Muffin Catcher I

Review — where we got to 5 min

The Shark Game finished Level 1’s first arc. Three ideas from it come back today.

• Clones. One sprite made many copies of itself, and each copy ran the same script.
• Touching. A fish asked touching [Shark v]? and deleted itself when the answer was yes.
• The controller. A forever with an if <> then inside it for each key.

Quick-fire

In the Shark Game every fish swam about in a random direction. Today’s muffins do something much simpler. What is it?

Today’s Topic 3 min

• Falling objects — clones that start at the top and travel down
• go to x: () y: (170) with (pick random (-200) to (200)) in the x slot
• change y by (-10) inside a repeat (10)
• A catcher that only moves sideways

Learning outcome

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

1. Make a sprite that drops from a random place along the top of the Stage.
2. Delete a clone when it is caught, and when it reaches the ground.
3. Steer a catcher left and right while things are falling.

Learnt — a falling muffins 27 min

Nearly every game has something falling in it. Rain, apples, bombs, snow. They are all built the same way, and it is today’s whole lesson.

Three questions

Any falling thing has to answer three questions. Nothing else.

1. Where do I start?
2. How do I go down?
3. When do I disappear?

Question 1 — where do I start?

At the top. The top edge is y = 180, so 170 puts the muffin just inside it.

The x is the interesting part. If it were always 0, every muffin would fall down the middle and the game would be over in a second.

1. Drag go to x: (0) y: (0) from Motion.
2. Open Operators and drag (pick random (1) to (10)) into the x slot.
3. Type −200 and 200 into its two boxes.
4. Type 170 into the y box.
5. Click the block a few times. The muffin jumps about along the top.

Question 2 — how do I go down?

In small steps, inside a loop. One big jump would be over before anyone saw it. This is the same reason the Shark Game moved five pixels at a time.

change y by (-340)
              

repeat (34)
  change y by (-10)
end
              

Thirty-four steps of ten is 340, which is the height of the Stage from top to bottom. So the loop ends exactly as the muffin reaches the ground.

Question 3 — when do I disappear?

There are two endings, and a falling thing needs both.

• Caught. It touched the bowl, so it goes.
• Missed. It reached the ground, so it goes.

A clone that never deletes itself is a real problem. Scratch stops making new ones once 300 are on the Stage, and your game quietly dies.

The whole muffin, in two stacks

The original sprite hides and makes clones. Each clone falls.

when flag clicked
hide
forever
  wait (1) seconds
  create clone of [myself v]
end

when I start as a clone
show
go to x: (pick random (-200) to (200)) y: (170)
repeat (34)
  change y by (-10)
  if <touching [Bowl v]?> then
    delete this clone
  end
end
delete this clone
            

What the computer actually does

1. Hide the original. Nobody ever sees it.
2. Wait a second. Make a clone.
3. The clone shows itself and jumps to the top, at a random x.
4. Move down ten. Is the bowl touching me? No.
5. Move down ten. Is the bowl touching me? No.
6. …thirty-four times, and then delete myself anyway.

The catcher

You built this in Topic 1.6. The bowl is that controller with the up and down parts removed, because a bowl on a table does not fly.

when flag clicked
go to x: (0) y: (-140)
forever
  if <key [right arrow v] pressed?> then
    change x by (10)
  end
  if <key [left arrow v] pressed?> then
    change x by (-10)
  end
end
            

Basic — build the stall 25 min

Build it in this order and test after every step. Do not skip the testing.

Step 1 — the stall

1. Start a new project and delete the cat.
2. Choose the backdrop Colorful City. That is our market.
3. Add the Bowl sprite. Add the Muffin sprite for the muffin.

The Scratch library has no muffin, so the muffin stands in for one. Your own game can use anything you like.

Step 2 — the bowl slides

1. Click the Bowl. Build the controller from the Learnt section.
2. Click the flag, click the Stage, then hold each arrow key.
3. The bowl should slide along the bottom and never move up or down.

Step 3 — one muffin, falling

1. Click the Muffin. Build the maker stack: hide, then a forever with a wait and a clone.
2. Build the falling stack under when I start as a clone.
3. Leave the touching-if out for now. Just get it falling.
4. Click the flag. A muffins should appear every second and drop to the ground.

Step 4 — catching

1. Drop an if <> then inside the repeat, under the change y.
2. Drag touching [mouse-pointer v]? into the diamond and change the dropdown to Bowl.
3. Put delete this clone inside the if.
4. Play it. A caught muffins should vanish the moment it lands in the bowl.

Step 5 — clear the leftovers

1. Check there is a delete this clone below the repeat as well.
2. Let three or four muffins hit the ground without catching them.
3. They should disappear, not pile up in a row along the bottom.

Step 6 — make it fair

1. Play for thirty seconds. Too easy? Change the wait to 0.6.
2. Too hard? Make the repeat 68 and the change y −5.
3. That is the same fall, at half the speed. Pick the numbers your class enjoys.

Your own stall

Build the same game out of sprites nobody else in the room has picked. A basket of plum, a hat catching raindrops, a plate under falling pancake.

• Your catcher moves left and right only.
• Your falling thing starts somewhere different every time.
• It vanishes when it is caught, and when it lands.

Two kinds of muffins

Add a second falling sprite. It must look different from the first, and it must fall at a clearly different speed.

• Both kinds fall from random places along the top.
• One is quicker than the other, and you can see which without being told.
• Both can be caught, and both disappear when they land.

The whole stall

Three kinds of muffins, three speeds, all falling at once. And a bowl that cannot be pushed off the edge of the Stage.

Try it as it is first: hold the right arrow for five seconds. The bowl walks off the Stage and the game is unplayable. Your job is to stop that.

when flag clicked
go to x: (0) y: (-140)
forever
  if <key [right arrow v] pressed?> then
    change x by (10)
  end
  if <key [left arrow v] pressed?> then
    change x by (-10)
  end
  if on edge, bounce
end
            

The muffin that dodges

One kind of muffins in your stall does not fall straight. On the way down it slides sideways, so being underneath it is not enough.

• It still starts at a random place along the top.
• It moves sideways as well as down, and the sideways part is not the same every run.
• It can still be caught, and it still disappears at the bottom.
• The other kinds of muffins are unchanged.

repeat (34)
  change y by (-10)
  change x by (pick random (-12) to (12))
  if <touching [Bowl v]?> then
    delete this clone
  end
end
            

Summary 4 min

A falling thing is a clone that answers three questions.

1. It starts at the top, at (pick random (-200) to (200)).
2. It comes down in small steps, inside a repeat (10).
3. It deletes itself twice over: once when caught, once when it lands.

The catcher is last lesson’s controller with half of it removed. Most of a new game is old blocks in a new shape.

Falling object

A clone that starts at the top of the Stage and moves down in small steps.

Spawn point

Where a clone appears. Here it is a random x along the top.

delete this clone

Removes one copy. Every clone needs a route to one, or the Stage fills up.

Catcher

The sprite the player steers. In this game it only moves sideways.

Next lesson: the game keeps score, and you can lose it.