Bug Hunters

Every program has bugs. Great programmers find them.

Year 7 Computer Science — Debugging

The Buggy Sandwich

Teacher makes a sandwich — but with bugs:

Puts filling between slices first
Then spreads butter on the outside of the bread
Eats it before cutting

What types of errors were these? Which steps were wrong order? What was missing?

Three Types of Bugs

Wrong Order

All steps present, wrong sequence

Missing Step

An essential step has been left out

Logic Error

A condition or value is incorrect

Your worksheet has one of each. Find them all!

Algorithm 1: Making Toast

Step 1: Put bread in the toaster
Step 2: Eat the toast
Step 3: Wait for the toaster to pop
Step 4: Spread butter on the toast
Step 5: Take the toast out of the toaster

Trace through this exactly. What goes wrong?

Algorithm 2: Combination Lock

Step 1: Turn dial RIGHT to first number
Step 2: Turn dial LEFT to second number
Step 3: Pull the lock open

Try this on a real combination lock. What's missing?

(Hint: a real combination lock needs 3 numbers...)

Algorithm 3: Biggest Number

numbers = [3, 7, 2, 9, 1]
biggest = 0

FOR EACH number IN numbers:
    IF number < biggest:
        biggest = number

PRINT biggest

Trace with [3, 7, 2, 9, 1]. What does it print? What should it print?

The Debugging Process

1. Trace carefully — follow every step exactly
2. Note what ACTUALLY happens — not what you expect
3. Find the divergence — where does actual ≠ expected?
4. Fix the minimum — change only what's broken
5. Test — does it work now? Are there more bugs?

Key Takeaway

Debugging requires systematic tracing — follow every step, check every value, never assume.

Professional programmers spend ~50% of their time debugging.

A bug is a problem to solve — not a reason to start over.