Programming Logic and Debugging: Finding and Fixing Errors in Code

Programming Logic and Debugging: Finding and Fixing Errors in Code

Grades 9–12 · CS & Digital Fluency · NYS 9-12.CT.4 · 55 Minutes

NYS-Aligned Standard

9-12.CT.4 — Implement a program using a programming language that includes sequencing, selection, and iteration, and use debugging strategies to identify and fix errors. NYS Computer Science and Digital Fluency Learning Standards (2020)

Learning Objectives — “I Can” Statements

• I can identify the three core programming structures: sequencing, selection (if/else), and iteration (loops).
• I can recognize common programming errors: syntax errors, logic errors, and runtime errors.
• I can apply debugging strategies (print statements, trace tables, rubber duck debugging) to find and fix errors.

Essential Question

When your code doesn’t work, what is the most systematic way to find out why?

Background — Pseudocode Convention Used

This lesson uses language-neutral pseudocode. Teacher may adapt examples to the language used in their course (Python, Java, Scratch, etc.).

Lesson Sequence

Hook / Warm-Up (8 min)

Show students this pseudocode:

SET total = 0
FOR each score in scores:
    SET total = score        ← Bug is here
PRINT "Average = " + total / 5

“What does this code try to do? What is the bug? What will happen when it runs?”

Direct Instruction (12 min)

1. Three programming structures:
   • Sequencing — instructions run in order
   • Selection — IF/ELSE branches based on a condition
   • Iteration — loops that repeat instructions
2. Three error types:
   • Syntax error — code violates language rules; caught before running
   • Runtime error — code crashes while running (e.g., dividing by zero)
   • Logic error — code runs but produces wrong output (hardest to find)
3. Debugging strategies:
   • Print/trace statements — output variable values at each step
   • Trace table — track variable values by hand, step by step
   • Rubber duck debugging — explain code aloud line by line; often reveals the flaw

Debugging Practice — Station Rotation (25 min)

Three buggy pseudocode programs. Students diagnose error type and correct:

Program 1 (Logic Error):

SET count = 0
FOR i from 1 to 10:
    IF i / 2 = 0:            ← should be i MOD 2 = 0
        SET count = count + 1
PRINT count

What should it count? What does it actually count? Fix it.

Program 2 (Logic Error — Wrong Variable):

SET highScore = 0
FOR each score in [45, 72, 91, 33, 88]:
    IF score > highScore:
        SET highScore = score
PRINT "Highest: " + score   ← should be highScore

Program 3 (Off-by-One Error):

SET sum = 0
FOR i from 1 to 9:           ← should be 1 to 10
    SET sum = sum + i
PRINT sum

What total does the program print? What should it print?

Reflection (6 min)

Students write a “Debug Report”: For one program, state: error type, where it was found, how they fixed it, and what strategy helped most.

Closure (4 min)

Exit ticket: “What is the difference between a syntax error and a logic error? Give an example of each.”

SDI & Differentiation Block

Supports for MLLs/ELLs

Entering/Emerging (NYSESLAT Levels 1–2):

• Provide a visual reference card: sequencing (numbered list), selection (Y/N branch), iteration (circular arrows)
• Allow student to identify the bug by pointing or circling rather than writing an explanation
• Provide word bank: bug, error, loop, fix, output, variable

Transitioning/Expanding (NYSESLAT Levels 3–4):

• Pre-teach: sequencing, selection, iteration, syntax, logic, debug, trace
• Sentence frame for Debug Report: “I found an error in Program ___. It was a ___ error because ___. I fixed it by ___.”

Supports for Students with IEPs

SDI Adaptation Dimensions: content, methodology, delivery

• Content: Provide only Program 1; provide a trace table template already partially filled in
• Methodology: Use the rubber duck strategy with a physical object; model trace table for one iteration
• Delivery: Allow partner work on Program 1; extended time; reduce Debug Report to 2 sentences

Suggested Placement: ICT

Answer Key

Warm-Up: Bug: SET total = score should be SET total = total + score. The program replaces the total instead of accumulating it.

Program 1: Error: logic. i / 2 does not detect even numbers — need i MOD 2 = 0. Fix: change / to MOD.

Program 2: Error: logic (wrong variable). PRINT uses score (the last loop variable) instead of highScore. Fix: change score to highScore in PRINT.

Program 3: Error: off-by-one (logic). Loop goes 1–9, missing 10. Sum = 45 instead of 55. Fix: change 9 to 10.

Exit ticket model: “A syntax error happens when code breaks the rules of the language (like a missing colon) — the program won’t even start. A logic error means the code runs, but the output is wrong because the logic is flawed (like using + instead of * in a calculation).”

Alignment Record