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An intro to programming concepts with Scratch

Session 3of 10 sessionsRepetition and

variations

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Session 3 goals

• Learn how to program with repetition• Changing costumes • Controlling execution (behavior) using

special keys

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The sneezing cat script

• We repeat 20 times

Say “AAAHHHH” for a short time Then grow the cat 5% bigger

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Exercise: bring the cat size down to normal

Repeat 20? times Briefly say “CCHHEEEW” Then shrink the cat’s size by 5%

Repeat N times and repeat forever are in the Control menu

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Program to compute the average

An optional piece: to omit this, proceed to slide #18

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A better average algorithm ( do blackboard work first)

• Let’s average N numbers• Say 5 numbers { 12, 14, 15, 10, 10}• Variable sum will be used to add them• After we add all the numbers, we then

divide by N• In the example above, sum =0, 12, 26, 41,

51, and finally 61• Then average = 61/5 = 12.2

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Exercise at your seat with paper

• Write SUM = 0• Write N = ____• Repeat with the class get the NEXT number __ __ __ __ __ add it to the SUM __ __ __ __ __• Compute the average by dividing the sum by N _____

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Doing it in a Scratch program

• Ask the user how many numbers (N)• Set our SUM to 0• Repeat the following N times ask the user for the NEXT number reset SUM to SUM + NEXT• Set AVERAGE to SUM / N• Tell the user the AVERAGE

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Making the script

• We need variables N, SUM, and NEXT• We need a repeat N loop• We need to ask the user for a number NEXT

inside the repeat loop• We need to add NEXT to SUM each time• We need to say the answer

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AVG : making the variables

Make variables for:

N: how many to average

NEXT: the next number

SUM: the sum of the

numbers

AVERAGE: the average

of the numbers

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AVG : asking for the numbers

Try this loop first and watchTry this loop; watch the value of NEXT change with your input.

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Let’s write the program in 3 steps, and slowly execute it

Step 1: get NStep 2: ask the N numbers and add them

upStep 3: compute and say the average

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AVG 1: get N from user

3 parts of our program: (1) ask user for N; (2) add up the N numbers; (3) compute and say the average. Click on each part to see it run!

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AVG 2: add N numbers to SUM

We just added 12 and 14 and are ready to add 15.

We added 12 and 14 to sum and are asking for 15.

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AVG 3: execute the 3rd part

Check all the variables to see if they are correct for this problem

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Exercise: make a single script and run it on new numbers

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Save your average program

You can always use it again in the future. That is what programs are for.

So, file it.

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Exercise: have the cat do a flip by rotating 20 degrees 18 times.

• Repeat from the Control menu: set the number of repetitions to 18

• Rotate from the Motion menu: set the size of each small rotation to 20 degrees

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Changing costumes

Another way to change the appearance of a sprite.

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Making a new costume

1) Get bat2-a sprite from the “sprite box”.

2) Click on sprite

3) Click on “Costumes”

4) Click on “Import”

5) Now get bat-2b sprite from the sprite box

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Use a loop to make the bat fly!

1) When and repeat from Control menu and 2) next costume from Looks menu.

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Changing coordinates

• We can randomly set the location of the bat so it will “flutter”.

• Set X to a random number• Set Y to a random number• Move the bat to location (X,Y)• Of course, the bat should remain on stage!

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Using pick random

Click the stop sign at the stage upper right to stop the forever loop.

Do we need a wait in the loop? Try it to see the change in flying.

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Controlling the bat’s direction

• Let’s create multiple scripts.• Click space bar for random moves• Click right arrow to move right• Click left arrow to move left• The bat will behave differently depending

upon which key is typed!• (So, could a gamer catch the bat?)

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Multiple interactive bat flight

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Exercise:

• Add a behavior (script) to move the bat up with the up arrow.

• Add a behavior to move the bat down with the down arrow.

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Adding a background

• Click on the Stage icon at lower right

• Click on Backgrounds

• Click on bat icon (Sprite1)

• Click on Scripts• FLY AGAIN!

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Changing background

• Click on the stage icon• New menu of operations appears• Click each menu operation to see what it does.• How can we program changes of background?• More later.

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Exercise: can we make Cassie do jumping jacks?

• Search the sprite locker to find different costumes for Cassie.

• Make a script that makes her move by contolling the changes in costumes.

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End of Session 3: outcomes

Student should be able to write a program controlled by a loop, and execute a script by clicking special keys. The student should also know how to use “costumes” to change the

appearance of a sprite.