeing
the numbers. Person B gives A the smaller number
card first (precise operations).
   Person A can only ask Person B which of two cards
has the smaller number. Person A never ever looks at
any numbers.
   To solve the sorting puzzle, person A must start from
an unordered list of cards (this is the INPUT).

ITEC-I5 Scratch Aug 2009          10
Sorting by selecting minimum
   Spread out list of number cards
31, 22, 13, 32, 11, 21, 23, 33, 12
   Select the minimum and begin a new list
11
   Select the min of what’s left and add it to the
end of the new list. 11, 12,
   Repeat this process until the original list is
empty and the new list has all the cards.
11, 12, 13, 21, 22, 23, 31, 32, 33
ITEC-I5 Scratch Aug 2009      11
Problem: what is the average age
of our class?

   How do we define this? ..compute this?
   An algorithm computes some
output
from the given input
using precise steps
that a machine can follow

ITEC-I5 Scratch Aug 2009   12
Students use Octave (10 min)

Verify the work and problem
solutions as below [while the

ITEC-I5 Scratch Aug 2009   13
Compute the average of two
numbers using Octave
   sum is a variable;
   sum = 12 + 15                   set its value to
     sum = 27                      12+15
   average = sum/2                average is another
     average = 13.500              variable; set its
value to the value of
sum divided by 2

ITEC-I5 Scratch Aug 2009             14
Octave has many useful ops

   It can operate on numbers -- +, /, sqrt
   It can operate on lists of numbers – min,
max, length, mean
   Use Octave as needed in camp and after!

ITEC-I5 Scratch Aug 2009     15
gKids = 10               13            11           12       13
   >> length(gKids)
Precise steps to
You can use the
   ans = 5         ages of the actual            compute the
group present.                output from the
   >> sum(gKids)                                 input.
The input is the
   ans = 59                                      list of ages; the
output is the
   >> avgAge =
sum(gKids)/length(gKids)                      average of all the
ages in the list.
   avgAge = 11.8000
ITEC-I5 Scratch Aug 2009                      16
sort a list into order
   >> gKids

   gKids = 10    13    11    12              What algorithm does
13
MATLAB use to sort the
   >> sort(gKids)                            list?
   ans = 10     11    12    13    13
What algorithm finds
   >> min(gKids)                             the min?
   ans = 10                                  What algorithm finds
   >> max(gKids)                             the max?
   ans = 13

   >>
ITEC-I5 Scratch Aug 2009            17
There are MANY sorting
algorithms.

Recursively selecting the min is a BAD algorithm
when the list is big. Computer scientists know
many different algorithms, some good for small
and some good for big lists.

ITEC-I5 Scratch Aug 2009        18
Start scratch and let’s go!
(90 min)

   Click on the cat icon
from www.scratch.mit.edu
   Scratch programming
environment comes up quickly
   We will first do simple ops

ITEC-I5 Scratch Aug 2009   19
Goals for Scratch

   Learn about objects and behavior
   Programming motion and sound
   Programming user interaction
   Creating your own story, game,
entertainment piece, or simulation
(starting Wednesday)

ITEC-I5 Scratch Aug 2009   20
PREFACE
   These slides are intended to be used in an
active lab demo and discussion and are
therefore not complete by themselves.
   Scratch has a nice facility for working with
sounds; however, during the intro only the
instructor’s computer should have speakers.
When students get to individual work later,
they can use headsets and microphones.

ITEC-I5 Scratch Aug 2009        21
Simple ops first

   Sprites are objects
   We operate on their color, size, position
   We move them
   We have them say things

ITEC-I5 Scratch Aug 2009    22
Click on the
“Looks”
button at
the top left.

