Space Invaders III

Review — the laser 5 min

• (laser ready) is a switch: 1 means fire, 0 means wait.
• The invader notices the hit and tells the laser with a broadcast.
• (((6) - (my row)) * (10)) makes the back row pay the most.
• We count invaders up as they are built and down as they are hit.

Quick-fire

Forty invaders must all take one step at exactly the same moment. What can reach all forty at once?

Today’s Topic 3 min

• A drumbeat: one broadcast moves forty clones
• A flag: any invader can tell the whole army to turn
• Bombs from a random invader
• Three lives

Learning outcome

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

1. Move many clones in step with one broadcast.
2. Use a flag variable so one clone can change what all of them do.
3. Pick one clone at random by its row and column.

Learnt — forty invaders, one beat 28 min

1 · The wrong way first

The first idea is to let every invader march by itself. Each one gets its own direction and turns when it reaches the edge. Watch what happens.

when I start as a clone
switch costume to (my row)
set [my direction v] to (1)
show
forever
  wait (0.3) seconds
  change x by ((6) * (my direction))
  if <<(x position) > (215)> or <(x position) < (-215)>> then
    set [my direction v] to ((0) - (my direction))
    change y by (-16)
  end
end
            

An army must turn together. So one invader reaching the edge has to turn everybody. That needs two new things: a drum, and a flag.

2 · The drum

The Stage keeps the beat. Every half second it broadcasts march. Every invader hears it and takes one step.

forever
  wait (0.5) seconds
  broadcast [march v] and wait
end
              

when I receive [march v]
change x by ((step) * (army direction))
              

• (step) is how far one beat moves. We use 6.
• (army direction) is 1 for right and −1 for left. Six times −1 is −6: a step left.

3 · The flag

A flag is a variable that says “something happened”. Ours is (hit edge). It starts at 0.

1. After its step, each invader checks: am I past 215, or past −215?
2. If so, it sets (hit edge) to 1. It does not turn itself.
3. After the beat, the Stage looks at the flag. If it is 1, the Stage flips (army direction), broadcasts drop, and lowers the flag.

when I receive [army ready v]
set [army direction v] to (1)
set [hit edge v] to (0)
forever
  wait (0.5) seconds
  broadcast [march v] and wait
  if <(hit edge) = (1)> then
    set [army direction v] to ((0) - (army direction))
    broadcast [drop v] and wait
    set [hit edge v] to (0)
  end
end
            

when I receive [march v]
if <(my row) > (0)> then
  change x by ((step) * (army direction))
  if <<(x position) > (215)> or <(x position) < (-215)>> then
    set [hit edge v] to (1)
  end
end

when I receive [drop v]
change y by (-16)
            

The builder gets two lines at the very end:

when flag clicked
hide
set [invaders left v] to (0)
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 [invaders left v] by (1)
    change [my column v] by (1)
  end
  change [my row v] by (1)
end
set [my row v] to (0)
broadcast [army ready v]
            

when flag clicked
set [step v] to (6)
            

4 · Bombs

The army fires back. Each beat, the Stage picks a random row and a random column. The invader standing on that spot drops a bomb. If that spot is empty, nobody fires.

1. Make fire row, fire column, bomb x and bomb y, all For all sprites.
2. In the Stage’s loop, before the march: set [fire row v] to (pick random (1) to (5)) and the same for the column, 1 to 8.
3. Give the Invader a second march stack that checks: is it me?
4. Paint a small Bomb sprite, and give it a clone stack that falls.

when I receive [march v]
if <<(my row) = (fire row)> and <(my column) = (fire column)>> then
  set [bomb x v] to (x position)
  set [bomb y v] to (y position)
  create clone of [Bomb v]
end
            

when flag clicked
hide

when I start as a clone
go to x: (bomb x) y: (bomb y)
show
repeat until <(y position) < (-170)>
  change y by (-6)
  if <touching [Ship v]?> then
    broadcast [ship hit v]
    delete this clone
  end
end
delete this clone
            

5 · Three lives

1. Make lives. On the Stage’s flag stack, set [lives v] to (3).
2. On the Ship, when I receive [ship hit v]: lose a life, then flash.
3. At no lives left, broadcast [game over v].
4. Paint a Banner sprite with a big GAME OVER costume. It shows itself and stops everything.
5. Add show to the Ship’s flag stack. A game can end while the ship is flashing — hidden.

when I receive [ship hit v]
change [lives v] by (-1)
if <(lives) < (1)> then
  broadcast [game over v]
end
repeat (3)
  hide
  wait (0.1) seconds
  show
  wait (0.1) seconds
end
            

when flag clicked
hide
go to x: (0) y: (40)

when I receive [game over v]
go to [front v] layer
show
stop [all v]
            

when flag clicked
set [lives v] to (3)
set [step v] to (6)
            

Basic — make it march and fight back 24 min

Open last week’s game. Work in this order and test after every step.

Step 1 — the drum

1. Make step, army direction and hit edge, For all sprites.
2. Add the two lines to the end of the builder.
3. Build the Stage’s flag stack and its drum.
4. Give the Invader its march and drop stacks.
5. Press the flag. Watch one whole edge-turn-drop.

Step 2 — the bombs

1. Paint the Bomb and make the four new variables.
2. Add the two random picks to the drum.
3. Add the Invader’s second march stack and the Bomb’s stacks.
4. Sit still under the army. A bomb should reach you within a few seconds.

Step 3 — the lives

1. Add lives, the Ship’s hit stack and the Banner.
2. Lose three lives on purpose. GAME OVER must appear.
3. Press the flag. The ship must be back, with three lives.

In step

• The whole army marches together, one step a beat.
• When any invader reaches an edge, all of them turn.
• All of them drop together.
• The columns stay straight, however long it runs.

Legs

Every invader has a second costume with its legs out. Use it.

• On every beat, the whole army switches legs together.
• Each row keeps its own look while its legs move.
• The legs match across the whole army — all in, or all out.

set [frame v] to ((5) - (frame))
                

switch costume to ((my row) + (frame))
                

Three lives

• Bombs fall from random invaders — never from an empty spot.
• A bomb costs one life, and the ship flashes.
• At no lives left, GAME OVER shows and everything stops.
• The flag gives back three lives, a visible ship and a full army.

Shields that wear away

Bring back the shields from Lesson 2-41’s Extra. If you did not build them then, build them now. This time, they take damage.

when I start as a clone
show
wait until <<touching [Bomb v]?> or <touching [Laser v]?>>
wait (0.05) seconds
delete this clone
            

if <touching [Shield v]?> then
  delete this clone
end
                

repeat until <<(y position) > (170)> or <touching [Shield v]?>>
  change y by (10)
end
                

Summary 5 min

• One broadcast reaches every clone. A drum on the Stage moves the whole army.
• The hidden original hears it too. Give it row 0 and leave it out.
• A flag is a message: any clone raises it, the Stage reads it and lowers it.
• broadcast [march v] and wait lets everyone answer before the Stage looks.
• Pick a random row and column, and only the clone on that spot acts.

Beat

One tick of the drum. Everything that marches moves once.

Flag

A variable that says “this happened”. Raised, read, then lowered.

In step

Moving at the same moment by the same amount.

Broadcast and wait

Send a message, and carry on only when everyone has finished with it.

Next lesson: the army speeds up as it thins, waves come one after another, and the game gets a real ending.