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Session 3: Loops — doing things over and over

Phase 1 — Scratch · Session 3 of 9

What we’re learning today

Today you’ll learn one of the most important ideas in all of programming: how to tell the computer to do something many times, without writing the same instruction over and over. This is called a loop. By the end of class, you’ll have used a loop to make your cat draw a perfect square — and then you’ll see how a tiny change to the loop turns the square into a triangle, a hexagon, or a flower.

You’ll need to remember from last time

  • Sequences — code runs in order, top to bottom.
  • The green flag starts your program.
  • The blocks move, turn, wait, and go to x: y:.
  • Order matters.

Part A: Why loops exist

Open Scratch and start a new project. Add a when green flag clicked block, like always.

Now imagine you wanted to make the cat march in a straight line — take ten little steps, one after another. With what you know so far, you’d have to drag ten move 10 steps blocks and snap them all together.

Try it. Drag ten of them. It’s tedious. Ten is annoying. A hundred would be ridiculous. A thousand would be impossible to build by hand.

Programmers ran into this problem about three seconds after they invented programming. The solution they came up with is the loop: a way to say “do this a bunch of times” without writing it out a bunch of times.

The repeat block

Delete the ten move blocks (right-click → delete, or drag them off to the left).

Now go to the Control category (orange blocks). Find this one:

repeat [10]
   <empty>

It’s shaped differently from the blocks you’ve used so far — it has a mouth in the middle. Blocks go inside the mouth, and they’ll run as many times as the number you put in the top.

Snap a move 10 steps block inside the mouth, and snap the whole repeat block under your when green flag clicked block. You should end up with this:

when [green flag] clicked
repeat [10]
   move 10 steps

Click the green flag. The cat takes ten steps in quick succession.

That’s a loop. The move 10 steps block ran ten times.

Try changing the number

Click the 10 in the repeat block and change it to 100. Click the green flag. The cat zooms across the stage.

Change it to 3. Three little steps.

Change it to 1. Just one step. (A loop that runs once is silly, but the computer will obey.)

Change it to 0. The cat doesn’t move. The loop runs zero times, which is allowed.

The number controls how many times the inside runs. That’s the whole concept.

Multiple blocks inside a loop

You can put more than one block inside the mouth. Try this:

when [green flag] clicked
repeat [4]
   move 50 steps
   turn ↻ 90 degrees

Click the green flag.

Watch carefully. The cat moves 50 steps, turns 90 degrees, moves 50 steps, turns 90 degrees… four times.

What shape did the cat just walk?

A square.

You just told the computer to draw a square in three lines of code. Without the loop, you would have written the same two blocks four times — eight blocks instead of three. With a loop that ran 100 times, that would be 200 blocks instead of 3.

This is why loops are one of the most important ideas in programming.

Checkpoint: Every student should have built a repeat loop with at least one block inside it and clicked the green flag to see it run. This is the natural stop point if class is cut short.

Part B: Drawing with loops

Now we’ll do something more visually satisfying than walking the cat in invisible squares. We’ll have the cat actually draw the square as it walks.

Adding the Pen extension

Scratch has extra block categories called extensions that aren’t shown by default. We need one called Pen to draw.

Look at the bottom-left corner of the Scratch window. There’s a blue square with a + icon — the “Add Extension” button. Click it.

A list of extensions appears. Find the one called Pen and click it.

A new category called Pen (green) now appears in your blocks panel. You’ll see blocks like pen down, pen up, set pen color, erase all, and a few others.

The way pen works: when pen down is active, the sprite leaves a trail behind it as it moves. When pen up, no trail.

Draw a square

Modify your loop program so it looks like this:

when [green flag] clicked
erase all
go to x: 0 y: 0
pen down
repeat [4]
   move 100 steps
   turn ↻ 90 degrees
pen up

Three new blocks at the top — erase all (so old drawings don’t stick around), go to x: 0 y: 0 (so the cat starts in the middle), and pen down (so the trail starts). And one new block at the end — pen up (so we don’t keep drawing if we add more code later).

Click the green flag. Watch the cat draw a square.

That, right there, is the moment programming starts to feel like a superpower.

Try the stretch

The base goal is the square above. If that works:

  • Stretch: Change the repeat to 3 and the turn to 120 degrees. What shape do you get? Now try 5 and 72. Now try 6 and 60. There’s a pattern. Can you figure it out?
    • Hint: Look at the two numbers and how they relate. (If you want a really sneaky hint: 4 × 90 = 360. 3 × 120 = 360. 5 × 72 = 360. The total turning is always 360 degrees, because that’s a full trip around.)

Or the extension

If you finished stretch and want more:

  • Extension: Use a nested loop — a loop inside a loop. The inner loop draws a shape; the outer loop turns a little and draws it again. Something like this: 
    repeat [12]
   repeat [4]
      move 50 steps
      turn ↻ 90 degrees
   turn ↻ 30 degrees
    Click the green flag. You should get a beautiful flower-of-squares pattern.

Wrap-up

Before we leave, share with the room:

  • What’s the coolest shape you got the cat to draw?
  • Did you try changing numbers and discover something unexpected?
  • If you wanted to draw a 100-sided shape, what would the two numbers in your loop be? (You don’t have to actually try it unless you want to. But predict.)

You’ve now used your first programming concept that isn’t in the peanut butter rule. Loops are the first big idea where the computer does something you didn’t tell it to do explicitly — it does the inside part many times because you set it up to. That’s a real shift.

If you missed this session

Open Scratch and start a new project. Then:

  1. Add a when green flag clicked block.
  2. From the Control category, drag out a repeat 10 block. Snap a move 10 steps block inside its mouth. Snap the whole thing under the green flag. Click the green flag. The cat moves ten times in a row.
  3. Change the inside to move 50 steps and turn 90 degrees, and change the repeat number to 4. Click the green flag. The cat walks a square.
  4. Click the Add Extension button (bottom-left). Pick Pen.
  5. Add erase all, go to x: 0 y: 0, and pen down to the top of your program (in that order). Click the green flag. The cat draws a square.

Then try Part B’s stretch goal: change the numbers to draw a triangle, a pentagon, a hexagon. About 25 minutes of work.

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

Stretch and extension ideas

  • Use set pen color to (Pen category) to change the color of your drawing.
  • Use set pen size to to make the lines thicker.
  • Make a spiral by combining move and turn outside a loop: 
    repeat [50]
   move 5 steps
   turn ↻ 10 degrees
    Each time around the loop, the cat takes a bigger and bigger swing. Actually no — wait, did it? Look carefully. The loop body is the same every iteration. So why does it look like a spiral and not a circle?
  • Try using a forever block instead of repeat. (Hint: you’ll need the red stop sign to make it stop.)

What’s next

Next week we’ll learn about events — how to make sprites react to keyboard presses, mouse clicks, and other things that happen while a program is running, not just when it starts.