Scroll World IV

Review — levels 5 min

• One level variable picks the screens and the sky through join.
• A rebuild deletes the old clones and makes the new ones.
• A list can decide the order of the levels.

Quick-fire

You want ten coins in level 1. You could make ten Coin sprites. What is wrong with that, once there are two levels?

Today’s Topic 3 min

• Two lists, coin x and coin y — one line per coin
• One clone per line, placed like a screen
• Collected coins delete themselves
• Coins that belong to a level

Learning outcome

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

1. Place any number of collectables in a world from lists.
2. Keep small things from gathering at the edge of the Stage.
3. Give each level its own set of coins.

Learnt — coins 28 min

1 · Coins from lists

Make two lists, coin x and coin y. Line 1 of each is the first coin’s world position, line 2 the second, and so on. The Coin sprite reads them one line at a time and leaves a clone at each:

when I receive [build v]
if <(is clone) = (1)> then
  delete this clone
else
  hide
  set [n v] to (0)
  repeat (length of [coin x v])
    change [n v] by (1)
    set [my x v] to (item (n) of [coin x v])
    set [my y v] to (item (n) of [coin y v])
    create clone of (myself v)
  end
end
            

when I start as a clone
set [is clone v] to (1)
set y to (my y)
set x to ((my x) - (scroll x))
wait until <touching (Pico v)?>
change [coins v] by (1)
delete this clone
            

2 · The edge, again

Lesson 3-33’s flag waited at the edge of the Stage. Coins do exactly the same, and there are ten of them. A coin can even tell it has been moved: its x position is not the x it asked for.

So a coin places itself exactly as a screen does, with the hide:

when I receive [scroll v]
if <(is clone) = (1)> then
  set x to ((my x) - (scroll x))
  if <([abs v] of ((my x) - (scroll x))) > (250)> then
    hide
  else
    show
  end
end
            

Basic — the coin run 24 min

Add coins to level 1 of your game.

Step 1 — draw a coin

1. Paint a small gold coin — no green and no blue in it.
2. Give it my x, my y, n and is clone, all for this sprite only.

Step 2 — design the coins

1. Make coin x and coin y for all sprites, and tick them to show them on the Stage.
2. Type ten positions in. Stand Pico where you want a coin, and read its world x from Lesson 3-34.
3. Put at least one where only a jump can reach it.

Step 3 — the machine

1. Build the Coin’s build script and clone script from Learnt part 1.
2. Add the scroll script from part 2, with its hide.
3. Make coins, set it to 0 when the flag is clicked, and untick the two lists to tidy the Stage.

Step 4 — play it through

1. Collect every coin once. Check the counter reaches your total.
2. Fall in the water and check the coins you already have stay collected.

A flag that waits for every coin

The finish flag stays down until every coin is collected — Cheese Maze’s locked cheese, in a scrolling world.

• It waits until coins equals (length of [coin x v]).
• Then it switches to its raised costume.
• Touching it while it is down does nothing.

Coins that belong to a level

Level 2 needs its own coins, and level 1’s must not follow it.

• The build clears both lists first, then fills them for the current level.
• Old coin clones delete themselves on build, like the screens.
• Show how many coins each level has.

delete all of [coin x v]
delete all of [coin y v]
if <(level) = (1)> then
  add (60) to [coin x v]
  add (-100) to [coin y v]
else
  add (100) to [coin x v]
  add (-100) to [coin y v]
end
            

Coins worth more

Make two of the coins stars, worth 5 each.

• A third list, coin worth, the same length as the other two.
• Each clone keeps its own worth and wears the star costume if it is 5.
• Collecting adds worth, not 1.

Ship it

Put the whole arc together into a scrolling game you would hand to somebody else.

• A title that explains the keys.
• Two levels, each with its own screens, sky and coins.
• Water, three lives, and a card between the levels.
• A best coin total that survives a new game.

Summary 5 min

• Coins are one sprite, two lists, and one clone per line.
• Every clone places itself with ((my x) - (scroll x)) and hides when it is off the Stage.
• A collected coin deletes itself, so it can never be counted twice.
• Each level fills the lists with its own coins.
• A third list can give every coin its own worth.

Collectable

Something in a level that is picked up by touching it.

Level data

The lists that say where things go — separate from the code that puts them there.

Record

One line across several lists, all describing the same thing.

Clamped

Pulled back to the edge because Scratch will not let a sprite leave the Stage.

Hand in

Save your game. You have finished the Scroll World arc — a world bigger than the Stage, where everything knows where it lives.