Falling Apples: Random X — Scratch L2

Where this arc is heading: a basket-carrying cat catches kuih raining from the sky. The Score climbs with every catch; miss too many and it is game over. Each lesson adds one piece until the whole game runs on its own.

Learning Goals 3 min

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

Use create clone of [myself v] inside a forever loop to spawn copies of a sprite on a steady beat.
Write a clone-only script under when I start as a clone that controls one falling kuih from spawn to delete.
Pick a random horizontal spawn point with pick random (-220) to (220) so no two kuih fall down the same column.

Warm-Up — the kuih that only falls once 7 min

Aisyah from KL is making a catch-the-kuih game for her little brother. She drags in a kuih sprite, drops it at the top of the Stage, and writes the smallest possible "fall" script:


when flag clicked
go to x: (0) y: (170)
repeat (70)
  change y by (-5)
end

Aisyah's first attempt. The kuih starts at the top centre and slides down to the bottom.

She clicks the flag. The kuih slides down once. Then the Stage just sits there. Her brother yawns.

What's missing for a game?

A game needs many kuih, not one. And they need to come from different x positions so the player has to move the basket. Aisyah needs to (a) make a fresh kuih every second or so, and (b) pick a new random x each time. Today's lesson is the block that does both — clones.

Now look at this second sketch. Predict what happens before you read the reveal:


when flag clicked
forever
  go to x: (0) y: (170)
  repeat (70)
    change y by (-5)
  end
end

Aisyah wraps the whole thing in a forever. One sprite, no clones.

Reveal

One kuih teleports to the top, slides down, teleports back, slides down — over and over. You only ever see one kuih at a time. There is nothing to "catch many of". A single sprite can only be in one place at a time. That is exactly the problem clones solve.

New Concept — clones 15 min

A clone is a temporary copy of a sprite that runs its own scripts on its own. The original sprite stays put. The clone starts with the same costume, size and position — then does whatever its when I start as a clone hat tells it. When it is no longer needed, you call delete this clone and it vanishes cleanly.

Two hats, two jobs

A clone-based sprite almost always has two scripts working as a team:

The spawner script sits under when flag clicked. It hides the original (we don't want to see the "mother kuih") then loops, making one clone every so often.
The clone script sits under when I start as a clone. Every clone runs this the moment it is born — show itself, pick a position, fall, delete.


when flag clicked
hide
forever
  wait (1) seconds
  create clone of [myself v]
end

The spawner. The original kuih hides forever and just keeps stamping out clones.

The 480×360 Stage and "off-edge" rules

The Stage goes from x: −240 on the far left to x: 240 on the far right. Top is y: 180, bottom is y: −180. We use −220 to 220 for x so kuih don't spawn half-off the edge, and y: 170 for the spawn height so they appear just on-screen.


when I start as a clone
go to x: (pick random (-220) to (220)) y: (170)
show
repeat until <(y position) < (-170)>
  change y by (-5)
end
delete this clone

The clone script. Pick a random column, show up, fall 5 pixels at a time, vanish when off the bottom.

Why repeat-until and not forever

A forever loop never ends. Put the falling code in a forever and the clone keeps "falling" off the bottom forever, while Scratch stacks thousands of invisible clones until everything freezes. A repeat until <> with the boolean () < () checking y position gives a clean exit — the moment the kuih's y drops below −170, the loop ends and we hit delete this clone.

Worked Example — Aisyah's kuih shower 12 min

Open Scratch. We will build the falling-kuih half of the game in eight steps. No basket yet — that is next lesson.

Stage plan. The kuih sprite (the only one you script today — see the ✏️ badge) spawns clones along the top at a random x, and each clone falls straight down toward where the basket will sit next lesson.

Step 1 — Start a fresh project

New Scratch project. Delete the default cat. Pick the Apple sprite from the library to stand in as a kuih (or paint a kuih-coloured ball). About 60×60 pixels works well.

Step 2 — The spawner hat

On the kuih, drag in when ⚑ clicked. Snap a hide right under it. The original kuih disappears the instant the flag is clicked.

Step 3 — The forever spawn loop

Add forever. Inside, add wait (1) seconds then create clone of [myself v]. Read it out loud: "Every one second, make a new copy of me."

Step 4 — Click the flag and see nothing

Click the flag. Nothing visible happens. Why? No clone has a script yet — they are born hidden, inheriting the original's hide. Invisible kuih are piling up in space.

Step 5 — Add the clone hat

Start a brand-new script beside the spawner. Drag in when I start as a clone. This hat only fires for clones, never for the original.

Step 6 — Random x, fixed y, show

Snap on go to x: () y: (). Into the x slot, drop pick random (-220) to (220). Type 170 in the y slot. Add show so the new clone becomes visible.

Step 7 — Fall until the bottom

Add repeat until <>. Into the diamond, drop () < () with y position on the left and -170 on the right. Inside, put change y by (-5). The kuih now slides down 5 pixels per frame.

Step 8 — Clean up

Below the repeat-until, snap on delete this clone. The moment a kuih falls below y: −170, it disappears. No invisible pile-up.

The full assembled stack


when flag clicked
hide
forever
  wait (1) seconds
  create clone of [myself v]
end

when I start as a clone
go to x: (pick random (-220) to (220)) y: (170)
show
repeat until <(y position) < (-170)>
  change y by (-5)
end
delete this clone

Two scripts, twelve blocks total. A complete kuih-shower.

