The Answer Checker Explains the Missing Question

The practice page grades answer submissions with a single line of code: parse($( #practice-answer ).value).eq(answer). If answer is undefined, the line throws an error, instead of marking the answer as incorrect. This code is responsible for the absence of vertical lines (undefined slope) in the question bank, as well as three of the 43 blog articles that explain this concept.

The author discovered this issue by feeding the question bank into the site's own function, rather than reading through the code. The investigation involved copying the question array from two JavaScript files (38,731 bytes and 9,985 bytes) and testing the slope() function with both regular and vertical pairs of points. The grader's parse().eq() chain was also tested with eight inputs, revealing that a vertical pair would cause the function to throw an error instead of returning an undefined slope.

The practice set contains ten questions, eight of which have distinct answers, meaning that two answers make up 20% of the bank each. This distribution could be problematic for students who struggle with understanding the sign rule, as they might guess the correct answer based on round numbers, rather than understanding the concept.

The question bank is limited to two distinct sets of five questions, cycling through them indefinitely, which means that a student who practices twice has seen every question. The source code contains 151 comment lines in Chinese, which is unusual for an English-language site. Additionally, the page is not strictly thin, as it has five questions, a hint button, a show-answer button, and a score display.

The author suggests that fixing the issue of missing vertical lines would require updating the grader to recognize undefined slopes, which would necessitate changing the answer type.