Teacher Guide: Debugging Strategies

Student page: Debugging Strategies IB Syllabus: B2.1.4 Estimated periods: 1 (90 min) Prerequisites: Variables & Data Types, Exception Handling

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Lesson Plans

Period 1: Trace Tables and Bug Hunting

Phase	Time	Activity	Student Page Section
Warm-up	10 min	Give students a 5-line code fragment — trace by hand before running	—
Teach	20 min	Trace table methodology, 3 types of bugs (syntax, runtime, logic)	Key Concepts
Practice	45 min	Core #1-3, then Extension #4-5	Practice Exercises
Wrap-up	15 min	Discuss: “Which type of bug is hardest to find? Why?”	—

Teaching notes:

• Trace table habit: read code before executing it — essential for exam conditions
• Print debugging: have students deliberately introduce a bug, then use print statements to find it
• Bug taxonomy: syntax (won’t compile), runtime (crashes during execution), logic (wrong output, no error)

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Differentiation

Supporting weaker students:

Strategy	When to use	Example
Pre-filled trace table headers	Can’t set up trace table	Provide table with column headers: Line, a, b, c, Output
Bug hint cards	Stuck on “Find the bug”	Hint: “Look at the loop condition — what value should i reach?”
One bug at a time	Overwhelmed by Fix Three Bugs	Give bugs separately, fix one, then reveal the next
Paired tracing	Can’t trace independently	One student reads code aloud, partner fills in trace table

Extending stronger students:

Strategy	When to use	Example
Create a buggy program	Finishes exercises early	Write a program with 3 deliberate bugs, swap with a partner to find
Advanced trace	After Core #1	Trace a program with nested if-else inside a loop
Debugger tool introduction	Extra time	Brief IDE debugger walkthrough (stepping, breakpoints, variable watch)
Bug journal	Ongoing	Keep a log of bugs encountered and how they were fixed — builds metacognition

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Answer Keys

Core 1: Trace practice

The code:

int a = 8;
int b = 3;
int c = a / b;
a = a % b;
b = c + a;

Trace table:

Line	a	b	c
int a = 8	8	—	—
int b = 3	8	3	—
int c = a / b	8	3	2
a = a % b	2	3	2
b = c + a	2	4	2

Final values: a=2, b=4, c=2

Key point: 8 / 3 = 2 (integer division truncates), 8 % 3 = 2 (remainder)

Core 2: Find the bug

The code:

for (int i = 1; i < 10; i++) {
    System.out.println(i);
}

Bug: < 10 should be <= 10 — the loop stops before printing 10.

Fixed code:

for (int i = 1; i <= 10; i++) {
    System.out.println(i);
}

Common student error: Some students suggest < 11 which also works but <= 10 is clearer and more intentional.

Core 3: Add debug prints

Given a program that calculates array average:

int[] arr = {10, 20, 30, 40, 50};
int sum = 0;
for (int i = 0; i < arr.length; i++) {
    sum += arr[i];
    System.out.println("DEBUG: i=" + i + " arr[i]=" + arr[i] + " sum=" + sum);
}
double avg = (double) sum / arr.length;
System.out.println("Average: " + avg);

Expected debug output:

DEBUG: i=0 arr[i]=10 sum=10
DEBUG: i=1 arr[i]=20 sum=30
DEBUG: i=2 arr[i]=30 sum=60
DEBUG: i=3 arr[i]=40 sum=100
DEBUG: i=4 arr[i]=50 sum=150
Average: 30.0

Key point: Debug prints show the running total, making it easy to spot if sum is accumulating correctly.

Marking notes: Accept any reasonable placement and format of debug prints. Key is that they track the variables changing each iteration.

Extension 4: Trace a nested loop

The code:

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print(i * j + " ");
    }
    System.out.println();
}

Trace:

i	j values	Output for this row
1	1	1
2	1, 2	2 4
3	1, 2, 3	3 6 9

Complete output:

1
2 4
3 6 9

Key point: Inner loop runs i times (not a fixed count). When i=2, j goes from 1 to 2, so output is 2*1=2 and 2*2=4.

Common mistakes: Thinking j <= i means j goes from 1 to 3 for every row.

Extension 5: Fix three bugs

The original code:

int[] data = {10, 20, 30, 40, 50}
int sum = 0;
for (int i = 0; i <= data.length; i++) {
    sum += data[i];
}
double avg = sum / data.length;
System.out.println("Average: " + avg);

Three bugs:

1. Syntax bug: Missing semicolon after array declaration — add ;
2. Runtime bug: i <= data.length — change to i < data.length (ArrayIndexOutOfBoundsException at index 5)
3. Logic bug: sum / data.length is integer division — use (double) sum / data.length

Fixed code:

int[] data = {10, 20, 30, 40, 50};
int sum = 0;
for (int i = 0; i < data.length; i++) {
    sum += data[i];
}
double avg = (double) sum / data.length;
System.out.println("Average: " + avg);

Expected output: Average: 30.0

Marking notes: All three bugs must be identified and fixed. Students should name the bug type for each (syntax, runtime, logic).

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Integration Notes

Item	In Class	Homework	Link
Worked Examples	Walk through trace table method	—	Worked Examples
Quick Code Check	Mid-lesson checkpoint	—	Quick Code Check
Core #1-3	Individual practice	—	Practice Exercises
Extension #4-5	Later in period	Complete for homework if needed	Practice Exercises
Trace Exercises	Wrap-up activity	—	Trace Exercise
GitHub Classroom	—	Due end of week	GitHub Classroom

Additional Resources

• Blank trace table templates (printable)
• Bug taxonomy poster: Syntax / Runtime / Logic with examples
• Partner debugging activity: write buggy code, swap, find bugs