From Scratch Programming Class

Session 5: Conditionals — making decisions

Phase 1 — Scratch · Session 5 of 9

What we’re learning today

So far your programs have always done the same thing every time they ran. Today we’ll learn how to make a program check something about the world — “is the cat touching the apple?” — and decide what to do based on the answer. This is called a conditional, and combined with everything you already know, it’s enough to build your first real game.

You’ll need to remember from last time

• Events — when [key] pressed, when this sprite clicked.
• Each sprite has its own scripts.
• Sequences run top to bottom; loops repeat.
• The arrow-key cat you built last week (or its equivalent in the catch-up exercise).

Part A: “If something, then do this”

Open Scratch and start a new project.

A conditional is the programming idea of “check first, then decide.” In English you use them all the time: “if it’s raining, take an umbrella.” The “check” is is it raining? The “decision” is take an umbrella, or don’t.

Scratch has a block for this. It’s in the Control category (orange):

if < > then
   <empty>

Like the repeat block, it has a mouth. Things go inside the mouth, and they only run if the condition (the thing in the diamond at the top) is true.

The diamond shape at the top of the if block is a slot for a condition — a question Scratch can answer with yes or no. To fill the slot, you need a block that’s also shaped like a diamond.

Take a look at the Sensing category (light blue). Notice that some blocks there are diamond-shaped:

• <key [space] pressed?> — answers “is the space key being pressed right now?”
• <touching [Sprite1]?> — answers “is this sprite touching that other sprite?”
• <touching color []?> — answers “is this sprite touching that color?”
• <mouse down?> — answers “is the mouse button being held?”

Each of those answers yes-or-no. You drop one into the diamond slot of an if block, and the inside runs only when the answer is yes.

Smooth movement (a callback to last week)

Remember how your cat felt a little choppy when you held down the arrow key? Conditionals fix that.

Last week you built four scripts, one per arrow key. This week we’ll build one script, with conditionals inside a forever loop.

Add a cat to your project (it should be there by default). Build this:

when [green flag] clicked
forever
   if <key [right arrow] pressed?> then
      change x by 5
   if <key [left arrow] pressed?> then
      change x by -5
   if <key [up arrow] pressed?> then
      change y by 5
   if <key [down arrow] pressed?> then
      change y by -5

Four if blocks, all stacked inside a forever loop, all under one green flag.

The forever block (Control, orange) is the loop equivalent of “keep doing this until I stop you.” Inside it, every fraction of a second, Scratch checks all four if blocks. If an arrow key is being held down, the corresponding change block runs — that fraction of a second, every fraction of a second, while the key is held.

Click the green flag. Hold an arrow key. The cat moves smoothly, because every fraction of a second the program is asking “is this key pressed?” and acting on the answer.

That’s the same arrow-key movement you built last week, but now it feels right. That’s the power of conditionals plus loops.

Checkpoint: Your cat moves smoothly with the arrow keys when you hold them down, using a single forever loop with four if blocks inside. This is the natural stop point if class is cut short.

Part B: Catch the apple

Now we’ll build something that actually feels like a game.

What you’re building

An apple sprite falls from the top of the stage. Your cat moves along the bottom (using your smooth arrow-key movement from Part A). When the cat catches the apple, the apple says “Yum!” and goes back to the top to fall again.

It’s a tiny game. You’ll know if you can catch the apple or not. That’s the whole point.

Set up the stage

You already have your cat with smooth arrow-key movement. Two small adjustments:

• Move the cat to the bottom of the stage. Add go to x: 0 y: -130 inside the when green flag clicked script (above the forever loop). Now the cat starts near the bottom every time.
• For this game, the cat only needs to move left and right. You can delete the up arrow and down arrow if blocks if you want, or leave them — your choice.

Add the apple

Click the Choose a Sprite button (bottom right). In the search box, type “apple.” Pick any apple-looking sprite. (Don’t see one? Any small sprite works. Pick a fruit, a ball, anything that can “fall.”)

