Space Invaders I — Scratch L2 Lesson 41

Learnt27 min · a grid of clones that know their place, and a ship
Basic25 min · build the army and the ship
Challenge 1–327 min · five looks, two edges, your own army
Extrabonus · shields

WHAT IT SHOULD LOOK LIKE

What we are making today. Forty invaders from ONE sprite, and a ship at the bottom to face them.

the army builds itself

forty invaders, one ship

panels run left to right

Review — the Penalty Shootout 5 min

A power meter is a number that climbs and falls in a loop.
A key press freezes it. The number at that moment is the power.
The keeper chose a side by itself, with pick random (1) to (3).
The best streak stayed, because the green flag never reset it.

Quick-fire

You need forty invaders on the screen. How many sprites do you make?

Reveal the answer

One. Forty clones of it, made by loops. In Level 1’s Paddle Bounce one loop made a row. Today two loops make rows of rows.

Today’s Topic 3 min

A repeat inside a repeat
Clones that know their own row and column
A costume chosen by the row
A ship that stays on the Stage

Learning outcome

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

Build a grid of clones with two loops, one inside the other.
Give every clone its own row and column number.
Keep a sprite on the Stage with two comparisons.

Learnt — one sprite, forty invaders 27 min

1 · A repeat inside a repeat

In Level 1 you built one row of targets with one loop. An army is five rows. So we put the row loop inside a second loop.

WHAT IT SHOULD LOOK LIKE

The inside loop makes one row of eight. The outside loop does that five times, one row lower each time.

inside loop: one row

outside loop: five rows

panels run left to right

Two counters keep track of where we are:

(my row) counts the rows, 1 to 5.
(my column) counts along a row, 1 to 8.

Every invader’s place comes from those two numbers. Column times 40 gives the x. Row times 32 gives the y.

Column	x	Row	y
1	40 − 180 = −140	1	172 − 32 = 140
2	80 − 180 = −100	2	172 − 64 = 108
…	40 more each time	…	32 lower each time
8	320 − 180 = 140	5	172 − 160 = 12

Make a sprite called Invader. Give it five costumes, one per row.
Open Variables. Make my row and pick For this sprite only. Do the same for my column.
On the flag: hide, then set [my row v] to (1).
Add repeat (5). Inside it, set [my column v] to (1) and a repeat (8).
Inside the small loop: the go to x: () y: () with the two sums, create clone of [myself v], and change [my column v] by (1).
Under the small loop, still inside the big one: change [my row v] by (1).


when flag clicked
hide
set [my row v] to (1)
repeat (5)
  set [my column v] to (1)
  repeat (8)
    go to x: (((my column) * (40)) - (180)) y: ((172) - ((my row) * (32)))
    create clone of [myself v]
    change [my column v] by (1)
  end
  change [my row v] by (1)
end

The original never shows. It walks the grid and leaves a clone at every stop.

Each clone only has to pick its look and appear:


when I start as a clone
switch costume to (my row)
show

Row 1 wears costume 1, row 5 wears costume 5. One block chooses all forty looks.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch. Nothing to press — the army builds itself one invader at a time.

Watch the small loop finish a row, then the big loop start the next one.

2 · Each clone keeps its own numbers

This is the clever part. A clone is born as a copy of the original — its variables too. So a clone made at row 2, column 5 keeps a 2 and a 5 for ever.

Add when this sprite clicked to the Invader.
Under it: say (join [row ] (my row)) for (1) seconds.
Then say (join [column ] (my column)) for (1) seconds.
Press the flag. Click invaders all over the army.


when this sprite clicked
say (join [row ] (my row)) for (1) seconds
say (join [column ] (my column)) for (1) seconds

A test, not part of the game. It proves every clone knows its place.

WHAT IT SHOULD LOOK LIKE

Click an invader and it tells you where it lives. Every clone kept its own copy of my row and my column.

click row 2, column 5

a second later

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and wait for the army. Then click any invader: it says its row, then its column.

Every invader answers with its own row and column.

WHAT IT SHOULD LOOK LIKE

The same army with “For all sprites”. There is only one box, so every invader gives the last number the loop wrote.

click the top-left one

click the bottom-right one

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and wait for the army. Then click any invader: every one says row 6, column 9.

The same army built with “For all sprites”. Click any two invaders and compare.

