Space Race II

Review — where we got to 5 min

Two lanes, two keys, one finish line. Two ideas come back today.

• if <key [d v] pressed?> then inside a forever loop.
• <() = ()>, from the Quiz Show.

Quick-fire

Hold your key down BEFORE anybody presses the green flag. What happens, and why is that a problem?

Today’s Topic 3 min

• A race on variable used as a switch, not as a number
• A referee sprite that counts down and turns the switch on
• An if inside an if: the race is on AND the key is down
• The winner switching the race back off

Learning outcome

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

1. Use a variable to turn other scripts on and off.
2. Run a countdown that every player has to wait for.
3. Freeze the race the moment somebody wins it.

Learnt — a switch, a referee, a freeze 28 min

1 · A variable that means yes or no

Every variable so far has been a number that counted something. This one only ever holds 0 or 1, and those stand for no and yes.

1. Make a variable called race on.
2. Leave it showing for now, so you can watch it flip.
3. It starts at 0. That means the race has not started.
4. Something will set it to 1 later. That means GO.

2 · The referee

Add one more sprite whose whole job is to start the race. It counts down and then turns the switch on.

1. Add a sprite near the top of the Stage and name it Referee.
2. Give it when flag clicked.
3. First block: set [race on v] to (0).
4. Then three say blocks, one second each: 3, 2, 1.
5. Then set [race on v] to (1).
6. Then say [GO!] for (1) seconds.

when flag clicked
set [race on v] to (0)
say [3] for (1) seconds
say [2] for (1) seconds
say [1] for (1) seconds
start sound [Win v]
set [race on v] to (1)
say [GO!] for (1) seconds
            

3 · One more question inside the controller

The switch does nothing on its own. Every controller has to ask about it, and the answer is an if inside an if.

1. Open your rocket’s controller.
2. Drag a new if <> then onto the empty canvas.
3. Put <(race on) = (1)> in its diamond.
4. Drag your WHOLE existing key-if into its mouth.
5. Drag the pair back into the forever loop.

forever
  if <(race on) = (1)> then
    if <key [d v] pressed?> then
      change x by (4)
    end
  end
end
            

Do the same on the second rocket. Both controllers must be gated, or one player can start early and the other cannot.

4 · The winner turns it off

One more line in the winning script. Without it the loser keeps flying while the result is on screen, which looks like the game did not notice.

when flag clicked
wait until <touching [Finish v]?>
set [race on v] to (0)
start sound [Cheer v]
say [Blue wins!] for (3) seconds
stop [all v]
            

Basic — a race with a proper start 25 min

Your race from last week. Three passes over it.

Step 1 — the switch and the referee

1. Make the race on variable and leave it showing.
2. Add a referee sprite at the top of the Stage.
3. Build its countdown script.
4. Press the flag and watch the number flip. Do not touch the rockets yet.

Step 2 — gate the first rocket

1. Build the outer if on the empty canvas.
2. Drag the existing key-if inside it.
3. Drag the whole thing back into the forever loop.
4. Test by holding your key from before the flag. Nothing should move.

Step 3 — gate the second

1. Do exactly the same on rocket 2.
2. Test both players holding their keys early.

Step 4 — the freeze

1. Add set [race on v] to (0) to both winning scripts.
2. It goes above the sound and the message.
3. Race properly and watch the loser stop dead when the winner crosses.

Step 5 — hide the number

1. Once it all works, untick the race on box.
2. A switch is for your project, not for the player.
3. Race five more times to check nothing broke.

A countdown worth waiting for

Three say blocks is a countdown. Make yours something players actually feel.

• A sound on each number, or one that builds.
• The GO is bigger, louder or brighter than the numbers.
• It is the same length every time, so players can learn it.

A rematch

Losing should be followed by racing again, not by reaching for the green flag. Add a way to run it back.

• A rematch starts without touching the green flag.
• Both rockets go back to their start lines.
• The countdown runs again.
• Nobody can move during it.

The photo finish

Last lesson you found the bug: cross together and both winning messages fire. Fix it. Only one player may win.

• Two rockets crossing in the same instant produce exactly one message.
• The same player does not always get it.
• The race still ends properly.

when flag clicked
wait until <touching [Finish v]?>
if <(race on) = (1)> then
  set [race on v] to (0)
  say [Blue wins!] for (3) seconds
  stop [all v]
end
            

A false start costs you

Pressing early currently does nothing. Make it send the rocket backwards, so guessing the GO is a real risk.

• Pressing during the countdown moves the rocket backwards.
• The player is told why.
• A rocket cannot be pushed off the left of the Stage.
• Pressing after GO still works normally.

forever
  if <key [d v] pressed?> then
    if <(race on) = (1)> then
      change x by (4)
    else
      start sound [Bonk v]
      say [Too early!] for (0.6) seconds
      change x by (-20)
    end
  end
end
            

Summary 4 min

One variable, holding nothing but 0 or 1, decided what every other script was allowed to do.

1. A switch is a variable that only ever holds 0 or 1.
2. The referee owns the switch. Nothing else turns it on.
3. Every controller asks about it, with an if inside an if.
4. The winner turns it off, which freezes everybody at once.

Switch

A variable holding 0 or 1, standing for no and yes.

Referee

A sprite whose job is to run the rules rather than to be played.

Gate

An if wrapped round something, so it only happens when a condition is met.

False start

Moving before the GO. A game has to decide what that costs.

Next lesson: asteroids in the way, and a star worth flying for.