Programming Basics: Find and Fix a Bug

Compare what you wanted with what your instructions actually did.

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A bug is a mistake in a program. Our goal is exactly three dots. The faulty plan says DRAW DOT twice. Trace it, compare the result with the goal, change one thing, and check again.

Worked example

Two DRAW DOT instructions make two dots. Add one more DRAW DOT instruction to make exactly three. Finding a mistake helps improve the plan; it is not a reason to give up.

The goal has three dots. A faulty program with two DRAW DOT instructions makes two dots. Add one DRAW DOT instruction and trace again to reach three.

Would you believe it?

Computer science can be explored without a computer. Paper, cards and puzzles can help learners practice giving instructions and reasoning about what happens next.

University of Canterbury: CS Unplugged

Question 1 How many dots does the faulty plan draw? 1 2 3

Question 2 How many dots does the goal require? 3 2 4

Question 3 Which change reaches the goal? Delete one DRAW DOT instruction Add three DRAW DOT instructions Add one DRAW DOT instruction

Question 4 What should you do after the change? Hide the instructions Trace the plan again Assume every problem is fixed

Question 5 What is the bug in this plan? One drawing instruction is missing The dot color is wrong There are too many instructions

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