Per-Level Enemy Patterns — Scratch L4

Where we are now: one Enemy sprite gives the player a manageable boar on level 1, two fast-jumping bats on level 2, and three homing hawks on level 3 — all from the same patrol script, gated by (level).

Learning Goals 3 min

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

Gate enemy behaviour with if <> then else chains on the (level) variable so one sprite acts differently on level 1, level 2, and level 3.
Scale the number of enemies per level using repeat (level) + create clone of [myself v].
Use point towards (Player v) + set rotation style [don't rotate v] to build a chasing enemy for the hardest level without the sprite flipping upside-down.

Warm-Up — the one enemy that's wrong on every level 7 min

In L04-19 you added one Enemy sprite per backdrop, positioned by when backdrop switches to. But every enemy moves the same boring way:


when flag clicked
forever
  move (2) steps
  if <touching [edge v] ?> then
    turn cw (180) degrees
  end
end

The L04-19 enemy. Slow patrol, edge-to-edge, forever. Fine on level 1. Way too easy on level 3.

Predict — if the player can already dodge this on level 1, what should happen on level 3 to keep the game interesting?

Reveal the answer

Make it harder. The same enemy should move faster, or chase the player, or multiply, or all three. Reusing one sprite with different behaviour per level is way less work than building three separate enemy sprites — and it keeps your project clean.

Today: one Enemy sprite, three personalities. Slow patrol on level 1. Fast patrol + a jumper on level 2. Chasers that follow you on level 3. All gated by the (level) variable.

New Concept — gating behaviour with the level number 15 min

You already know if <> then else (L02-07) and you know how to compare numbers with () = () (L02-08). Today we chain them. A chain of if-else-if is how one script picks one behaviour out of many, based on a single number.

The level-gate pattern


if <(level) = (1)> then
  move (2) steps
else
  if <(level) = (2)> then
    move (5) steps
  else
    point towards (Player v)
    move (4) steps
  end
end

One sprite, three behaviours. Level 1: slow patrol. Level 2: faster patrol. Level 3: chase the player.

Read it top-down. Scratch checks (level) = (1) first. If true, slow move. If false, drop into the else and check the next question. If that's also false, fall through to the last branch — the chaser.

Scaling enemy count with repeat

Different number of enemies per level too. Level 1: 1 enemy. Level 2: 2. Level 3: 3. The (level) variable is the count we want:


when backdrop switches to [level1 v]
delete this clone

when I receive (spawn-enemies v)
repeat (level)
  create clone of [myself v]
end

Two scripts. Wipe the old clones first, then spawn one clone per level number. Level 3 = 3 clones.

The chaser — point towards plus don't rotate

point towards (Player v) turns the sprite to face the player. Add move (4) steps in a forever loop and it slowly homes in. Problem: by default the enemy tilts to face the player — it'll be upside-down half the time. Fix with set rotation style [don't rotate v] at the top of the sprite's flag-click.


when flag clicked
set rotation style [don't rotate v]

Run once at the start. Now point towards still steers the motion direction but the sprite's picture stays the right way up.

Worked Example — the three-personality Lipan 12 min

We'll turn the Enemy into a Lipan (centipede). Slow on the forest floor, fast in the cave, swarming chasers in the sky.

Stage plan. Level 1: one Lipan clone on the forest floor, patrolling at 2 steps. The cat (✏️ badge) can walk or jump over it. Level 2 spawns two faster clones; level 3 spawns three chasers.

Step 1 — Sprite + rotation style

Rename the Enemy sprite to Lipan. Add one flag-click hat:


when flag clicked
set rotation style [don't rotate v]
hide

Hide the original — only the clones will ever be visible. Cleaner.

Step 2 — Spawn on level start

One hat per level. Each one clears old clones, then spawns the right number:


when backdrop switches to [level1 v]
broadcast (clear-enemies v)
repeat (level)
  create clone of [myself v]
end

Level 1 → repeat 1 → one Lipan clone. Same script copy-pasted for level2 and level3 hats.

Step 3 — Clones wipe themselves


when I receive (clear-enemies v)
delete this clone

On the sprite. Every clone hears the broadcast and removes itself. The original (which is hidden) doesn't, because delete this clone does nothing on the original.

Step 4 — Clone's first moments


when I start as a clone
go to x: (pick random (-150) to (150)) y: (-100)
show

Spawn at a random spot on the ground, then become visible.

Step 5 — The level-gated behaviour loop


when I start as a clone
forever
  if <(level) = (1)> then
    move (2) steps
    if <touching [edge v] ?> then
      turn cw (180) degrees
    end
  else
    if <(level) = (2)> then
      move (5) steps
      if <touching [edge v] ?> then
        turn cw (180) degrees
      end
    else
      point towards (Player v)
      move (4) steps
    end
  end
end

21 blocks. One forever loop with a level-gate inside. Each level picks one branch.

Step 6 — The level-2 jumper

Level 2 also has a jumper. Easy add to the level-2 branch — give a small upward boost on a timer:


when I start as a clone
forever
  if <(level) = (2)> then
    change y by (8)
    wait (0.3) seconds
    change y by (-8)
    wait (0.3) seconds
  end
end

Second forever loop on the clone, only does anything on level 2. The clone hops 8 pixels up and back. Combine with the patrol loop and it walks-and-hops.

Step 7 — Test the chase

Flip to level 3 (via your NextLevelButton from L04-19). Three clones spawn, each turns toward the cat, each crawls slowly forward. Now move the Player — every clone tracks you. The faster you move, the more obvious the homing is.

