Space Invaders II

Review — the army from one sprite 5 min

• A repeat inside a repeat builds a grid of clones.
• (my row) and (my column) are “For this sprite only”. Every clone keeps its own copy.
• switch costume to (my row) gives each row its own look.
• Two comparisons keep the ship on the Stage.

Quick-fire

You make the space bar fire a laser. You hold it down. What goes wrong?

Today’s Topic 3 min

• A switch: one laser at a time
• An invader that knows when it is hit
• Points that come from the row
• Counting what is left

Learning outcome

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

1. Use a variable as a switch: 1 for yes, 0 for no.
2. Remove a clone and the thing that hit it, with one broadcast.
3. Work out a score from a clone’s own variable.

Learnt — a laser with rules 27 min

1 · One laser at a time

A switch is a variable with only two values. We call ours (laser ready). 1 means “you may fire”. 0 means “wait”.

1. Paint a sprite called Laser: a thin bar, about 6 wide and 18 tall.
2. Make laser ready, For all sprites. The Ship and the Laser both need it.
3. On the Ship, a second flag stack: set [laser ready v] to (1) and a forever.
4. Inside: an if <> then with an <<> and <>> — space pressed, and laser ready is 1.
5. Inside the if: set [laser ready v] to (0) first, then create clone of [Laser v].

when flag clicked
set [laser ready v] to (1)
forever
  if <<key [space v] pressed?> and <(laser ready) = (1)>> then
    set [laser ready v] to (0)
    create clone of [Laser v]
  end
end
            

The laser’s job is to fly up, then switch the ship back on:

when flag clicked
hide

when I start as a clone
go to [Ship v]
show
repeat until <(y position) > (170)>
  change y by (10)
end
set [laser ready v] to (1)
delete this clone
            

2 · A hit

When a laser touches an invader, both must go. But only one of them should do the noticing. If the laser deleted itself first, the invader would never see it.

So the invader notices. Then it tells the laser with a broadcast.

1. In the invader’s clone stack, after show, add wait until <touching [Laser v]?>.
2. Then broadcast [laser hit v], the points, and delete this clone.
3. On the Laser: when I receive [laser hit v], switch the ship back on, and delete.

when I start as a clone
switch costume to (my row)
show
wait until <touching [Laser v]?>
broadcast [laser hit v]
change [score v] by (((6) - (my row)) * (10))
change [invaders left v] by (-1)
delete this clone
            

when I receive [laser hit v]
set [laser ready v] to (1)
delete this clone
            

Points by row

The back row should pay the most. Each invader already knows its row, so it can work out its own price:

3 · Counting what is left

Scratch cannot count clones for you. And a deleted clone cannot count itself. So we count as we make them, and count down as they go.

1. Make invaders left and score, both For all sprites.
2. In the builder, before the loops: set [invaders left v] to (0).
3. Next to create clone of [myself v]: change [invaders left v] by (1).
4. On the Stage: when flag clicked then set [score v] to (0).

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
            

when flag clicked
set [score v] to (0)
            

When it reaches 0

The invader that brings the count to 0 is the last one. It can say so, just before it goes:

change [invaders left v] by (-1)
if <(invaders left) = (0)> then
  broadcast [you win v]
end
delete this clone
            

when I receive [you win v]
say [You win!]
            

Basic — arm the ship 25 min

Open last week’s project. The army and the ship are there already.

Step 1 — the laser

1. Paint the Laser sprite. Make it thin and bright.
2. Give it the flag stack and the clone stack.
3. Make the switch and the Ship’s fire stack.
4. Hold space. Count the lasers in the air: it must always be one.

Step 2 — the hit

1. Add the wait-until, broadcast and delete to the invader’s clone stack.
2. Add the Laser’s when I receive [laser hit v] stack.
3. Shoot one invader. Both must vanish, and you must be able to fire again.

Step 3 — the score

1. Make score and reset it on the Stage.
2. Add the points sum to the invader.
3. Shoot a front-row invader: +10. Now a back-row one: +50.

Step 4 — the counter and the win

1. Add the two counting blocks to the builder.
2. Add the count-down and the if <> then to the invader.
3. Add when I receive [you win v] to the Ship.
4. Delete the click test from last week. It has done its job.

One at a time

• Space fires one laser from the ship.
• While it is in the air, space does nothing.
• The moment it is gone, the next one can fire.

The back row pays

• The front row is worth 10.
• Each row further back is worth 10 more.
• The back row is worth 50.
• The score starts at 0 every game.

Clear the army

• invaders left starts at 40 and counts down on every hit.
• When the last invader goes, the game says you won.
• Press the flag again and everything starts fresh.

The mystery saucer

Now and then a saucer crosses the very top of the Stage. Hit it for a surprise prize.

when flag clicked
hide
forever
  wait (pick random (5) to (10)) seconds
  go to x: (-225) y: (165)
  create clone of [myself v]
end

when I start as a clone
show
repeat until <(x position) > (225)>
  change x by (3)
  if <touching [Laser v]?> then
    broadcast [laser hit v]
    change [score v] by ((pick random (1) to (6)) * (50))
    delete this clone
  end
end
delete this clone
            

Summary 4 min

• A switch is a variable with two values: 1 for yes, 0 for no.
• Switch off as you fire. Switch on when the laser is gone.
• When two sprites touch, let one notice and tell the other with a broadcast.
• A clone can work out its own price from its own variable: (((6) - (my row)) * (10)).
• Deleted clones cannot be counted. Count up as you make them, down as they go.

Switch

A variable that is only ever 1 or 0 — on or off.

Hit

The moment two sprites touch in a game.

Counter

A variable that goes up or down by one to keep count.

Checkpoint

A version of your game that works and is worth saving.

Next lesson: the army marches — all forty together — and fires back.