Debugging and Testing

Active debugging and testing show students that errors are not just mistakes to avoid but puzzles to solve. Working with real buggy code in these activities helps students connect theory to practice, building confidence and competence in computational thinking through hands-on problem solving.

• Year 7
• Computing
• 4 activities
• 30 min–45 min

Learning Objectives

1. Identify common types of errors in Scratch programs, such as infinite loops and logic flaws.
2. Analyze the execution flow of a Scratch program with conditional statements to predict outcomes.
3. Design a systematic testing plan to verify all branches of a Scratch program have been executed.
4. Evaluate the effectiveness of different debugging strategies for resolving specific program errors.
5. Create a corrected version of a Scratch program after identifying and fixing bugs.

Problem-Based Learning

⏱ 30 min·Pairs

Pair Swap: Buggy Code Exchange

Pairs create a simple Scratch program with one deliberate bug, such as a misplaced conditional. They swap programs, predict paths using flowcharts, test systematically, and note fixes. Debrief as a class on shared solutions.

Prepare & details

How would you test every possible path through a program with multiple branches?

Facilitation Tip: During Pair Swap, circulate to ensure partners are verbalizing their thought processes, not just pointing at blocks.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Stations Rotation

⏱ 45 min·Small Groups

Stations Rotation: Error Types Hunt

Set up stations for syntax, logic, and runtime bugs in Scratch files. Small groups test each, record symptoms and fixes on worksheets, then rotate. End with groups presenting one fix to the class.

Prepare & details

Identify common types of errors in Scratch programs and propose solutions.

Facilitation Tip: For Station Rotation, provide a clear checklist at each station so students focus on the error type, not just the activity itself.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 35 min·Whole Class

Whole Class: Path Testing Challenge

Display a branched Scratch program on the board. Students individually list all paths, then vote on test cases as a class. Run the code live, debugging live errors together while noting why exhaustive testing matters.

Prepare & details

Design a systematic approach to debugging a complex Scratch project.

Facilitation Tip: In the Path Testing Challenge, model step-through execution using a projector so students see how to isolate branches before testing.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 40 min·Individual

Individual: Debug Journal

Students receive a complex Scratch project file. They log tests, errors found, and fixes in a journal template, iterating until it runs perfectly. Share one entry in pairs for feedback.

Prepare & details

How would you test every possible path through a program with multiple branches?

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Teaching This Topic

Teach debugging as a detective skill, emphasizing that bugs are clues, not failures. Avoid rushing to fixes; instead, guide students to slow down and test hypotheses. Research shows that students benefit most when they verbalize their reasoning, so require explanations at every step. Avoid demonstrations of perfect code; show messy, buggy processes to normalize the struggle.

What to Expect

Students will confidently identify, explain, and fix logic-based errors in Scratch programs. They will also develop the habit of testing all possible paths in code, not just the obvious ones. By the end, students should articulate their debugging process and value iteration as a core part of programming.

Watch Out for These Misconceptions

Common Misconception

During Pair Swap, watch for students assuming the first visible error is the only one present.

What to Teach Instead

During Pair Swap, require students to write down all errors they find before swapping back, using the buggy code printout to mark each issue and its location.

Common Misconception

During Station Rotation, watch for students treating errors as one-time mistakes to be corrected immediately.

What to Teach Instead

During Station Rotation, have students record each error on a sticky note with the path they took to reach it, reinforcing that errors are part of the process.

Common Misconception

During Path Testing Challenge, watch for students stopping after the first successful path they find.

What to Teach Instead

During Path Testing Challenge, ask students to use colored highlighters to mark every path they test, ensuring they account for all possible branches in the code.

Assessment Ideas

Quick Check

After Pair Swap, collect the written descriptions of bugs from each pair. Check that their explanations include the incorrect behavior and the specific blocks or lines they would change to fix the issue.

Exit Ticket

During Station Rotation, as students finish each station, ask them to write one new debugging strategy they learned at that station and how they will apply it in their next project.

Peer Assessment

After the Path Testing Challenge, have students pair up to review each other’s filled-in path diagrams. They should check that all branches were tested and that fixes were justified with clear reasoning.

Extensions & Scaffolding

• Challenge: Ask students to intentionally create a program with a hidden bug, then challenge a peer to find and fix it within a time limit.
• Scaffolding: Provide a partially completed checklist for the Error Types Hunt with error categories pre-listed and space for students to note examples.
• Deeper exploration: Have students write a short reflection on a bug they fixed, describing the steps they took and what they learned about their own debugging habits.

Key Vocabulary

Bug	An error or flaw in a computer program that causes it to produce an incorrect or unexpected result, or to behave in unintended ways.
Debugging	The process of finding and resolving defects or problems within a computer program that prevent correct operation.
Testing	The process of evaluating a program by running it with specific inputs to check if it behaves as expected and to find bugs.
Infinite Loop	A sequence of instructions that repeats endlessly, often due to a faulty condition in a loop structure.
Logic Error	A mistake in the program's design or algorithm that causes it to produce incorrect results, even if the syntax is correct.