Step 8 — Tune the difficulty

Numbers to tweak after testing: chase speed (move (4) — try 3 for easier, 6 for brutal), clone count cap, jumper wait time, patrol speed. Stay under the L4 stack cap of 30 blocks per script — if a script grows past 30, split it into a my-block (L03-15).

What you just built: a difficulty curve. One sprite, one variable, three feels. Add level 4? Add an else-if branch. No new sprites needed.

Try It Yourself — three behaviour drills 15 min

🟢 Easy

Goal: On level 1, change the Lipan's colour every two seconds so it pulses gently. On levels 2 and 3, no pulse. Use a second forever loop on the clone, gated by (level) = (1).


when I start as a clone
forever
  if <(level) = (1)> then
    change [color v] effect by (25)
    wait (2) seconds
  end
end

Think: The pulse adds personality without changing difficulty. Visual flavour is per-level too — not just numbers.

🟡 Medium

Goal: On level 3 only, make every chaser say its distance from the player as it moves. Use the distance reporter distance to (Player v) from Sensing inside say.


when I start as a clone
forever
  if <(level) = (3)> then
    say (distance to (Player v))
  end
end

Each clone shows its own distance. Three clones = three speech bubbles, all updating live.

Think: Speech bubbles in a chaser tell the player who's closest. Could become a real gameplay clue — the nearest enemy says BOO! when distance drops below 30.

🔴 Stretch

Goal: Add a level-4 boss preview. Only one clone spawns. The boss has its own behaviour branch in the gate — moves slowly toward the player but stops when distance < 50, then says Wait your turn!.


when I start as a clone
forever
  if <(level) = (4)> then
    if <(distance to (Player v)) > (50)> then
      point towards (Player v)
      move (2) steps
    else
      say [Wait your turn!]
    end
  end
end

Think: A boss is just an enemy with a different gate branch. As your platformer grows, every new enemy type is one more else-if. Beyond level 4, refactor the chain into a my-block (L03-15) so you don't outgrow 30 blocks.

Mini-Challenge — the cave enemy that never stops 5 min

"Hakim's overlapping behaviours"

Hakim wrote two scripts on his Lipan clone — one for patrol, one for chase. He thought the chase would only fire on level 3. It fires on every level.


when I start as a clone
forever
  move (2) steps
  if <touching [edge v] ?> then
    turn cw (180) degrees
  end
end

when I start as a clone
forever
  point towards (Player v)
  move (4) steps
end

Hakim's clone scripts. Two forever loops, both running on every level.

What did he forget, and what's the smallest fix?

Reveal one valid solution

Both when I start as a clone hats fire on every clone, every time, on every level. There's no gate. The patrol script moves 2 steps and the chase script moves 4 steps — so the clone does both, drifting toward the player at random speed. Looks buggy.

Fix: wrap each forever's inside in a level gate:


when I start as a clone
forever
  if <(level) < (3)> then
    move (2) steps
    if <touching [edge v] ?> then
      turn cw (180) degrees
    end
  end
end

when I start as a clone
forever
  if <(level) = (3)> then
    point towards (Player v)
    move (4) steps
  end
end

Lesson: when you have two scripts that should be mutually exclusive, both have to know about it. Splitting behaviour across two scripts is fine — but each script still needs its own gate.

Recap 3 min

You turned one enemy sprite into three. The pattern is the level gate: an if <> then else chain that picks one behaviour based on the (level) variable. Number of enemies scales with the level number using repeat (level). The chaser uses point towards with set rotation style [don't rotate v] so the picture stays upright while the motion direction tracks the player. Same sprite. Same script structure. Different difficulty per biome.

Level gate: An if <> then else chain that runs different code depending on the (level) variable. The pattern that makes one sprite behave like many.
Chase: A movement style where the sprite faces the player every frame and steps forward. Built with point towards (Player v) + move () steps inside a forever.
Rotation style: How a sprite handles being pointed in a new direction. All around tilts the picture; left-right mirrors it; don't rotate keeps it upright. Set with set rotation style [...].
Spawn count: The number of enemies that exist at once. Scaled by repeat (level) + create clone of [myself v] — level 3 spawns 3 enemies, level 7 would spawn 7.
Mutually exclusive: Two pieces of code that should never run at the same time. In a level gate, only one branch fires per level — the others are skipped.

Homework 2 min

The Three-Personality Lipan. Continue last lesson's project. Replace the boring shared enemy with a properly gated one.

One Enemy sprite called Lipan. Hidden original, clones visible.
Three when backdrop switches to hats — one per level — each broadcasts clear-enemies then spawns repeat (level) clones.
One when I receive (clear-enemies v) + delete this clone.
One when I start as a clone with the level-gate forever from the worked example — slow on level 1, fast on level 2, chase on level 3.
Add the level-2 jumper: a second clone-forever that only does anything when (level) = (2).
Test by clicking the NextLevelButton: level 1 = one slow Lipan; level 2 = two fast hopping Lipans; level 3 = three chasers.

Save as HW-L4-20-Lipan-Personalities.sb3.

Bring back next class:

The .sb3 file.
Your answer to: "You can already 'die' if a Lipan touches the cat. On level 3 with three chasers, dying sends you back to the start of the level — which feels brutal. What would feel fair instead?" (Hint: that's exactly L04-21.)

Heads up for next class: SCR-L04-21 adds checkpoints — a flag at the level midpoint that saves your position so dying only kicks you back to the checkpoint, not the start of the whole level.