Car Race Game II

Review — where we got to 5 min

• Two cars, two sets of keys, both live at once.
• A car turns, then moves forward — turn cw (15) degrees and then move (10) steps, in that order.
• Checkpoints in order stop the finish line being cheated.

Quick-fire

Your car does exactly 4 steps whenever you hold up, and stops dead the instant you let go. What does that feel like?

Today’s Topic 3 min

• Acceleration — speed that builds up
• Friction — speed that leaks away
• Steering as a number, and what a negative turn means
• Checkpoints as a sprite, placed by your own block

Learning outcome

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

1. Make a sprite that keeps moving after you let go of the key.
2. Use one variable, positive or negative, to turn either way.
3. Place five checkpoints with one My Block and count laps from them.

Learnt — giving the car weight 28 min

What is missing

A real car does not go from still to fast in an instant. It accelerates — it gets quicker while you keep your foot down. And when you lift off, it does not stop; it rolls, and slowly loses speed.

Your car does neither. Let’s fix both with one variable.

Step 1 — the key stops moving the car

This is the strange bit, and it is the whole idea: the up arrow no longer moves the car. It makes a number bigger.

if <key [up arrow v] pressed?> then
  change [speed v] by (0.8)
end
if <key [down arrow v] pressed?> then
  change [speed v] by (-0.8)
end
            

Step 2 — one block does all the moving

move (speed) steps
            

Try it. Hold up for a second, then let go. The car keeps going for ever, because nothing ever makes speed smaller again.

Step 3 — friction

Take 10% off the speed every time round the loop:

set [speed v] to ((speed) * (0.9))
            

Steering, the same way

Do it again with a second variable. And here is the neat part — there is only one turn block:

if <key [left arrow v] pressed?> then
  change [steering v] by (-1.2)
end
if <key [right arrow v] pressed?> then
  change [steering v] by (1.2)
end
turn cw (steering) degrees
set [steering v] to ((steering) * (0.85))
            

Checkpoints, properly

Last lesson the checkpoints were colours painted on the track. That works, but it means every track you draw has to use exactly the right colours.

Better: make them a sprite, with five costumes numbered 1 to 5. Post 5 is the finish line.

Placing five clones is five near-identical jobs — so make a block:

define Post (x) (y) (number)
go to x: (x) y: (y)
switch costume to (number)
create clone of [myself v]
            

when flag clicked
hide
Post (145) (62) [1] :: custom
Post (145) (-62) [2] :: custom
Post (-145) (-62) [3] :: custom
Post (-145) (62) [4] :: custom
Post (0) (100) [5] :: custom
            

One rule for all five

Every post runs the same check:

1. Am I touching the car?
2. Is my number exactly one more than the last checkpoint collected?
3. If yes, I am the new last checkpoint.
4. And if I am number 5, that is a lap — count it and go back to 0.

when I start as a clone
show
forever
  if <touching [Red car v]?> then
    if <(costume [name v]) = ((P1 checkpoint) + (1))> then
      set [P1 checkpoint v] to (costume [name v])
      if <(P1 checkpoint) = (5)> then
        change [P1 lap v] by (1)
        set [P1 checkpoint v] to (0)
      end
    end
  end
end
            

Basic — upgrade your race 24 min

Open your Lesson 5 race.

Step 1 — two variables per car

Make P1-Speed and P1-Steering. Set both to 0 when the flag is clicked.

Step 2 — rewrite the controls

The four key checks now only change numbers. Then, still inside the loop: turn cw (P1-Steering) degrees, move (P1-Speed) steps, then the two ×-blocks.

Step 3 — feel it

Drive it. Try 0.9, then 0.98, then 0.7 for the friction. Bigger is more slippery. Pick what you like.

Step 4 — the checkpoint sprite

New sprite, five costumes, the Post My Block, and the clone script above.

Step 5 — player 2

Duplicate the car, change the four keys, and make P2-Speed, P2-Steering, P2 checkpoint and P2 lap. Add a second check to each post for the blue car — one sprite watches both.

Give it weight

• Holding up makes the car faster and faster, up to a top speed.
• Letting go makes it roll on and slow down by itself.
• Turning has the same feel.

Grass, not a wall

The deck’s mission. Going off the track must not send you back to the start.

• On the grass, the car crawls.
• Back on the road, it picks up speed normally.
• Nothing teleports, and nothing resets.

Five posts, two cars

• One checkpoint sprite, five clones, placed by a My Block.
• Skipping a post does nothing at all.
• Both cars are counted, separately, by the same five posts.

Three tracks

Next lesson the game gets a menu, and a menu needs something to choose between. Draw three tracks.

• An easy one — wide, gentle corners.
• A hard one — narrow, twisty.
• One of your own design. A figure of eight is a good one.

Summary 5 min

• Acceleration: the key changes a number, and one move (speed) steps does the moving.
• Friction: multiply by 0.9 every loop and the car coasts to a stop by itself.
• One turn block does both directions, because a negative turn goes the other way.
• Checkpoints as a sprite work on any track, and a My Block places all five in five lines.
• A post counts only if its number is one more than the last one — which is what makes a lap honest.

Acceleration

Speed going up while you keep pushing.

Friction

Speed leaking away when you stop pushing.

Negative

Below zero. In motion blocks it means the opposite direction.

Clone

A copy of a sprite made while the game runs. All five posts are one sprite.

Hand in

Share your race and send the link to your teacher — and bring three tracks next week.