Unit 11.1A · Term 1

Tracing & Debugging

Tracing means stepping through code line by line, recording how variables change — a critical exam skill. Debugging is the process of finding and fixing errors. Together, they make you a confident, effective programmer.

Learning Objectives

11.1.2.3 Trace program code
11.1.2.6 Debug a program

Lesson Presentation

11.1A-lesson-08-tracing-debugging.pdf · Slides for classroom use

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Conceptual Anchor

The Detective Analogy

Tracing code is like being a detective examining a crime scene step by step. A trace table is your evidence notebook — you record every variable change as it happens. Debugging is finding which "suspect" (line of code) caused the "crime" (bug). Methodical, line-by-line investigation always beats random guessing.

Rules & Theory

Trace Table Method

A trace table has one column per variable plus an Output column. You add a new row each time a variable changes value.

# Code to trace:
x = 5
y = 3
total = 0
while x > 0:
    total = total + y
    x = x - 1
print(total)

Step	x	y	total	x > 0?	Output
init	5	3	0	True
1	4	3	3	True
2	3	3	6	True
3	2	3	9	True
4	1	3	12	True
5	0	3	15	False	15

Three Types of Errors

Error Type	When Detected	Cause	Example
Syntax Error	Before running	Violates Python rules	`print("hi"` — missing `)`
Runtime Error	While running	Impossible operation	`10 / 0` — ZeroDivisionError
Logic Error	After running	Wrong algorithm/formula	Using `+` instead of `*`

Common Python Errors

Error Name	Cause	Fix
`SyntaxError`	Missing colon, bracket, quote	Check punctuation
`IndentationError`	Wrong whitespace	Use consistent 4-space indentation
`NameError`	Undefined variable	Check spelling, declare before use
`TypeError`	Wrong type in operation	Use `int()`, `str()` conversion
`ValueError`	Invalid conversion	`int("abc")` — validate input
`ZeroDivisionError`	Division by zero	Check divisor != 0
`IndexError`	Invalid list/string index	Check bounds

Reading a Traceback

Python tracebacks read bottom to top. The last line tells you the error type and message. The line above it shows the exact line number and code that caused the error. Always start reading from the bottom!

Worked Examples

1 Trace Table — Swapping Variables

a = 10
b = 20
temp = a
a = b
b = temp
print(a, b)

Step	a	b	temp	Output
1	10	20	—
2	10	20	10
3	20	20	10
4	20	10	10
5				20 10

2 Finding and Fixing Bugs

# BUGGY CODE — Find 3 errors:
num = input("Enter a number")      # Bug 1: missing colon in prompt
total = 0
for i in range(1, num):            # Bug 2: num is a string!
    total += i
    print("Sum:", totl)             # Bug 3: typo "totl"

# FIXED CODE:
num = int(input("Enter a number: "))  # Fix 1: added ": ", Fix 2: int()
total = 0
for i in range(1, num + 1):           # Fix: include num in range
    total += i
print("Sum:", total)                   # Fix 3: fixed typo, moved outside loop

3 Trace — Fibonacci Sequence

a = 0
b = 1
for i in range(6):
    print(a, end=" ")
    a, b = b, a + b

i	a (before)	b (before)	Output	a (after)	b (after)
0	0	1	0	1	1
1	1	1	1	1	2
2	1	2	1	2	3
3	2	3	2	3	5
4	3	5	3	5	8
5	5	8	5	8	13

Output: 0 1 1 2 3 5

4 Debugging Strategy — print() Debugging

# If you can't find a bug, add print() statements:
def average(numbers):
    total = 0
    for n in numbers:
        total += n
        print(f"DEBUG: n={n}, total={total}")  # Temporary debug print
    avg = total / len(numbers)
    print(f"DEBUG: final total={total}, count={len(numbers)}")
    return avg

result = average([10, 20, 30])
print("Average:", result)

# Remove debug prints when the bug is fixed!

Pitfalls & Common Errors

Logic Error — Hardest to Find

Logic errors don't crash the program — they just give wrong results. The code runs fine, but the answer is incorrect. Only trace tables and careful testing reveal them.

Skipping Steps in Trace Tables

In exams, you must show every variable change, not just the final values. Each row should represent one iteration or one statement execution.

Confusing Error Types

A NameError is a runtime error (program crashes), not a syntax error. Syntax errors prevent the program from running at all. Runtime errors occur while the program is running.

Pro-Tips for Exams

Trace Table Tips

Draw columns for every variable mentioned + an Output column
Execute one line at a time, left to right, top to bottom
For loops: check the condition before and after each iteration
Write values exactly as Python computes them (int vs float matters!)
If the code uses print(), record in the Output column at that step

Debugging Strategy

Read the error message carefully (especially the last line)
Locate the line number mentioned in the traceback
Add print() statements to check variable values at suspicious points
Test with simple, known inputs where you can calculate the expected output

Graded Tasks

Remember

Name the three types of errors and give one characteristic of each.

Understand

Classify each error: (a) primt("Hello") (b) int("abc") (c) Using n + 1 instead of n - 1 in a formula.

Apply

Create a complete trace table for: n = 4; result = 1; while n > 1: result *= n; n -= 1; print(result)

Apply

The following code should print the average of 3 numbers but has 3 bugs. Find and fix them:
total = 0; for i in range(3); n = input("Number: "); total += n; avg = total / 2; print(avg)

Analyze

Trace: s = ""; for i in range(1, 5): if i % 2 == 0: s += str(i); else: s += "*"; print(s)

Create

Write a deliberately buggy program (with 1 syntax error, 1 runtime error, and 1 logic error). Then provide the corrected version with explanations.

Self-Check Quiz

1. What type of error is a misspelled variable name like pritn instead of print?

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2. What type of error gives a wrong answer but doesn't crash?

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3. What columns should a trace table always include?

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4. How do you read a Python traceback?

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5. What error does int("hello") cause?

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