ITEC-I5 Scratch Aug 2009   23
Major components
   At right: the stage with sprite[s] or
objects or actors
   At left: operations and attributes for the
sprites
   Center: scripts or program[s] for the
behavior[s] of the sprites
   Your sprites are actors that you direct
ITEC-I5 Scratch Aug 2009   24
The “hello” script
   Can do it in 57 languages – java, C++,
… Scratch
   Easy in Scratch: select “Looks”
operations and drag the “hello operation”
   Then double click on this “lego block”:
check your sprite behavior at the right

ITEC-I5 Scratch Aug 2009                  25
Make the cat 50% larger

1. Select “Looks” operations
2. Drag the “change size” operator into your
script
3. Click and edit for a 50% change (increase)
4. Double click your one operation script
5. Did your cat sprite get 50% bigger?
ITEC-I5 Scratch Aug 2009            26
Putting a sprite in a location

   Motion ops                                X is +
X is –
   Go to x , y    Y is +
Y is +

   Set x to                             Origin is (0, 0)

   Set y to
X is –                   X is +
   Glide to        Y is -                   Y is -

ITEC-I5 Scratch Aug 2009                  27

Try a “move op” to see changes.
Try “rotation ops” as well.
Choose “Motion” ops
Click on “attitude variables”

ITEC-I5 Scratch Aug 2009                      28

“glide to” operation
glide to (200, -100)

ITEC-I5 Scratch Aug 2009              29
Scripting a sequence of ops

   Do ops in the following order by
dragging operation blocks into a single
connected block
   Say hello
   Move 200 steps forward
   Grow 50% bigger
   Make the “meow sound”
ITEC-I5 Scratch Aug 2009     30
Result of 4 operation script

ITEC-I5 Scratch Aug 2009   31
Some new operations
•color
change
(Looks)
• wait
(Control)
• move
(Motion)

ITEC-I5 Scratch Aug 2009               32
Exercise: write a script to
   Make the cat move along a square path
   Say “hi” at all four corners
   Wait 3 seconds at each corner
   Change color at all four corners
   Double size when back to the original
starting location.
   Say “That’s all folks” when done.
Show an instructor that you have achieved this.

ITEC-I5 Scratch Aug 2009   33
Using variables

   Script might have to remember
information
   How far is the cat from the dog?
   How fast is the rocket moving?
   Where did the user click on the stage?

ITEC-I5 Scratch Aug 2009    34
Let’s implement an algorithm
to average two numbers
   Make a
variable
“number1”
(click and
drag and
set)
   Make
another
one
“number2”
ITEC-I5 Scratch Aug 2009   35
Compute average first as sum
   Make variable
average
   Drag a “set
operation” to script
area
   Drag a + operation
   Drag variables
number1 and
number2 to
parameters
   Click to execute
ITEC-I5 Scratch Aug 2009   36
Average script as 4 operation
sequence.

Change the two
numbers and click
the sequence to
execute the block
again.

ITEC-I5 Scratch Aug 2009                   37
Elements of Scratch: objects
   Colors
   Sounds                    We want to use Scratch
to program with
   Locations in 2D space     multimedia.

   Sprites
   Costumes
   Variables (to remember the state of things)
   Events: that are broadcast for communication

ITEC-I5 Scratch Aug 2009       38
Elements of Scratch: control
   Sequence of operations
   Loops or repetition
   Detecting events (key or mouse
pressed, sprites overlapping each other,
sprites hitting edge of stage, sensor
giving value)

ITEC-I5 Scratch Aug 2009   39
Loops are for repeating sequences

   Bethoven’s 5th: bump-bump-bam;
bump-bump-bam; …
   Running the bases in baseball or driving
around the block.
   Milling around waiting for some event
to occur (in the aquarium example, the
fish sprites mill around forever)

ITEC-I5 Scratch Aug 2009   40
Loop constructs in Scratch
Try these!
   Repeat N times
   Repeat forever
   Repeat forever if
some condition
exists (suppose I’m
a sprite wandering
question)
ITEC-I5 Scratch Aug 2009    41
Exercise: use a loop to

   Move sprite around the 4 corners of the
square
   Wait 2 seconds at each corner
   Say something at each corner
   Double size when done
   Change color when done
Show an instructor that you have achieved this.

