Tower Defence I

Review — the Scroll World arc 5 min

• Everything in a big world keeps its own place in a variable made for this sprite only.
• A list can hold a level’s design: where the coins go, which level comes next.
• The original sprite hears broadcasts too, so it has to be one of the copies.

Quick-fire

A bug has to walk a winding path to the castle. Could you paint the path and have the bug follow the colour, like the Line Follower in Level 2?

Today’s Topic 3 min

• A path as two lists: path x and path y
• A glide whose seconds are a sum: distance ÷ speed
• A wave: clones, each with its own next
• Lives lost at the castle gate

Learning outcome

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

1. Make a clone walk any path described in two lists.
2. Send a wave of clones that each keep track of their own progress.
3. Change how fast, how many and how tough the enemies are with numbers alone.

Learnt — the path and the wave 28 min

1 · The path is a list of corners

The road has seven corners. Put their x values in a list called path x and their y values in path y, in order. A bug starts at corner 1 and glides to each next corner until there are none left:

when I start as a clone
go to x: (item (1) of [path x v]) y: (item (1) of [path y v])
show
set [next v] to (2)
repeat until <(next) > (length of [path x v])>
  set [leg v] to (([abs v] of ((item (next) of [path x v]) - (x position))) + ([abs v] of ((item (next) of [path y v]) - (y position))))
  glide ((leg) / (speed)) secs to x: (item (next) of [path x v]) y: (item (next) of [path y v])
  change [next v] by (1)
end
if <(lives) > (0)> then
  change [lives v] by (-1)
end
delete this clone
            

leg is the distance to the next corner: how far along x plus how far along y. On a straight leg one of those is always 0 — Vending Machine’s abs, earning its keep. And lives never goes below 0, because bugs already on the path keep arriving after the castle has fallen.

2 · A wave is clones

One Bug sprite, hidden, makes the whole wave. It sets the health, makes a clone, waits, and does it again. Every clone runs the walk above for itself — which only works because next is for this sprite only.

when I receive [wave v]
repeat (bugs to send)
  set [health v] to (bug health)
  change [bugs alive v] by (1)
  create clone of (myself v)
  wait (0.8) seconds
end
            

Basic — the march 24 min

Start a new project. You are building the field and the enemy — no towers yet.

Step 1 — paint the field

1. A grass backdrop, a road that turns only at right angles, and a castle at the end.
2. Paint the road in one flat colour — Lesson 3-38’s Extra will ask about it.

Step 2 — the corners

1. Make lists path x and path y.
2. Hover the mouse over each corner of your road and read its x and y under the Stage. Type them in, in order, start to castle.

Step 3 — the bug

1. Add a bug sprite at size 40, hidden when the flag is clicked.
2. Give it next, health and speed, for this sprite only. Set speed to 100.
3. Build the clone’s walk from Learnt part 1.

Step 4 — the wave

1. Make lives, bugs alive, bugs to send and bug health, for all sprites.
2. Build the wave script from Learnt part 2.
3. From the flag: set lives to 10, bugs to send to 8, and broadcast [wave v].

One next for everybody

Make next for all sprites instead, send four bugs, and count the damage.

• A watcher in each clone counts the moments it is not touching the road colour.
• Watch where the bugs go.
• Explain what went wrong, using the word “share”.

Two kinds of bug

Add beetles that walk at 60 a second beside ladybugs at 120. Send the beetles first.

• The same walk; only speed is different.
• Each bug adds its kind to an arrivals list as it reaches the gate.
• Read the list afterwards.

Waves that grow

Send three waves. Wave n has 2 + 2n bugs, and each waits for the last to finish.

• A wave counter.
• wait until <(bugs alive) = (0)> between waves.
• Start with 20 lives and count how many get through.

set [wave v] to (0)
repeat (3)
  change [wave v] by (1)
  set [bugs to send v] to ((2) + ((2) * (wave)))
  broadcast [wave v] and wait
  wait until <(bugs alive) = (0)>
end
            

A boss

After the wave, send one boss: bigger, slower, tougher, and worse for the castle.

• A Boss sprite: the beetle at size 70, speed 70, health 80.
• It walks the same lists.
• Reaching the gate costs three lives, not one.

Summary 5 min

• A path is two lists of corners; a bug glides from one to the next.
• A leg’s glide time is its length divided by the bug’s speed.
• A wave is one hidden sprite making clones; each keeps its own next.
• bugs alive says when a wave is really over.

Waypoint

A corner on the path that an enemy heads for next.

Wave

A group of enemies sent one after another.

Spawn

Making a new enemy — here, a clone.

Boss

A single, tougher enemy, usually at the end of a wave.

Hand in

Save your field. Next lesson the castle fights back.