Kart Racer IV

Review — Kart Racer III 5 min

• The gates were two lists, gate x and gate y.
• One Gate sprite stood on the next gate, waited for the kart, then hopped on.
• A lap time was timer − lap start, and best survived the green flag.

Quick-fire

The Gate always stood exactly where the kart had to go next. If a kart had no driver, how could that same trick help it?

Today’s Topic 3 min

• A rival that uses your own drive block — only the driver is a script
• Waypoints in two lists, and a hidden Pin on the next one
• Slowing down for corners, from a third list: way speed
• Start lights, a race position and a finish card

Learning outcome

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

1. Write a script that drives a sprite by choosing its inputs.
2. Use a list to tell a computer player how to behave in each place.
3. Work out who is winning from two numbers.

Learnt — a rival kart 28 min

1 · Same kart, different driver

Duplicate your kart and call it Rival. It keeps the whole drive (gas) (steer) :: custom block — same push, same friction, same steering. A computer player that cheats on speed is no fun to beat.

Only the key blocks go. In their place, a script decides gas and steer every tick.

2 · Waypoints and the Pin

The rival needs to know where to go. That is eight points, one at each corner of the track, in two lists: way x and way y. The last point is the finish line.

A hidden sprite called Pin stands on the rival’s next point. It is the Gate trick again:

when flag clicked
hide
set [rival wp v] to (1)
set [rival laps v] to (0)
forever
  go to x: (item (rival wp) of [way x v]) y: (item (rival wp) of [way y v])
  wait until <(distance to [Rival v]) < (60)>
  change [rival wp v] by (1)
  if <(rival wp) > (length of [way x v])> then
    set [rival wp v] to (1)
    change [rival laps v] by (1)
  end
end
            

3 · Steering towards the Pin

The rival needs a steer, not a new direction. So it peeks: point at the Pin, remember that direction, then turn back. The gap between the two, wrapped like the slip in Kart Racer II, says which way to steer.

set [old v] to (direction)
point towards [Pin v]
set [want v] to (direction)
point in direction (old)
set [turn v] to (((((want) - (old)) + (540)) mod (360)) - (180))
set [steer v] to (0)
if <(turn) > (3)> then
  set [steer v] to (1)
end
if <(turn) < (-3)> then
  set [steer v] to (-1)
end
            

4 · Slowing for corners

Flat out into every corner, the rival runs wide onto the grass. So each corner gets a speed limit, in a third list, way speed: 4.5, 5, 4, 4.5, 4.5, 5, 4.5 — and 10 for the finish straight.

set [limit v] to (item (rival wp) of [way speed v])
set [gas v] to (1)
if <<(distance to [Pin v]) < (100)> and <(rival speed) > (limit)>> then
  set [gas v] to (-1)
end
drive (gas) (steer) :: custom
            

5 · Start lights

A Lights sprite with five costumes (off, 1, 2, 3, go) and one variable, racing. Both karts start with wait until <(racing) = (1)>.

when flag clicked
show
set [racing v] to (0)
set [light v] to (0)
switch costume to [lights-off v]
wait (0.6) seconds
repeat (3)
  change [light v] by (1)
  switch costume to (join [lights-] (light))
  wait (0.8) seconds
end
switch costume to [lights-go v]
set [racing v] to (1)
say [GO!] for (1) seconds
hide
            

6 · Who is winning?

Count gates passed. You pass 4 a lap. The rival passes 8 waypoints a lap — and every second waypoint is a gate, so halve it.

set [my gates v] to (((laps) * (4)) + ((next gate) - (1)))
set [rival gates v] to ([floor v] of ((((rival laps) * (8)) + ((rival wp) - (1))) / (2)))
if <(my gates) > (rival gates)> then
  set [position v] to [1st]
end
if <(my gates) < (rival gates)> then
  set [position v] to [2nd]
end
            

7 · The finish

Whoever finishes lap 3 first sets winner — but only if it is still empty. Then a Card sprite shows you-win or you-lose.

if <(laps) = (3)> then
  if <(winner) = []> then
    set [winner v] to [you]
  end
  broadcast (finish v)
end
            

Basic — race the rival 25 min

Open your race from Kart Racer III.