ITEC-I5 Scratch Aug 2009     42
   Choose the “file” option at the top of
the window
   Choose “open”, then “examples”
   Choose “stories”
authors instructions
   Click OK, wait for load, click green flag

ITEC-I5 Scratch Aug 2009      43
   How many actors (sprites)?
   What is the role of the girl?
   How are the answers you give 'remembered'
and then used in later actions?
   What is the role of the little whale?
   What makes the little whale flip around?
   What makes the big whale spout?

ITEC-I5 Scratch Aug 2009      44
Tuesday: computing distances
and directions (30 min)

   How to go from here (A) to there (B)?
   Computing distance
   How useful in Scratch or navigation,
etc.?

ITEC-I5 Scratch Aug 2009   45
Angles are important
180 degrees
degrees                            (straight angle)

degrees

90 degrees
(right angle)
360 deg
complete
circle
http://www.mathsisfun.com/geometry/degrees.html
To see animation of angle measure.
ITEC-I5 Scratch Aug 2009             46
Using a protractor to measure angles

ITEC-I5 Scratch Aug 2009   47
Measure some angles with a
protractor

C                          Q

P

B
A
ITEC-I5 Scratch Aug 2009           48
The treasure is buried under an oak tree
42 degrees to the right of the line
between the statue and post

post

42
deg

statue

ITEC-I5 Scratch Aug 2009   49
Right triangles are special
What is the distance
B                                  from point A to point B?
Do this on graph paper.
Draw the triangle ABC
3                                 with side CB = 3 and
side CA = 4.
Cut a “ruler” out of
C                    A        graph paper to measure
4
the side AB.

ITEC-I5 Scratch Aug 2009                         50
Measure another triangle
C   12
B

5

A person walks north 5 miles from A to C
A
and then east 12 miles from C to B for a
total of 17 miles. How far would the
person walk going directly from A to B?
(Do this on a scale drawing.)

ITEC-I5 Scratch Aug 2009                51
Pythagorean theorem
   Long ago Pythagorus discovered                  a
b

that in a right triangle, with
short sides a and b, the long                            c
side c is such that c times c is                    hypotenuse
equal to axa + bxb
The scare crow said it in the
cxc=axa+bxb

Wizard of Oz: “In a right                 This is of great use in
triangle, the square of the               engineering and navigation.

hypotenuse is equal to the sum
of the squares of the other two
sides”.
ITEC-I5 Scratch Aug 2009                       52
Shortest distance from A to B?
C
25   B

   It’s 65 miles
going north from
40
A to C and then
east from C to B.
   What if we go the
direct path from
A to B along a                   A
straight line?
ITEC-I5 Scratch Aug 2009            53
Scratch script:
distance A to B
   Pick point A
with the mouse
   Pick point B
with the mouse
   Compute the
distance from A
to B
   Test separately!
ITEC-I5 Scratch Aug 2009   54
Test the first part separately

We ask the user to click the mouse. We use variables Ax and Ay to save the
mouse click position, then move our sprite there (156, 82) and change its color.

ITEC-I5 Scratch Aug 2009                         55
Test the second part (point B)
After double
clicking on our
second code
segment, our
sprite has
moved from A,
where both x
and y were
positive, to B
where both x
and y are
negative. Our
variables are
displayed.
B = (-99,-51)
ITEC-I5 Scratch Aug 2009                     56
Test the final distance comp.

Executing the above code, results in
the stage at the right. Our sprite has
moved a distance of 287.6 in going
from point A to point B. (Extra: In
what direction did it move?)

ITEC-I5 Scratch Aug 2009   57
exercises
   Solve the distance             Find a distance on
problem by                      the earth using your
changing the script             actual GPS reading
to not use mouse                from your
input.                          geocaching unit.
   Just set (Ax, Ay) to
(0,0) and (Bx, By) to
(25, 45) and
execute!
ITEC-I5 Scratch Aug 2009            58
Shortest distance from A to B?
C
25             B

