Sean's Technology and Writing Blog

Debugging in Scratch: Download my slides from Scratch Conference

27 August 2019


Title slide from my presentation on debugging Scratch at Scratch ConferenceI'm back at my desk following a fabulous weekend at Scratch Conference, hosted by the Raspberry Pi Foundation at Churchill College, Cambridge. I delivered a presentation on debugging in Scratch, which covered tips and techniques to help educators to find errors in their students' projects more quickly.

The talk included my run down of the most common errors students made, based on my experience volunteering at a Code Club. Here they are, briefly:

  1. Local variable created for all sprites, not just one sprite
  2. Lookalike blocks (for example, confusing set and change blocks, or x and y blocks)
  3. Wrong blocks in wrong brackets (check at the top and bottom of brackets to confirm the right blocks are inside)
  4. Blocks in the wrong order (especially with regard to initialising variables after starting to use them)
  5. Rogue spaces in text fields ("hello" and "hello " are not equal)
  6. Scripts on the wrong sprite (check too for fragments left behind when this is fixed)
  7. Duplicate scripts created instead of extending existing scripts (this is unique to using worksheets)
  8. Not changing the numbers in blocks (and also, not changing the default menu option in blocks with a dropdown)
  9. "It worked a minute ago!" errors (failing to reset to a known state at the start of execution, so the project keeps visibility, variable, costume etc settings from the previous run)
  10. Timing issues (not waiting for blocks triggered by a broadcast to finish; multiple green flag scripts without explicit coordination; see my blog on race conditions in Scratch)

I have now published my slides, together with my speaker notes, for this presentation, with more information on the above. The presentation also includes a debugging framework (reproduce, diagnose, fix, learn), and tips for testing a script and exposing what's going on inside it at different points in time. Download my Scratch debugging talk here (PDF). In the future, I plan to write an article about debugging Scratch, but I hope you find these speaker notes a useful format until then.

You can also find additional Scratch resources in my new Scratch resource centre here!

Get free chapters from my Scratch books here.


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