Loop Comparison¶

Run the Loop Comparison MicroSim Fullscreen
Edit in the p5.js Editor

About This MicroSim¶

This MicroSim animates three Scratch loop blocks side by side: repeat (a counted loop that runs a fixed number of times), forever (a continuous loop that never stops on its own), and repeat until (a conditional loop that ends when a condition becomes true). Watching a sprite move through each loop type helps students see how choosing a loop changes program behavior, from drawing a square to continuous game-style movement to moving until a sprite reaches an edge.

How to Use¶

Click one of the three loop buttons (🔁 repeat (10), ∞ forever, or 🎯 repeat until < >) to choose which loop type to visualize.
Press ▶ Start to run the animation, ⏸ Pause to stop it partway, and 🔄 Reset to return to the beginning.
Drag the Speed slider to make the animation run faster or slower.
Watch the Iteration counter and the info panel in the upper left, which show the loop's description and a short code example.

Iframe Embed Code¶

You can add this MicroSim to any web page by adding this to your HTML:

<iframe src="https://dmccreary.github.io/scratch-textbook/sims/loop-comparison/main.html"
        height="485px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan¶

Grade Level¶

Grades 4-6 (ages 9-12)

Duration¶

10-15 minutes

Prerequisites¶

Understands that scripts run in sequence, top to bottom, and has been introduced to the idea that a loop repeats a set of blocks (earlier in this chapter). Knows what the touching edge? sensing block checks, since the repeat-until example uses it. No prior experience choosing between loop types is assumed — this is the first side-by-side comparison in the course.

Activities¶

Exploration (5 min): Students select each of the three loop buttons in turn — repeat (10), forever, repeat until \<touching edge?> — and press Start to watch each one run at the default speed, noting whether and how it stops.
Guided Practice (5 min): Using the Speed slider, students slow down the repeat (10) loop and count iterations aloud with the on-screen counter, then compare it to forever, which keeps going until Pause is pressed.
Assessment (5 min): Before pressing Start on repeat until, students predict when the sprite will stop, then run it to check, and explain what would need to be added to a forever loop to make it stop the same way.

Assessment¶

Student can correctly choose which loop type to use for (a) drawing a shape with an exact number of sides, (b) an action that should run continuously during a game, and (c) an action that should stop automatically once a condition is met — and can explain why an unguarded forever loop never stops on its own.