   Solve the distance
problem by
changing the script             40
How far
to not use mouse                                A to B?
input.
   Just set (Ax, Ay) to
(0,0) and (Bx, By)
to (25, 45) and
A
execute!
ITEC-I5 Scratch Aug 2009                      59
Using Octave to solve it
(You can use a scale drawing too.)
   >> b = 40                 % declare variable b to have value 40 miles

   b =40

   >> a = 25                 % declare variable a to have value 25 miles

   a =25

   >> cSquared = a * a + b * b      % compute the square of the hypotenuse (long side)

   cSquared =2225

   >> c = sqrt(cSquared)            % we need the square root of 2225

   c = 47.1699

   >> c * c                          % the square root times itself must be 2225 it is!

   ans =2225               % the direct route is about 47 and 1/6 miles; much less than 65

ITEC-I5 Scratch Aug 2009                                   60
Tuesday: Scratch more

More about control in Scratch. How to
control sprite behaviors?
How to control user interactions?

ITEC-I5 Scratch Aug 2009   61
Conditions can be checked
   Do something if
sprite k hits sprite m
   Do something if a
certain key is
pressed
   Do something is
some variable takes
a certain value

ITEC-I5 Scratch Aug 2009   62
story
   Using mouse
   Entering a character
   Asking the user a question

ITEC-I5 Scratch Aug 2009   63
Sprite follows the mouse

Try changing
the number of
steps or the
wait time.

ITEC-I5 Scratch Aug 2009                   64
Sprites can interact with each other
   Can detect when colors overlap in space
   Can detect when sprites bump into edge of
the stage
   Later: See “bouncing balls” example under
Simulations under Examples
Interact with this simulation
Check out the rather complex scripts

ITEC-I5 Scratch Aug 2009       65
Simple communicating sprites
   Dog talks to cat in “forever” (infinite) loop
   Dog sprite sends a message to the cat sprite
   Cat sprite says “hello” and gives the floor back to the dog

ITEC-I5 Scratch Aug 2009                  66
Cat sprite reacts to dog

ITEC-I5 Scratch Aug 2009   67
exercises
   A) modify the above scripts so that a global
variable is used to count how many times the
dog says “Hello”
   B) change the control so that when “Hello” is
said 10 times by the dog, the script stops
with the dog saying “That’s all folks”.
   C) change the scripts so that each sprite
moves to a random position on the stage
before saying “Hello”

Show your work to an instructor when you get finished with each part .

ITEC-I5 Scratch Aug 2009                      68
Check out the break dance
example
   Open examples; music and dance;
break dance
   How does break dancing begin?
   What are the roles of the sprites?
   What events are in the scripts?
   What should happen when the boom
box is clicked?

ITEC-I5 Scratch Aug 2009   69
Wednesday Goals
   Computing directions or headings in Scratch
or Octave (30 min)
   Studying Scratch examples done by others
(50 min).
   Design a project: story, game,
simulation, or entertainment/art to do
and discuss with instructor.)

ITEC-I5 Scratch Aug 2009     70
Wednesday: practical trig

1) Scale drawings
2)    Angles and distances
3)    Using trig functions

ITEC-I5 Scratch Aug 2009   71
Heading from point A to point B
Suppose my GPS
(22, 10)
                                                  *

says I am at x=10;
y=5                        N

   My treasure is at
x=22 and y=10
   What is my heading         **              E
and how far do I       (10, 5)

travel?
ITEC-I5 Scratch Aug 2009                      72
Make a scale drawing and
measure the angle and distance.

ITEC-I5 Scratch Aug 2009   73
Tangent of an angle
B
Do you see THREE similar
triangles? All share the vertex
and angle A. Are there more?                  B2

30
B3                            20

10
20

A                     C3                      C2           C
60

ITEC-I5 Scratch Aug 2009        74
Tangent of an angle
10/20 = 0.5 = ½                                                      B
20/40 = 0.5 = ½
30/60 = 0.5 = ½                                         B2
15/30 = 0.5 = ½ where is this
triangle?                                                            30
5/10 = 0.5 = ½ where is this    B3                           20
triangle?
10
20

