Computer Science · 9th Grade

Active learning ideas

Identifying and Debugging Logic Errors

Logic errors hide in plain sight because the program runs without crashing, so students need active strategies to expose them. Hands-on practice with pseudocode and test cases turns abstract reasoning into visible, correctable patterns.

• Common Core State Standards
• CSTA: 3A-AP-15
• CSTA: 3A-AP-17

• 20–35 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Think-Pair-Share
• 20 min · Pairs

Think-Pair-Share: Spot the Bug

Provide a pseudocode algorithm with a planted logic error (an off-by-one condition or a flipped comparison operator). Students individually identify what they think is wrong, then compare findings with a partner and build consensus before the class discusses the correct diagnosis.

Explain various ways logic errors can be identified before code is even written.

Facilitation Tip

During Think-Pair-Share: Spot the Bug, circulate and listen for students describing the algorithm’s intended behavior before they look for mistakes.

What to look for

Present students with a simple pseudocode algorithm designed to calculate the average of three numbers, but with a logic error (e.g., dividing by 2 instead of 3). Ask: 'What is the expected output for inputs 10, 20, 30? What is the actual output of this algorithm? Identify the specific line causing the logic error.'

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 02

• Inquiry Circle
• 30 min · Small Groups

Inquiry Circle: Test Case Builders

Groups receive an algorithm and design four test cases that would reveal whether it handles edge cases correctly, including an empty input, the maximum value, and two typical cases. Groups share their test cases; the class votes on which is most likely to catch a hidden bug and explains why.

Construct a test case that reveals a specific logic error in an algorithm.

Facilitation Tip

In Collaborative Investigation: Test Case Builders, ask groups to articulate why a single test case is insufficient and what additional inputs expose the error.

What to look for

Provide students with a pseudocode algorithm for determining if a number is even or odd. Include a logic error. Ask students to write one test case (input and expected output) that would reveal this error and briefly explain why their test case works.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 03

• Gallery Walk
• 35 min · Small Groups

Gallery Walk: Broken Algorithms

Post five flowcharts or pseudocode snippets around the room, each containing a different type of logic error. Students rotate with annotation sheets, writing the error they find at each station and one specific fix. Stations include off-by-one, wrong comparison operator, missing base case, and incorrect loop condition.

Differentiate between syntax errors and logic errors in algorithmic design.

Facilitation Tip

During the Gallery Walk: Broken Algorithms, have students annotate each algorithm with sticky notes that name the error and suggest a fix.

What to look for

In pairs, students exchange pseudocode algorithms they have written for a simple task. Each student reviews their partner's algorithm, identifying one potential logic error and suggesting a specific test case to expose it. They then discuss their findings.

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Activity 04

• Peer Teaching
• 25 min · Pairs

Peer Teaching: Trace My Logic

Partners each write a short algorithm. They swap and must explain aloud, step by step, exactly what their partner's algorithm does for three specific inputs, including one unexpected edge case. The act of explaining aloud reliably surfaces mismatches between the author's intent and the algorithm's actual behavior.

Explain various ways logic errors can be identified before code is even written.

Facilitation Tip

For Peer Teaching: Trace My Logic, insist that the explainer uses a dry-erase board to map each variable’s state step by step while the listener follows with their own tracing sheet.

What to look for

Present students with a simple pseudocode algorithm designed to calculate the average of three numbers, but with a logic error (e.g., dividing by 2 instead of 3). Ask: 'What is the expected output for inputs 10, 20, 30? What is the actual output of this algorithm? Identify the specific line causing the logic error.'

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

A few notes on teaching this unit

Teach tracing as a habit, not an event: model it on the board with colored markers for each variable, and insist students do the same. Avoid rushing to syntax fixes; instead, pause on the reasoning step where the algorithm’s intent diverges from its steps. Research shows that novices benefit from explicit flowcharts before code, so pair pseudocode with visual walkthroughs.

Students will confidently locate logic errors in algorithms, explain why the code fails for specific inputs, and propose test cases that reveal the flaw before writing any code.

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Watch Out for These Misconceptions

• During Think-Pair-Share: Spot the Bug, watch for students who assume that any warning message points to a logic error.

  Redirect by asking, 'Does the program crash or just give the wrong answer?' Then have them trace the algorithm with inputs 0, 1, and 10 to see the discrepancy between expected and actual behavior.

• During Collaborative Investigation: Test Case Builders, watch for students who create only 'happy path' inputs like positive whole numbers.

  Prompt groups to add edge cases: zero, a negative value, and an empty list, then explain why each one is necessary to expose logic flaws.

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Methods used in this brief

• Think-Pair-Share
• Inquiry Circle
• Gallery Walk
• Peer Teaching