BeginnerCoder
DonateLesson 5 — Spot the bug: off-by-one
The most common bug in the world
An off-by-one bug gives an answer that is wrong by exactly one: one page too few, one item skipped, a loop that stops early. AI assistants write them too, because the code looks right.
Example: how many pages do you need to show 11 items, 5 per page? 11 // 5 is 2 — but the 11th item needs a third page. You need to round up:
(items + per_page - 1) // per_page # rounds up without floats
To find off-by-one bugs, test the boundaries: exactly full (10 items), one more (11), less than one page (4), and zero.
Why it matters: boundary tests take a minute to write and catch the bug that "looks right" in every code review.
The AI's page_count() is off by one when items do not fill the last page. Fix it so all four lines are correct (11 items → 3 pages, 0 items → 0 pages).
Run your code — when the output matches the task, you will see a green check. Then mark the lesson complete.
Test your knowledgeA quick 3-question quiz on this lesson — instant feedback.