3 · The ship

The ship slides along the bottom. The arrow keys move it. Two comparisons stop it at the edges.

Add the Rocketship sprite from the library. Set its size to 30.
On the flag: go to x: (0) y: (-150) and a forever.
Inside: two if <> then blocks with key [left arrow v] pressed? and key [right arrow v] pressed?.
Add two more: if (x position) is past −215 or 215, put it back with set x to (-215) or set x to (215).


when flag clicked
go to x: (0) y: (-150)
forever
  if <key [left arrow v] pressed?> then
    change x by (-6)
  end
  if <key [right arrow v] pressed?> then
    change x by (6)
  end
  if <(x position) < (-215)> then
    set x to (-215)
  end
  if <(x position) > (215)> then
    set x to (215)
  end
end

Move first, then check. If it went too far, put it straight back.

WHAT IT SHOULD LOOK LIKE

The arrow keys slide the ship. Two comparisons stop it at −215 and 215, so it never slides off.

hold ←

it stops at the edge

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag, click the stage once, then hold ← or → to slide the ship. Try to drive it off the edge.

Try to drive the ship off the Stage. It stops with its whole body still showing.

Basic — the army and the ship 25 min

New project. Pick the Stars backdrop. Then two sprites.

Step 1 — the invader’s costumes

Paint a small invader. Squares are enough — make it about 30 wide.
Right-click it and duplicate it four times. Recolour each copy.
Or choose costumes from the library instead: Crab, Beetle, Ghost, Dragonfly, Bat.
Check the order: costume 1 is the back row, costume 5 the front.

Step 2 — the two counters

Make my row and my column, both For this sprite only.
Untick their boxes so they do not cover the army.

Step 3 — the grid

Build the flag stack with the two loops.
Build the clone stack: costume, then show.
Press the flag and count. Five rows of eight, nothing overlapping.

Step 4 — the click test

Add the click stack.
Click the four corners. You should hear 1,1 then 1,8 then 5,1 then 5,8.
Keep it for now. You can delete it later.

Step 5 — the ship

Add the Rocketship and its flag stack.
Hold → for five seconds. The ship must stop with its whole body showing.
Now hold ←.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, click the stage once, then hold ← or → to move the ship under the army.

Where you should be at the end of Basic: an army and a ship. Nobody can fire yet.

Only one invader appears.

The go to x: () y: () is outside the small loop. Every clone is made in the same spot, piled on top of each other.

I get one long row that runs off the Stage.

set [my column v] to (1) is missing, or it sits outside the big loop. The column must go back to 1 at the start of every row.

Every row has the same costume.

The clone stack says switch costume to (1) instead of switch costume to (my row). Drag the orange variable into the slot.

Every invader I click says row 6.

The variables are “For all sprites”. Delete them and make them again, this time “For this sprite only”.

An extra invader sits at the bottom right.

The original is showing. It needs hide at the top of the flag stack.

CHALLENGE 1 · L2-41-M1🪙 30 Adcoins8 min

Five rows, five looks

Every row has its own costume.
All eight invaders in a row match.
The back row looks the most dangerous — it will be worth the most.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — every row its own look, and the back row the scariest.

five rows, five looks

Done when: you can step back from the desk and name each row by its look alone.

CHALLENGE 2 · L2-41-M2🪙 50 Adcoins9 min

Two edges

The ship slides with ← and →.
It stops at both edges.
Its whole body stays on the Stage.
It never turns round or flips over.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — the ship reaches both edges and stays whole.

far left

far right

panels run left to right

Done when: you hold each arrow for five seconds and the ship stops whole at both ends.

Teacher note — why −215 and not −240

A sprite’s x is its middle. The ship is about 38 wide, so at x = −240 half of it is off the Stage. Ask pupils to find the number by trying: go too far, then come back five at a time until the whole ship shows.

CHALLENGE 3 · L2-41-M3🪙 70 Adcoins10 min

Your own army

Make a bigger army. Change numbers only — no new blocks.

Ten columns instead of eight.
The whole army still fits on the Stage, with room at both sides.
No two invaders overlap.
The click test still gives the right row and column.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the same two loops, a bigger army. Only the numbers changed.

ten columns

Done when: ten columns build, and clicking the last invader in the front row says row 5, column 10.

