Bubble Popper II

Review — last lesson 5 min

• create clone of [myself v] makes a copy; the copy runs when I start as a clone.
• Hide the original. It is the spawner.
• Every clone needs delete this clone, or the project silts up.
• Scratch stops at 300 living clones and says nothing about it.

Quick-fire

Your reef had twenty-six bubbles and every one of them rose at exactly 5 steps a tick. It looked wrong. What was missing?

Today’s Topic 3 min

• For all sprites — one box, shared by everything
• For this sprite only — one box per clone
• Why the shared version breaks so quietly
• Giving each bubble its own speed, size, wobble and colour

Learning outcome

By the end of this lesson you will be able to:

1. Choose the right kind of variable for a job, and say why.
2. Give every clone its own values so a crowd looks natural.
3. Recognise a shared-variable bug from the way it looks on screen.

Learnt — one box, or one each 28 min

1 · The dialog you have been clicking past

Every time you make a variable Scratch asks a question, and until now the answer has not mattered:

• For all sprites — one box in the whole project. Every sprite and every clone reads and writes the same box.
• For this sprite only — the box belongs to the sprite. And here is the part that matters: every clone gets its own copy of it.

Scores, lives and timers are shared — there is only one game. Anything that describes one bubble must be sprite-only.

2 · Watch the shared one break

Here is a reef where speed was made for all sprites. Every clone sets it as it is born:

when I start as a clone
set [speed v] to (pick random (2) to (8))
show
repeat until <(y position) > (165)>
  change y by (speed)
end
delete this clone
            

3 · The fix is one dropdown

Delete speed and make it again — same name, same blocks — choosing For this sprite only. Not one block changes. The behaviour changes completely:

4 · What a clone inherits

When a clone is born it takes a snapshot of the sprite:

• position, direction, size, costume, effects — copied
• sprite-only variables — copied, then independent
• shared variables — not copied, because there is only one

That snapshot is useful: set the costume in the spawner before the clone block and each bubble is born the right colour already.

Basic — its own everything 24 min

Open last lesson’s reef. You are giving each bubble a character of its own.

Step 1 — the variables

1. Make speed — for this sprite only.
2. Make wobble — for this sprite only.
3. Keep bubbles as it is — it counts the whole reef, so it is shared.
4. Untick all three readouts. They cannot show you a clone’s copy anyway.

Step 2 — roll the dice as it is born

1. At the top of the clone script, set speed to a random 2 to 8.
2. Set wobble to a random 1 to 4.
3. Add set size to (pick random (25) to (60)) %.

Step 3 — use them in the rise

1. Change the rise from change y by (5) to change y by (speed).
2. Add a sideways drift: change x by (wobble) inside the same loop.
3. Put if on edge, bounce after it so a drifting bubble turns back.

Step 4 — colour it in the spawner

1. In the spawner, before the clone block: switch costume to (pick random (1) to (5)).
2. Run it. Twenty bubbles, no two alike.

when I start as a clone
set [speed v] to (pick random (2) to (8))
set [wobble v] to (pick random (1) to (4))
set size to (pick random (25) to (60)) %
change [bubbles v] by (1)
show
repeat until <(y position) > (165)>
  change y by (speed)
  change x by (wobble)
  if on edge, bounce
end
change [bubbles v] by (-1)
delete this clone
            

A proper wobble

A straight diagonal is not a wobble. Make them weave.

• Each bubble has its own wobble and its own drift direction.
• The drift reverses every so often, so the path snakes.
• No two bubbles weave in step with each other.

change x by (wobble)
change [tick v] by (1)
if <(tick) > (12)> then
  set [wobble v] to ((0) - (wobble))
  set [tick v] to (0)
end
            

Big and slow

Stop rolling speed separately. Work it out from the size, so a big bubble is always a slow one.

• Size is rolled once, per bubble.
• Speed is calculated from it, not rolled.
• A big bubble is visibly slower than a small one, every single run.

set [my size v] to (pick random (25) to (60))
set size to (my size) %
set [speed v] to (round ((9) - ((my size) / (10))))
            

Bubble race

Five bubbles in five lanes. Find out which one wins.

• Five clones at fixed x positions, evenly spread.
• Each has its own speed and its own lane number.
• The first to the surface announces its lane, and the others do not overwrite it.

if <(winner) = (0)> then
  set [winner v] to (lane)
end
            

A shoal of thirty

Put everything together and fill the reef. Thirty bubbles, each with its own speed, size, wobble and colour — and no lockstep anywhere.

• Thirty made, comfortably under the 300 ceiling.
• Four things rolled per bubble, all sprite-only.
• The shared counter still returns to 0 at the end.

Summary 5 min

• For all sprites = one box for the project. Scores, lives, timers.
• For this sprite only = one box per clone. Speed, size, wobble, lane.
• A crowd that moves in lockstep is a shared variable. That is the symptom to learn.
• Scratch shows only the original’s copy of a sprite-only variable, so you debug these by watching the Stage.
• A variable cannot be converted. Delete it and make a new one.

Global variable

“For all sprites” — one box shared by everything.

Sprite-only variable

“For this sprite only” — and every clone gets its own copy.

Lockstep

When many clones move identically because they share a value.

Inherit

What a clone copies from the sprite at the moment it is born.

Hand in

Save your shoal. Next lesson you start popping them — and the score turns out to need both kinds of variable at once.