What you just built: the core of every falling-things game — Tetris, Galaga, every catch-the-X game ever. The spawner makes things appear on a beat. The clone script owns the life of one thing from birth to death. We add the basket in SCR-L02-38 and the scoring/game-over flow in SCR-L02-39.

Try It Yourself — three spawn tweaks 15 min

🟢 Easy

Goal: Make the kuih fall faster. Don't change the wait time — just change the falling speed inside the clone script.


when I start as a clone
go to x: (pick random (-220) to (220)) y: (170)
show
repeat until <(y position) < (-170)>
  change y by (-10)
end
delete this clone

One number changed: -5 became -10. The kuih now fall twice as fast.

Think: The spawn rate didn't change, but each kuih is on-Stage for less time, so there are fewer kuih visible at once. Speed and density are linked.

🟡 Medium

Goal: Spawn kuih more often — one every 0.5 seconds — but only between x: -100 and x: 100 (a narrow middle band). It looks like rain through a slot.


when flag clicked
hide
forever
  wait (0.5) seconds
  create clone of [myself v]
end

when I start as a clone
go to x: (pick random (-100) to (100)) y: (170)
show
repeat until <(y position) < (-170)>
  change y by (-5)
end
delete this clone

Two numbers changed in the spawner (1 → 0.5) and two in the random range.

Think: We cover the spawn rate as its own difficulty knob in L02-40 — the game speeds up the longer you survive. For now, feel how the wait number controls density.

🔴 Stretch

Goal: Random fall speed per kuih. Some drop slowly, some fast. Add a speed variable inside the clone script (set "For this sprite only" so each clone gets its own copy) and pick a random number from 3 to 9.


when I start as a clone
go to x: (pick random (-220) to (220)) y: (170)
set [speed v] to (pick random (3) to (9))
show
repeat until <(y position) < (-170)>
  change y by ((0) - (speed))
end
delete this clone

Each clone gets its own private speed. () - () turns the positive random number into the negative y change we need.

Think: Per-clone variables are a Level 3 idea, but a stretch student is ready. Each clone remembers its own speed — the spawner doesn't have to care.

Mini-Challenge — Daniel's frozen kuih 5 min

"Daniel's kuih never come down"

Daniel built the falling-kuih sprite for his teh tarik shop game. He clicks the flag. One kuih appears at the top — and just sits there. No falling. The Stage fills with frozen kuih at the top, one per second. Here is his clone script:


when I start as a clone
go to x: (pick random (-220) to (220)) y: (170)
show
repeat until <(y position) < (-170)>
  change x by (-5)
end
delete this clone

Daniel's clone script. Kuih appear, but they don't fall.

Spot the bug. Two things are wrong, and one means the repeat-until never finishes even if you fix the other.

Reveal the fix

Bug 1: The change inside the loop is change x by (-5), not change y by (-5). The kuih slides left, not down. It never changes its y, so the exit condition (y position) < (-170) never becomes true.

Bug 2: The loop body and the loop exit must talk about the same axis. The exit tests y but the body changes x — a mismatch. Always check the value you are actually changing.


when I start as a clone
go to x: (pick random (-220) to (220)) y: (170)
show
repeat until <(y position) < (-170)>
  change y by (-5)
end
delete this clone

Loop exit and loop body must talk about the same thing. If you change y inside the loop, check y in the exit. Mismatch = forever-stuck clone = laggy game.

Recap 3 min

You met clones — the way one sprite makes many copies that each live their own little life. A clone system always has a spawner (one forever loop calling create clone of [myself v] on a beat) and a clone script (under when I start as a clone) that owns one clone from birth to delete this clone. The original hides; the clones show. Each clone picks its own random x so the kuih spread across the Stage instead of stacking in one column.

Clone: A temporary copy of a sprite that runs its own scripts. Made with create clone of [myself v], removed with delete this clone.
Spawner: A script — usually a forever loop under when ⚑ clicked — whose only job is to keep producing clones on a steady beat.
Clone script: The script under when I start as a clone. Every new clone runs it from the top once. The original ignores it.
pick random: An Operators block that returns a different whole number each run, between the two numbers you give. Perfect for varying spawn positions.
repeat until: A control loop that runs its body until the boolean in its diamond becomes true. We use it to fall "until I'm off the bottom".

Homework 2 min

The Kuih Shower. Build the kuih sprite from this lesson on its own, with no basket. Hand it in as a stand-alone "rain" project. Tomorrow we add the basket.

New project. Delete the cat. Add a kuih sprite (or paint one).
Build the spawner script exactly as in Steps 2–3 of the worked example.
Build the clone script exactly as in Steps 5–8.
Click the flag. Kuih should spawn at random x positions and fall to the bottom, one per second.
Now tweak: change the wait time to 0.3 seconds and the fall speed to -3. Watch the Stage fill with slow kuih. Then change them back.

Save as HW-L2-37-Kuih-Shower.sb3.

Bring back next class:

The .sb3 file with the original settings (1 second wait, -5 fall speed).
Your answer to: "What happens if you delete the delete this clone block? Run it for 30 seconds and describe what you see."
One sentence: "My kuih spawn at x: ___ to ___ and fall at ___ pixels per frame."

Heads up for next class: SCR-L02-38 · Catch & Reset adds the basket sprite at the bottom and teaches each kuih clone to ask touching (Basket v) ?. Bring this kuih-shower project — we add to it directly.