Teacher reveal — the four numbers that change

The repeat goes to 10, and the gap has to shrink so ten fit. With a gap of 36, column 10 is at 360. Take away 198 and the army runs from −162 to 162.


repeat (10)
  go to x: (((my column) * (36)) - (198)) y: ((172) - ((my row) * (32)))
  create clone of [myself v]
  change [my column v] by (1)
end

A good question for the pupil: why did the 180 have to change? (Because the army must stay in the middle when it gets wider.)

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch ten columns build. Then click the stage once and hold ← or → to move the ship.

Ten columns from the same two loops. Only the numbers changed.

EXTRA MISSION · L2-41-MX🪙 100 Adcoinsbonus

Shields

The real game has four green shields above the ship. Each one is a small grid of bricks. Build them with loops, not by hand.

WHAT IT SHOULD LOOK LIKE

Extra — four shields between the ship and the army, built by loops inside loops.

shields up

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch four shields build under the army. Click the stage once, then hold ← or → to move the ship.

Four shields of eight bricks each, from one brick costume and three loops.

Teacher notes — three loops deep

The trick is putting the pen back after each row and each shield. After a row of four, step back 48 and down 9. After a whole shield, step right 110 and back up 18.


when flag clicked
hide
go to x: (-183) y: (-90)
repeat (4)
  repeat (2)
    repeat (4)
      create clone of [myself v]
      change x by (12)
    end
    change x by (-48)
    change y by (-9)
  end
  change x by (110)
  change y by (18)
end

when I start as a clone
show

Pupils who get lost here should say it aloud: “four shields, each two rows, each four bricks”. The loops are that sentence, outside to inside. In Lesson 2-43 bombs and lasers will chip these away.

Summary 4 min

A repeat inside a repeat makes a grid: rows of rows.
A clone is born with a copy of the original’s “For this sprite only” variables.
So every invader keeps its own (my row) and (my column).
switch costume to (my row) gives each row its own look.
Two comparisons keep the ship on the Stage.

Nested loop: A loop inside another loop. The inside one runs in full each time round.
Grid: Things laid out in rows and columns.
For this sprite only: A variable each sprite — and each clone — keeps for itself.
Clamp: Putting a number back inside its limits when it goes past them.

Next lesson: a laser. Hit an invader and it is gone — and the back row is worth the most.

Learnt27 min · a grid of clones that know their place, and a ship
Basic25 min · build the army and the ship
Challenge 1–327 min · five looks, two edges, your own army
Extrabonus · shields

WHAT IT SHOULD LOOK LIKE

What we are making today. Forty invaders from ONE sprite, and a ship at the bottom to face them.

the army builds itself

forty invaders, one ship

panels run left to right

Review — the Penalty Shootout 5 min

A power meter is a number that climbs and falls in a loop.
A key press freezes it. The number at that moment is the power.
The keeper chose a side by itself, with pick random (1) to (3).
The best streak stayed, because the green flag never reset it.

Quick-fire

You need forty invaders on the screen. How many sprites do you make?

Reveal the answer

One. Forty clones of it, made by loops. In Level 1’s Paddle Bounce one loop made a row. Today two loops make rows of rows.

Today’s Topic 3 min

A repeat inside a repeat
Clones that know their own row and column
A costume chosen by the row
A ship that stays on the Stage

Learning outcome

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

Build a grid of clones with two loops, one inside the other.
Give every clone its own row and column number.
Keep a sprite on the Stage with two comparisons.

Learnt — one sprite, forty invaders 27 min

1 · A repeat inside a repeat

In Level 1 you built one row of targets with one loop. An army is five rows. So we put the row loop inside a second loop.

WHAT IT SHOULD LOOK LIKE

The inside loop makes one row of eight. The outside loop does that five times, one row lower each time.

inside loop: one row

outside loop: five rows

panels run left to right

Two counters keep track of where we are:

(my row) counts the rows, 1 to 5.
(my column) counts along a row, 1 to 8.

Every invader’s place comes from those two numbers. Column times 40 gives the x. Row times 32 gives the y.