A                      C3                      C2           C
60

ITEC-I5 Scratch Aug 2009        75
Tangent of an angle
B
If C2 is distance 47 from A, how
long is side B2-C2? _____
B2
If B3-C3 has length 12, how
long is A-C3 ? _______
30
B3

12

A                       C3                         C2   C
60

ITEC-I5 Scratch Aug 2009        76
Tangent of an angle is its RISE
divided by its RUN
   Carpenters use the
terms RISE and RUN
to measure how           Tangent of angle =
steep a roof angle       rise/run
is.
   All roofs making the     = “slope”                  rise
same angle, have
the same ratio
RISE/RUN
   Size of the house                             run
doesn’t matter!
ITEC-I5 Scratch Aug 2009                77
Using the hypotenuse and one leg
   10 more minutes and a few slides                                    B
needed to do this
   >> rise = 6
   rise = 6                                                 Hyp
= 10
Rise
   >> hyp = 10                                                         =6
   hyp = 10

   >> angleA = asind(rise/hyp)                                         C
A
   angleA = 36.8699                               sin A = rise / hyp
   >> sind (36.8699) %in degree

   ans = 0.6000

ITEC-I5 Scratch Aug 2009                  78
What’s the distance from A to C?
   Use the cosine to
   Use the Pythagorean               compute it.
Theorem to compute               cosine A = run / hyp
it.
   run = hyp * cosine A
   10^2 = b^2 + 6^2

ITEC-I5 Scratch Aug 2009           79
Problem: angles and sides
   Draw any right triangle
   Measure one side
   Measure one angle
   What is the missing angle? ______
   What are the missing sides? ____ ____
   Check the Pythagorean relation ______
   Check the sind and cosd relations ___ ___
ITEC-I5 Scratch Aug 2009   80
Angles in Octave
   >> rise = 20

   rise =20

   >> run = 40

   run = 40

   >> angle = atand(rise/run)   % function or operator atand gives your angle in degrees

   angle = 26.5651

   >> rise = 30

   rise =30

   >> angle = atand(rise/run)

   angle = 36.8699

ITEC-I5 Scratch Aug 2009                                  81
Angles in Scratch

ITEC-I5 Scratch Aug 2009   82
Computing the angle

Execute each of the 3
code segments.
Slope is 0.5 or ½
degrees.

ITEC-I5 Scratch Aug 2009                     83
Heading from point A to point B
Suppose my GPS
(22, 10)
                                                 *

says I am at x=10;
y=5                       N

   My treasure is at
x=22 and y=10
   What is my heading        **              E
and how far do I      (10, 5)

travel?
ITEC-I5 Scratch Aug 2009                      84

   >> east = 22-10 % miles
   east = 12
   >> north = 10-5 % miles
   north = 5
   >> angle = atand(east/north)
   angle = 67.3801 % head 67 degrees east of north

ITEC-I5 Scratch Aug 2009          85
Using a different triangle
   >> rise = 10-5 %miles
   rise = 5
   >> run = 22-10 %miles
   run =12
   >> angle = atand(rise/run)
   angle = 22.6199 % head about 23 degrees north of east

ITEC-I5 Scratch Aug 2009       86
Exercises in computing angles
   A) Repeat the                               A

above Octave                      30
20
solutions in             C
Scratch                  24
C
   B) Find the                         A
45   36
angles A at     50
the right
(angle C=90)                  A                      C
ITEC-I5 Scratch Aug 2009                 87
Exercises in Scratch (50 min)
   Do some of the triangle exercises – any of
them from geocaching?
   Study the aquarium program to see how the
various fish behave; change fish color and
observe the change; change its speed and
observe; what other behavior changes can be
   Students study and modify existing Scratch
programs; students imagine creating a
program of their own.
ITEC-I5 Scratch Aug 2009    88
Student Scratch project

Pick a project: story, game, simulation,
or entertainment/art to do by Friday.
discuss with instructor.)

ITEC-I5 Scratch Aug 2009     89
Experiment with Scratch as
time permits Thur and Fri

   Try your own scripts: make moves, sounds,
interactions in simple cases
   Try the examples and learn what makes them
work