Step 1 — the rival

1. Duplicate the Kart. Rename it Rival and switch its costume to car-blue.svg.
2. Give it its own rival speed (For all sprites, so you can watch it), and use it everywhere in its drive block instead of speed.
3. Delete its key script. Put it on the lower grid box: x −37, y 113.

Step 2 — the lists and the Pin

1. Make three lists: way x = 170, 185, 120, 10, −100, −185, −170, 0; way y = 125, −10, −125, −60, −125, −40, 125, 125; way speed = 4.5, 5, 4, 4.5, 4.5, 5, 4.5, 10.
2. Add a small Pin sprite (any shape) and build its script from Learnt section 2.

Step 3 — the driver

On the Rival: wait until <(racing) = (1)>, then a repeat until <not <(winner) = []>> holding sections 3 and 4.

Step 4 — lights, position, finish

1. Add the Lights sprite from section 5.
2. In your Kart: wait until <(racing) = (1)> before its loop, and the position blocks from section 6 inside it.
3. Add the finish blocks from section 7 to the Gate and the Pin, and a Card sprite.

Pick how good the rival is

New players need an easy rival. Fast players need a hard one.

• A skill variable: 0.8 easy, 1 normal, 1.1 hard.
• While the lights are red, keys 1, 2 and 3 choose it.
• The rival’s corner limits are multiplied by skill.

set [limit v] to ((item (rival wp) of [way speed v]) * (skill))
            

when [1 v] key pressed
if <(racing) = (0)> then
  set [skill v] to (0.8)
end
            

A jump start

Pressing ↑ before green should cost you.

• Hold ↑ while any light is red: that is a jump start.
• On green, the kart says “Jump start! 1 second penalty.” and waits one second.
• The rival leaves on time.

set [penalty v] to (0)
repeat until <(racing) = (1)>
  if <key (up arrow v) pressed?> then
    set [penalty v] to (1)
  end
end
if <(penalty) = (1)> then
  say [Jump start! 1 second penalty.] for (1) seconds
end
            

A rubber band

A race you win by half a lap is boring. So is one you lose by half a lap.

• While you are 1st, the rival’s skill is 1.15.
• While it is ahead, its skill drops to 0.9.
• The race should stay close all the way.

if <(position) = [1st]> then
  set [skill v] to (1.15)
else
  set [skill v] to (0.9)
end
            

Race your own ghost

The best rival is you, from one lap ago.

• During lap 1, record where the kart is every tick, in three lists.
• When lap 1 ends, a see-through Ghost kart replays it.
• Lap 2 is you against your own lap 1.

if <(laps) = (0)> then
  add (x position) to [ghost x v]
  add (y position) to [ghost y v]
  add (direction) to [ghost dir v]
end
            

when I receive [ghost go v]
show
set [i v] to (1)
repeat (length of [ghost x v])
  go to x: (item (i) of [ghost x v]) y: (item (i) of [ghost y v])
  point in direction (item (i) of [ghost dir v])
  change [i v] by (1)
end
hide
            

Summary 5 min

• The rival has your kart. A script chooses its gas and steer.
• A hidden Pin on the next waypoint gives it something to point towards.
• A list of corner speeds tells it when to brake — and braking made it faster.
• Lights, a position from gates passed, and the first to three laps wins.

Waypoint

A point a computer player drives towards.

AI

Here: a script that plays the game by choosing the same inputs a person would.

Rubber band

Making the computer player easier when it is ahead and harder when it is behind.

Hand in

Save your finished Kart Racer — it is a whole game now. Next lesson starts something completely different: a quiz where every question is one line of text.