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Bug hunter: find the line that is wrong.

Play Bug Hunter

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Real programmers spend more time fixing code than writing it. Each round shows a short Python program that prints the wrong thing. Tap the buggy line and see why.

Ages 9+
3–8 min
🔊 Sound
debugging
Python
reading code
Free · no sign-up

How to play Bug Hunter

Choose a level: Beginner, Loops and functions, or Trickier programs.
Read what the program should print and what it actually prints.
Tap the line you think has the bug. A first-time find scores 20 points.
Read the fix and the reason, then move to the next bug.

Tips and strategy

Run the program in your head, line by line, and write down what each variable holds.
Find the first line where the value stops matching what you expect.
Check the usual suspects: the wrong operator (+ instead of ×), a loop that stops one too early, a counter that starts at the wrong number.
Read the goal carefully: sometimes the bug is that a line does a different job from the one asked.

What you'll learn

A bug is a mistake in a program. Debugging means finding it and fixing it, and it is one of the most useful skills in coding. This game trains the habit of tracing a program line by line, the same habit you use in Python lessons and in school computing.

Common beginner bugs

Off-by-one: range(1, 5) gives 1 to 4, not 1 to 5. To include 5 write range(1, 6).
Wrong operator: area uses *, not +.
Wrong starting value: a counter should start at 0, a product at 1.
Overwriting instead of adding: total = i replaces the total. total += i adds to it.
Comparing the wrong way: > misses the equal case and >= includes it.

A simple debugging method

Say what the program should do.
Run it in your head, or add print() lines to see the values.
Find the first place where reality and your plan differ.
Change one thing, then test again.

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FAQ

Bug Hunter: questions

What is a bug in coding?

A bug is a mistake in a program that makes it behave in a way you did not plan, such as printing the wrong number or stopping with an error.

How do I find a bug in my Python code?

Trace the code line by line, writing down what each variable holds, or add print() lines to show values. The bug is at the first place where a value is not what you expected.

What is an off-by-one error?

A loop or index that is one too few or one too many, such as range(1, 5) when you wanted 1 to 5. It is the most common beginner bug.

Is this game for beginners?

Yes. Level 1 uses only variables, print and simple loops. You do not need to write any code, only read it.

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