Bug Hunt

Every coder makes bugs - the skill is finding them. Learn to tell a syntax error from a logic error from a runtime error, and how testing and trace tables track a bug down.

⏱️ 18 min 🎯 14 activities Teachers Not yet rated Students Not yet rated

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What you'll cover

Every coder makes bugs 🐛

No programmer writes perfect code first time. The real skill is **debugging** - finding and fixing the mistakes. And to fix a bug fast, you first work out **which kind** it is. There are three: **syntax** errors, **logic** errors and **runtime** errors. Each looks different, and each is hunted differently.

Three kinds of bug 🗂️

The three error types, plus the two tools you use to find them.

It will not even run 🚫

A Python program has a missing colon after an if statement, so it refuses to run and reports a SyntaxError. What kind of error is this?

A syntax error
A logic error
A runtime error
No error at all

Match each error to its telltale sign 🔗

Syntax error
Logic error
Runtime error
Testing

The program will not run at all
The program runs, but the answer is wrong
The program runs, then crashes partway through
Running the program with data to check the output

Logic vs runtime ⚖️

The two trickiest bugs both let the program **start** running. The difference is what happens next.

It runs, but... 🤔

A program calculates an average, runs with no error message, but always prints a number that is too small. What kind of error is this?

A logic error
A syntax error
A runtime error
A hardware fault

How to hunt a bug 🔎

Two tools find bugs fast. **Testing**: run the program with chosen **test data** and compare the actual output to what you expected. Good test data is a mix of **normal**, **boundary** (edge) and **erroneous** (invalid) values. **Trace tables**: write out each variable and follow its value **line by line** as the program runs. The moment a variable holds a value you did not expect, you have found the faulty line.

Spot the syntax errors ✅

Select the TWO problems that are SYNTAX errors - ones that stop the program running at all.

A missing closing bracket
The keyword written as pirnt instead of print
Using minus where you meant plus
Dividing a number by zero

Tidy code, fewer bugs 🧹

Bugs hide in messy code. A few habits make them far easier to spot and to avoid. **Meaningful names** (total, not t) say what a variable is for. **Comments** explain the tricky bits. **Indentation** and **white space** show the structure at a glance. None of this changes what the program does - but it makes a hidden bug stand out, and helps the next person understand your code.

It crashes halfway 💥

A program runs fine until it reaches the line int('hello'), where it stops with an exception. What kind of error is this?

A runtime error
A syntax error
A logic error
A spelling mistake in a comment

The debugging loop 🪜

An interactive activity.

Diagnose it 🧭

An interactive activity.

The summary 📝

A _____ error breaks the language's rules, so the program will not run at all. A _____ error lets the program run but gives the wrong result, with no message. A _____ error lets it run and then crashes partway through. To find bugs you use _____ - comparing output to what you expected - and a _____ table to follow each variable line by line.

syntax logic runtime testing trace comment boundary exception indentation flowchart

Your turn ✍️

An interactive activity.