October 19th, 2005

Splinter: Sensei

Surprise Quiz - My Thoughts

[This entry refers to the logic problem in my previous entry.]

A lot of good comments; it's clear people were really thinking about it! This is a classic problem that shows up in different guises and is still debated.

First, some preliminaries. In the problem, I tried to define "surprise" as not knowing which day the quiz was. I should have added "at any time prior to the quiz day" just to be clear. Given the problem's particular definition, saying that there will be a "surprise" quiz sometime next week is not contradictory.

I'm also going to work from the assumption that the professor didn't intentionally lie to them. One might assume the opposite, but once you assume that the problem itself contains lies, you might as well assume that the problem didn't exist. (If you can demonstrate that there's an inherent contradiction, that's different.) Plus, the professor's original statements (there was a quiz and it's timing was a surprise) came true.

I'd like to consider two weaknesses within the problem.
(This was correctly identified by kuddlepup, traveller_blues, twopiearr, and bogglerat, at least.)
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(I think that ristin, pippinbear, tafyrn, and xolo were all heading in this direction. Some good explanations, too!)
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Anyhow, that's my analysis of this one. Please feel free to post alternate approaches, gaps in my reasoning, etc. And thanks for the great comments on the puzzle! (Yay! I've got all these clever readers! :)