Computer Science
Grade 9
20 min
Debugging Techniques
Debugging Techniques
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1
Introduction & Learning Objectives
Learning Objectives
Define what a 'bug' is and identify the three main types of programming errors: syntax, runtime, and logic.
Interpret basic error messages to locate the line number and type of error in their code.
Use print statements to trace the value of variables and understand the flow of a program.
Apply the technique of commenting out code to isolate the source of a bug.
Explain the 'Rubber Duck Debugging' method and how it helps solve logic errors.
Correct common 'off-by-one' errors in loops and list indexing.
Ever written a program that you were sure was perfect, only to have it crash or give a weird answer? 🐛 Let's learn how to become code detectives and hunt down those pesky bugs!
Debugging is the essential skill of finding...
2
Key Concepts & Vocabulary
TermDefinitionExample
BugAn error, flaw, or fault in a computer program that causes it to produce an incorrect or unexpected result, or to behave in unintended ways.A calculator program that shows 2 + 2 = 5 has a bug.
DebuggingThe process of finding and resolving bugs within computer programs, software, or systems.Adding print statements to see the values of variables and find out why your calculator program is adding incorrectly.
Syntax ErrorAn error caused by breaking the grammatical rules of a programming language. The program will not run at all.Typing `prnt("Hello")` instead of `print("Hello")` in Python. The missing 'i' is a syntax error.
Runtime ErrorAn error that occurs while the program is running. The program starts, but crashes or stops unexpectedl...
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Core Syntax & Patterns
Read the Error Message
Error messages typically provide three key pieces of information: the file name, the line number, and the error type.
Always start by carefully reading the entire error message. It is the computer's way of telling you exactly where it got confused. Don't be intimidated by it; use it as your first clue.
Isolate the Problem (Comment Out)
Use the comment symbol (e.g., `#` in Python) to temporarily disable lines or blocks of code.
If you are not sure where a bug is, comment out a section of code and re-run the program. If the error goes away, the bug is in the code you just commented out. This helps you narrow down the search.
Rubber Duck Debugging
Explain your code, line-by-line, out loud to an inanimate object (like a rubber duck) or anot...
4 more steps in this tutorial
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Challenging
A program is supposed to read scores, calculate an average, and assign a grade. It runs but always outputs the wrong grade. What is the most efficient, multi-step debugging plan based on the techniques from the tutorial?
A.First, rewrite the entire program from scratch. Second, run it again.
B.First, use Rubber Duck Debugging on the grading part. If that fails, comment out the averaging part.
C.First, check for syntax errors. Second, add print statements to every single line of the program.
D.First, print the calculated average right before the grading `if/elif/else` block. Second, if the average is correct, use print statements inside the grading block to trace the logic.
Challenging
A function `get_last_item(my_list)` is designed to return the last element using the code `return my_list[len(my_list) - 1]`. This works perfectly for lists with items but causes a runtime error for an empty list `[]`. This failure on a specific 'edge case' highlights which debugging principle?
A.Logic errors can be hidden until a program is tested with a wide range of inputs, including unusual ones.
B.Syntax errors are the most common type of error for list functions.
C.Commenting out code is the only way to solve runtime errors.
D.All programs with lists will eventually have an off-by-one error.
Challenging
Your program produces a complex financial calculation that is slightly incorrect (a logic error). The calculation involves over 20 lines of code. Which two debugging techniques from the tutorial would be most effective to use together here?
A.Reading the error message and commenting out code.
B.Fixing syntax errors and using Rubber Duck Debugging.
C.Print Statement Debugging and Rubber Duck Debugging.
D.Fixing off-by-one errors and reading the error message.
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