From Scratch Programming Class

Session 6: Variables — remembering things

Phase 1 — Scratch · Session 6 of 9

What we’re learning today

Today you’ll learn how to make Scratch remember a number — like a score, a count, or a level. Once you can do that, you can finally turn last week’s catch-the-apple game into a real game with a score on the screen that goes up every time you catch one. The thing that makes Scratch remember is called a variable, and it’s one of the ideas you’ll use in every program you ever write.

You’ll need to remember from last time

• Conditionals (if blocks) and the forever loop.
• Sensing blocks like <touching ...?>.
• The catch-the-apple game you built last week (or its catch-up version).

Part A: A box with a number in it

Open Scratch and start a new project. Keep the cat.

A variable is the programming word for “a labeled box that holds something.” You give the box a name, you put a number in it, and you can change the number whenever you want. The computer remembers what’s in the box for as long as your program is running.

Real-world example: the score in a basketball game is a variable. It starts at 0. Every time a team scores, the number goes up. The scoreboard remembers it. That’s all a variable is.

Make your first variable

Look at the Variables category in the blocks panel (orange — but a different orange from Control). At the top of it, there’s a button called Make a Variable. Click it.

Scratch asks for a name. Type clicks. Leave the rest at the defaults (For all sprites). Click OK.

Several things just happened:

• A new set of blocks appeared in the Variables category, all related to your clicks variable.
• A small display labeled “clicks” appeared in the top-left corner of the stage. That’s your variable’s value, shown in real time. Right now it shows 0.

Use the variable

Build this on the cat:

when [green flag] clicked
set [clicks] to 0

when this sprite clicked
change [clicks] by 1

Two scripts. One resets the variable to 0 when you click the green flag. The other adds 1 to the variable every time you click the cat.

Click the green flag. The display shows 0.

Click the cat. The display shows 1. Click again. 2. Again. 3.

You just made the computer remember something. Every click adds one. The variable holds the running total.

Click the green flag again. Back to 0. Click the cat. 1 again. The variable resets when you start over, then counts up from there.

Show the variable to the cat

You can also use the variable’s value inside other blocks. Add a third script:

when [space] key pressed
say (clicks) for 1 seconds

The (clicks) block (rounded, in Variables — it’s the rounded one right next to your variable name) gives you the current value.

Click the green flag. Click the cat a few times. Press space. The cat says the current value out loud. Click the cat more. Press space again. The cat says the new value.

That’s the three things you do with a variable: set it, change it, and read it.

Checkpoint: You have a variable that increases when you click the cat, resets when you click the green flag, and is displayed by the cat when you press space. This is the natural stop point if class is cut short.

Part B: Score in your catch-the-apple game

Now we’ll add a score to the game from last week.

Open last week’s project

Open the catch-the-apple project from Session 5. (If you don’t have it — for example because you missed last week — go back to Session 5 and build the base game first, or use the catch-up version.)

You should have a cat that moves with the arrow keys (smooth movement, with a forever loop and four if blocks) and an apple that falls from the top, says “Yum!” when the cat catches it, and resets to a random spot at the top.

Add a score variable

Click Make a Variable in the Variables category. Name it score. Click OK. The score display appears in the top-left of the stage.

Reset the score on green flag

The cat already has a when green flag clicked script. Add to it:

when [green flag] clicked
go to x: 0 y: -130
set [score] to 0
forever
   if <key [right arrow] pressed?> then
      change x by 5
   if <key [left arrow] pressed?> then
      change x by -5

(Just add set [score] to 0 somewhere before the forever loop.)

Make the apple update the score

Click the apple sprite in the bottom-right panel to switch to its scripts. Find the part where it says “Yum!” and resets to the top. Add change [score] by 1 to that block:

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

Click the green flag. Catch the apple a few times.

The score in the corner goes up every time. You’re keeping score in your own game.

That’s the base goal.

Stretch goal

Add a “missed” penalty. If the apple falls past the bottom of the stage without being caught, decrease the score by 1.

Find the if <(y position) < -180> block from last week’s stretch goal (or add it now). Inside it, add change [score] by -1. So:

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

Now your score reflects skill — catches add, misses subtract. Carelessness costs you points.

Extension

Add a second variable called best. Whenever score goes higher than best, update best. The display now shows two numbers: your current score, and the best score you’ve gotten this session.

Add this to a forever loop somewhere (maybe on the cat):

forever
   if <(score) > (best)> then
      set [best] to (score)

The <> comparison is in Operators (green). The (score) and (best) rounded blocks are in Variables.

Now you have a “personal best” tracker. It resets when you click the green flag, but within a single play session, it keeps the high score visible.

Wrap-up

Before we leave, share with the room:

• What’s the highest score you got? Did anyone beat it after another few tries?
• For the kids who did the missed-penalty stretch — does it feel like a different game when there’s a real consequence for missing?
• What’s another game where the score would matter?

You learned the simplest version of one of the biggest ideas in programming: state. Programs that remember something — anything — between one moment and the next are running on state. Every website that knows you’re logged in, every game that tracks your progress, every app that holds a counter — all of it is variables, behind the scenes.

You also closed the loop on something. Your game from last week was missing one important piece, and now it has it. Real software gets built that way: ship something, play with it, find what’s missing, add it, repeat.

If you missed this session

Open Scratch and start a new project. Then:

1. Click Make a Variable in the Variables category. Name it clicks. Click OK.
2. Build the two scripts from Part A: green-flag-resets-to-0, and sprite-clicked-changes-by-1. Click the cat to test.
3. Open your catch-the-apple project from Session 5 (or build the catch-up version of Session 5 first if you don’t have one).
4. Add a score variable. Reset it to 0 on green flag. Have the apple’s “if touching cat” block also change score by 1. Click the green flag and play. Score goes up each catch.

About 25 minutes of work total.

If you get stuck, ask your buddy at the start of next class.

Stretch and extension ideas

• Show the variable in a “large” view — right-click on the variable display on the stage and pick “large readout.” Looks more like a real scoreboard.
• Make a “level” variable that increases every time the score hits a multiple of 10. Display both score and level.
• Use hide variable [name] (Variables category) to hide a variable from the stage when you don’t want players to see it. Useful for variables you’re tracking internally but don’t want shown.
• Tinker with the set vs change distinction. Try set score to 10 and watch the score jump. The difference between “put this exact number in the box” and “add this number to whatever’s in the box” is worth feeling.

What’s next

Next week we’ll put everything together — sequences, loops, events, conditionals, sensing, variables — and build a single small game that uses all of it. After that, you’ll plan and build your own milestone project.