Make sure you’re now editing the apple’s scripts (click the apple in the bottom-right panel; the script area should be empty).

Build this on the apple:

when [green flag] clicked
go to x: pick random -200 to 200 y: 180
forever
   change y by -3
   if <touching [Sprite1] ?> then
      say [Yum!] for 1 seconds
      go to x: pick random -200 to 200 y: 180

A few things to notice in this script:

• pick random -200 to 200 is in the Operators category (the green blocks). It’s another diamond-shaped block, but it goes into the x slot of go to because that slot accepts numbers. It picks a random x-coordinate each time, so the apple appears somewhere different.
• Sprite1 is the cat’s name. Scratch may have renamed it; if your cat is called something else (like “Cat”), pick that name from the dropdown.
• The script flow: the apple starts at a random spot at the top (y = 180 is near the top), then forever it falls (changes y by -3), and on every fall it checks “am I touching the cat?” If yes, it says “Yum!” and resets to the top.

Click the green flag. The apple falls. Move the cat with the arrow keys. Catch it.

You just built a game.

That’s the base goal. If you got the apple to fall, the cat to catch it, and the apple to reset — you’re done for today.

Stretch goal

What happens when the apple reaches the bottom of the stage and the cat didn’t catch it? Right now: it just keeps falling forever and disappears off screen. That’s not great game design.

Add a “missed!” condition. Inside the forever loop, after the existing if, add another:

if <(y position) < -180> then
   say [Missed!] for 1 seconds
   go to x: pick random -200 to 200 y: 180

y position is in the Sensing category (it’s not a diamond — it’s a rounded block, because it returns a number, not a yes/no answer). The <> comparison is in the Operators category — look for < in the green blocks.

Now if the apple falls past the bottom of the stage (y < -180), it says “Missed!” and resets to the top. The game has a real “miss” condition.

Extension

Add a second falling object. A different fruit, a coin, anything. Make it fall faster (or slower) than the apple. The cat now has to catch both, possibly making choices about which to chase.

To add it: the new sprite is just another sprite. Build the same “forever fall + if touching cat then yum + reset” script on it. The only thing you change is the speed (change y by -5 instead of -3, for instance).

Two falling sprites at different speeds is twice as hard. Your buddy can play it.

Wrap-up

Before we leave, share with the room:

• Did anyone catch the apple ten times in a row? Five?
• What was the trickiest part of getting the apple to reset correctly?
• For the kids who did the stretch — what made you decide what number to use for “the bottom of the stage”?

You did something bigger today than just learning conditionals. You combined events (arrow keys), loops (forever), and conditionals (if touching) to build something that looks and feels like a real game. Every game you’ll ever play uses these three ideas, just with more sprites and more conditions.

You’re not even halfway through Phase 1, and you already have everything you need to build games.

If you missed this session

Open Scratch and start a new project. Then:

1. Build the smooth arrow-key cat from Part A (one forever loop, four if blocks). About 10 minutes.
2. Add an apple sprite (or any sprite that can “fall”).
3. Build the apple’s falling-and-resetting script described in Part B above. About 15 minutes.
4. Click the green flag. Move the cat. Try to catch the apple.

When you come next week, you’ll be ready. If anything’s confusing, ask your buddy at the start of class.

Stretch and extension ideas

• Use if/else (Control category — like if, but with two mouths). Lets you do “if X then this, otherwise that” in one block. More elegant than two if blocks for the same situation.
• Make the apple change costume when caught, then change back when it resets. (Look at the apple’s Costumes tab; try next costume.)
• Use start sound (Sound category) to play a sound when the cat catches the apple.
• Add a wait 0.05 seconds block at the end of the apple’s forever loop. What changes? Why?

What’s next

Next week we’ll learn about variables — how to make Scratch remember things, like a score. That’ll let you finally turn your catch-the-apple game into a real game with points.