Column	x	Row	y
1	40 − 180 = −140	1	172 − 32 = 140
2	80 − 180 = −100	2	172 − 64 = 108
…	40 more each time	…	32 lower each time
8	320 − 180 = 140	5	172 − 160 = 12

Make a sprite called Invader. Give it five costumes, one per row.
Open Variables. Make my row and pick For this sprite only. Do the same for my column.
On the flag: hide, then set [my row v] to (1).
Add repeat (5). Inside it, set [my column v] to (1) and a repeat (8).
Inside the small loop: the go to x: () y: () with the two sums, create clone of [myself v], and change [my column v] by (1).
Under the small loop, still inside the big one: change [my row v] by (1).


when flag clicked
hide
set [my row v] to (1)
repeat (5)
  set [my column v] to (1)
  repeat (8)
    go to x: (((my column) * (40)) - (180)) y: ((172) - ((my row) * (32)))
    create clone of [myself v]
    change [my column v] by (1)
  end
  change [my row v] by (1)
end

The original never shows. It walks the grid and leaves a clone at every stop.

Each clone only has to pick its look and appear:


when I start as a clone
switch costume to (my row)
show

Row 1 wears costume 1, row 5 wears costume 5. One block chooses all forty looks.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch. Nothing to press — the army builds itself one invader at a time.

Watch the small loop finish a row, then the big loop start the next one.

2 · Each clone keeps its own numbers

This is the clever part. A clone is born as a copy of the original — its variables too. So a clone made at row 2, column 5 keeps a 2 and a 5 for ever.

Add when this sprite clicked to the Invader.
Under it: say (join [row ] (my row)) for (1) seconds.
Then say (join [column ] (my column)) for (1) seconds.
Press the flag. Click invaders all over the army.


when this sprite clicked
say (join [row ] (my row)) for (1) seconds
say (join [column ] (my column)) for (1) seconds

A test, not part of the game. It proves every clone knows its place.

WHAT IT SHOULD LOOK LIKE

Click an invader and it tells you where it lives. Every clone kept its own copy of my row and my column.

click row 2, column 5

a second later

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and wait for the army. Then click any invader: it says its row, then its column.

Every invader answers with its own row and column.

WHAT IT SHOULD LOOK LIKE

The same army with “For all sprites”. There is only one box, so every invader gives the last number the loop wrote.

click the top-left one

click the bottom-right one

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and wait for the army. Then click any invader: every one says row 6, column 9.

The same army built with “For all sprites”. Click any two invaders and compare.

3 · The ship

The ship slides along the bottom. The arrow keys move it. Two comparisons stop it at the edges.

Add the Rocketship sprite from the library. Set its size to 30.
On the flag: go to x: (0) y: (-150) and a forever.
Inside: two if <> then blocks with key [left arrow v] pressed? and key [right arrow v] pressed?.
Add two more: if (x position) is past −215 or 215, put it back with set x to (-215) or set x to (215).


when flag clicked
go to x: (0) y: (-150)
forever
  if <key [left arrow v] pressed?> then
    change x by (-6)
  end
  if <key [right arrow v] pressed?> then
    change x by (6)
  end
  if <(x position) < (-215)> then
    set x to (-215)
  end
  if <(x position) > (215)> then
    set x to (215)
  end
end

Move first, then check. If it went too far, put it straight back.

WHAT IT SHOULD LOOK LIKE

The arrow keys slide the ship. Two comparisons stop it at −215 and 215, so it never slides off.

hold ←

it stops at the edge

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag, click the stage once, then hold ← or → to slide the ship. Try to drive it off the edge.

Try to drive the ship off the Stage. It stops with its whole body still showing.

Basic — the army and the ship 25 min

New project. Pick the Stars backdrop. Then two sprites.

Step 1 — the invader’s costumes

Paint a small invader. Squares are enough — make it about 30 wide.
Right-click it and duplicate it four times. Recolour each copy.
Or choose costumes from the library instead: Crab, Beetle, Ghost, Dragonfly, Bat.
Check the order: costume 1 is the back row, costume 5 the front.

Step 2 — the two counters

Make my row and my column, both For this sprite only.
Untick their boxes so they do not cover the army.

Step 3 — the grid

Build the flag stack with the two loops.
Build the clone stack: costume, then show.
Press the flag and count. Five rows of eight, nothing overlapping.

Step 4 — the click test

Add the click stack.
Click the four corners. You should hear 1,1 then 1,8 then 5,1 then 5,8.
Keep it for now. You can delete it later.

