Kart Racer III

Review — Kart Racer II 5 min

• A step is speed × sin(d) across and speed × cos(d) up.
• The kart moved by its heading, which followed its direction by a fifth of the gap each tick. The gap was the slip.
• The wrap — + 540, mod 360, − 180 — found the short way round.
• The Pen drew skids while the slip was over 15°.

Quick-fire

Level 2’s Car Race had coloured posts round the track, to stop people cheating a lap. How did it make sure they were driven in order?

Today’s Topic 3 min

• Checkpoint gates stored in two lists: gate x and gate y
• One Gate sprite that only stands on the next gate
• A lap time: timer − lap start
• A best lap that survives the green flag

Learning outcome

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

1. Make a track’s checkpoints out of data, not drawings.
2. Time something with the timer, without resetting it.
3. Keep a record that a new game does not wipe.

Learnt — a lap you cannot cheat 28 min

1 · The cheat

The easy way to count laps is “every time the kart touches the white line, add 1”. Watch what that does to a kart that just rocks over the line.

2 · Gates in two lists

A lap is four gates, taken in order. Their places go in two lists, one item per gate. Item 1 of gate x goes with item 1 of gate y.

3 · One Gate sprite

You do not need four sprites. One Gate stands on the next gate, waits for the kart, then jumps to the one after. A gate that is not there cannot be touched, so the order cannot be broken.

when flag clicked
set [next gate v] to (1)
set [laps v] to (0)
forever
  go to x: (item (next gate) of [gate x v]) y: (item (next gate) of [gate y v])
  wait until <touching [Kart v]?>
  change [next gate v] by (1)
  if <(next gate) > (length of [gate x v])> then
    set [next gate v] to (1)
    change [laps v] by (1)
  end
end
            

4 · Lap times and a best lap

The timer counts seconds from when the project starts. We never reset it. Instead, we write down the time a lap started, and take it away at the end.

set [lap time v] to (([round v] of (((timer) - (lap start)) * (100))) / (100))
add (lap time) to [lap times v]
if <<(best) = (0)> or <(lap time) < (best)>> then
  set [best v] to (lap time)
end
set [lap start v] to (timer)
            

Look at what the green flag does not do: it never sets best. So the record stays when you play again. 0 means “no best lap yet”.

Basic — a race against the clock 25 min

Open your kart from Kart Racer II. The kart itself stays the same.

Step 1 — the lists and variables

1. Make two lists, For all sprites: gate x and gate y. Type the four numbers from the table into each.
2. Make a third list, lap times, and the variables next gate, laps, lap start, lap time and best.

Step 2 — the Gate

1. Save our gate, gate.svg, and use Upload Sprite. Call it Gate and set its size to 110.
2. Build the script from Learnt section 3.
3. Put the lap-time blocks from section 4 inside its if, after change [laps v] by (1).
4. Under its green flag, add go to [back v] layer, delete all of [lap times v] and set [lap start v] to (timer).

A lap board

Tell the driver where they are in the race.

• Crossing the line after lap 1: the kart says “Lap 2 of 3”.
• After lap 2: “Final lap!”.
• After lap 3: “Finished!”, and it stays.

when I receive [new lap v]
if <(laps) = (1)> then
  say [Lap 2 of 3] for (1.5) seconds
end
if <(laps) = (2)> then
  say [Final lap!] for (1.5) seconds
end
if <(laps) = (3)> then
  say [Finished!]
end
            

Three laps and stop

A race has an end.

• After lap 3, the keys stop working.
• The kart rolls to a stop on its own friction.
• It says the whole race time, to one decimal place.

when flag clicked
set [race start v] to (timer)
repeat until <(laps) = (3)>
  drive (gas) (steer) :: custom
end
repeat (40)
  drive (0) (0) :: custom
end
say (join [Finished! 3 laps in ] (join (([round v] of (((timer) - (race start)) * (10))) / (10)) [ s.]))
            

A fifth gate

Prove the track really is data.

• Add a gate in the bottom-right corner, at (120, −125).
• It must come between gate 1 and the old gate 2.
• Do not change a single block.

insert (120) at (2) of [gate x v]
insert (-125) at (2) of [gate y v]
            

Split times

Real race timing does not wait for the end of the lap.

• On lap 1, write down the time at every gate.
• On lap 2, at each gate, show how far ahead (−) or behind (+) lap 1 you are.
• The Gate says it, for one second, as you go through.

set [split v] to ((timer) - (lap start))
if <(laps) = (0)> then
  add (split) to [best splits v]
else
  set [diff v] to (([round v] of (((split) - (item (next gate) of [best splits v])) * (10))) / (10))
  if <(diff) > (0)> then
    say (join [+] (diff)) for (1) seconds
  else
    say (diff) for (1) seconds
  end
end
            

Summary 5 min

• The gates are two lists. A new gate is two new numbers.
• One Gate sprite shows only the next gate, so they can only be taken in order.
• A lap time is timer − lap start. The timer is never reset.
• A best lap that the green flag does not set survives every new game.

Checkpoint

A place a racer must pass, so a lap cannot be cut short.

Lap time

How long one lap took: the time now, take away the time it started.

Split

The time at one checkpoint, part-way round a lap.

Hand in

Save your race. Next lesson you stop racing the clock: a rival kart lines up beside you, and it drives itself.