Step 5 — the ship

Add the Rocketship and its flag stack.
Hold → for five seconds. The ship must stop with its whole body showing.
Now hold ←.

Runs here in the page. Nothing to install.

How to use this: Click the green flag, click the stage once, then hold ← or → to move the ship under the army.

Where you should be at the end of Basic: an army and a ship. Nobody can fire yet.

Only one invader appears.

The go to x: () y: () is outside the small loop. Every clone is made in the same spot, piled on top of each other.

I get one long row that runs off the Stage.

set [my column v] to (1) is missing, or it sits outside the big loop. The column must go back to 1 at the start of every row.

Every row has the same costume.

The clone stack says switch costume to (1) instead of switch costume to (my row). Drag the orange variable into the slot.

Every invader I click says row 6.

The variables are “For all sprites”. Delete them and make them again, this time “For this sprite only”.

An extra invader sits at the bottom right.

The original is showing. It needs hide at the top of the flag stack.

CHALLENGE 1 · L2-41-M1🪙 30 Adcoins8 min

Five rows, five looks

Every row has its own costume.
All eight invaders in a row match.
The back row looks the most dangerous — it will be worth the most.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — every row its own look, and the back row the scariest.

five rows, five looks

Done when: you can step back from the desk and name each row by its look alone.

CHALLENGE 2 · L2-41-M2🪙 50 Adcoins9 min

Two edges

The ship slides with ← and →.
It stops at both edges.
Its whole body stays on the Stage.
It never turns round or flips over.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — the ship reaches both edges and stays whole.

far left

far right

panels run left to right

Done when: you hold each arrow for five seconds and the ship stops whole at both ends.

Teacher note — why −215 and not −240

CHALLENGE 3 · L2-41-M3🪙 70 Adcoins10 min

Your own army

Make a bigger army. Change numbers only — no new blocks.

Ten columns instead of eight.
The whole army still fits on the Stage, with room at both sides.
No two invaders overlap.
The click test still gives the right row and column.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the same two loops, a bigger army. Only the numbers changed.

ten columns

Done when: ten columns build, and clicking the last invader in the front row says row 5, column 10.

Teacher reveal — the four numbers that change

The repeat goes to 10, and the gap has to shrink so ten fit. With a gap of 36, column 10 is at 360. Take away 198 and the army runs from −162 to 162.


repeat (10)
  go to x: (((my column) * (36)) - (198)) y: ((172) - ((my row) * (32)))
  create clone of [myself v]
  change [my column v] by (1)
end

A good question for the pupil: why did the 180 have to change? (Because the army must stay in the middle when it gets wider.)

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch ten columns build. Then click the stage once and hold ← or → to move the ship.

Ten columns from the same two loops. Only the numbers changed.

EXTRA MISSION · L2-41-MX🪙 100 Adcoinsbonus

Shields

The real game has four green shields above the ship. Each one is a small grid of bricks. Build them with loops, not by hand.

WHAT IT SHOULD LOOK LIKE

Extra — four shields between the ship and the army, built by loops inside loops.

shields up

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch four shields build under the army. Click the stage once, then hold ← or → to move the ship.

Four shields of eight bricks each, from one brick costume and three loops.

Teacher notes — three loops deep

The trick is putting the pen back after each row and each shield. After a row of four, step back 48 and down 9. After a whole shield, step right 110 and back up 18.


when flag clicked
hide
go to x: (-183) y: (-90)
repeat (4)
  repeat (2)
    repeat (4)
      create clone of [myself v]
      change x by (12)
    end
    change x by (-48)
    change y by (-9)
  end
  change x by (110)
  change y by (18)
end

when I start as a clone
show

Summary 4 min

A repeat inside a repeat makes a grid: rows of rows.
A clone is born with a copy of the original’s “For this sprite only” variables.
So every invader keeps its own (my row) and (my column).
switch costume to (my row) gives each row its own look.
Two comparisons keep the ship on the Stage.

Nested loop: A loop inside another loop. The inside one runs in full each time round.
Grid: Things laid out in rows and columns.
For this sprite only: A variable each sprite — and each clone — keeps for itself.
Clamp: Putting a number back inside its limits when it goes past them.

Next lesson: a laser. Hit an invader and it is gone — and the back row is worth the most.