4 ms·

This is a good insight. To formalize it: each question segments the words into 2^max_word_length groups of various sizes, and we seek to minimize the worst case

by dlss 15y ago

This is a good insight. To formalize it: each question segments the words into 2^max_word_length groups of various sizes, and we seek to minimize the worst case number of questions still needed, regardless of which answer we get back. Which, for the ispell words list, appears to be 'e' :( edit: which was the solution found for the suboptimal method in the article, and which is not present for words in the corpus 25% of the time code at http://pastie.org/3693750 http://pastie.org/3693750

kd5bjo 15y ago
Not quite; you don't want to minimize the worst case, but the expected case. That comes from maximizing the entropy of the result set ( https://en.wikipedia.org/wiki/Entropy_(information_theory) https://en.wikipedia.org/wiki/Entropy_(information_theory) ). To calculate this, take the sum of -p * log(p) for all patterns of the guessed letter, where p is the proportion of words in the candidate set that have this pattern.
- eru 15y ago
  > Not quite; you don't want to minimize the worst case, but the expected case. That depends on your opponent. In game theory you often assume that your opponent plays against you and as good as they can, so you'd go with the worst case.
  - kd5bjo 15y ago
    That's only true if the worst case still lets you win the game. Otherwise, your opponent will pick the worst case word every time and you lose. As a zero-sum game, if the players agree on a dictionary beforehand, there is some Nash Equilibrium; I don't have the skill to figure out what it would be, but there would have to be some nonzero probability of guessing every word in the dictionary before running out of guesses.
    - dlss 15y ago
      Interesting. Thanks for your comment here and the one above. Although I firmly believe that hangman is a game, and hence the assumptions of game theory are in full effect, I had not considered the information theory take on things :) Since you asked about the Nash equilibrium for this case, here it is: In the event that you did not have enough questions to uniquely identify the possible words (and assuming that words with more letters than guesses are not allowed ie every word can be guessed with the right questions), the game becomes a matching game [http://en.wikipedia.org/wiki/Matching_pennies http://en.wikipedia.org/wiki/Matching_pennies]. First both players will work out the minimum number of question paths that can identify every allowed word. Any word in more than one set will be ignored as a dominated strategy by the hosting player. Then the guessing player will pick randomly between question paths, and the hosting player will pick randomly among the disjoint sets of words at the end of those question paths, with the specific word not mattering. You can see that once both players are picking between several options, with the entire game riding on their choice, the actual letter guessing becomes unimportant. The game itself resembles an unfair rock paper scissors. If the players were betting $1 per round, the expected value to the guesser is ($2 divided by the number of disjoint sets) - $1.
      - kd5bjo 15y ago
        How does this work if there are multiple covering sets with the same (minimal) number of question paths? Does it matter which minimal covering set you evaluate, or does your strategy need to be some kind of superposition derived from all the minimal sets? Also, if there are some words that are never picked by your strategy, doesn't that open the door for an alternative set of question paths that only covers the now-reduced dictionary? (cue reference to the Unexpected Hanging Paradox) In many ways, the entropy is a heuristic for finding a good question path given the known probabilities for each word in the dictionary. The formula that I quoted above can be trivially extended for nonuniform distributions of the chosen words. It sounds like the Nash equilibrium would be designed such that (after eliminating dominated strategies), the expected information gain at each step would be the same for all question paths. I wouldn't want to have to prove it, though.

by dlss 15y ago

kd5bjo 15y ago

Not quite; you don't want to minimize the worst case, but the expected case. That comes from maximizing the entropy of the result set ( https://en.wikipedia.org/wiki/Entropy_(information_theory) https://en.wikipedia.org/wiki/Entropy_(information_theory) ). To calculate this, take the sum of -p * log(p) for all patterns of the guessed letter, where p is the proportion of words in the candidate set that have this pattern.

eru 15y ago
> Not quite; you don't want to minimize the worst case, but the expected case. That depends on your opponent. In game theory you often assume that your opponent plays against you and as good as they can, so you'd go with the worst case.
- kd5bjo 15y ago
  That's only true if the worst case still lets you win the game. Otherwise, your opponent will pick the worst case word every time and you lose. As a zero-sum game, if the players agree on a dictionary beforehand, there is some Nash Equilibrium; I don't have the skill to figure out what it would be, but there would have to be some nonzero probability of guessing every word in the dictionary before running out of guesses.
  - dlss 15y ago
    Interesting. Thanks for your comment here and the one above. Although I firmly believe that hangman is a game, and hence the assumptions of game theory are in full effect, I had not considered the information theory take on things :) Since you asked about the Nash equilibrium for this case, here it is: In the event that you did not have enough questions to uniquely identify the possible words (and assuming that words with more letters than guesses are not allowed ie every word can be guessed with the right questions), the game becomes a matching game [http://en.wikipedia.org/wiki/Matching_pennies http://en.wikipedia.org/wiki/Matching_pennies]. First both players will work out the minimum number of question paths that can identify every allowed word. Any word in more than one set will be ignored as a dominated strategy by the hosting player. Then the guessing player will pick randomly between question paths, and the hosting player will pick randomly among the disjoint sets of words at the end of those question paths, with the specific word not mattering. You can see that once both players are picking between several options, with the entire game riding on their choice, the actual letter guessing becomes unimportant. The game itself resembles an unfair rock paper scissors. If the players were betting $1 per round, the expected value to the guesser is ($2 divided by the number of disjoint sets) - $1.
    - kd5bjo 15y ago
      How does this work if there are multiple covering sets with the same (minimal) number of question paths? Does it matter which minimal covering set you evaluate, or does your strategy need to be some kind of superposition derived from all the minimal sets? Also, if there are some words that are never picked by your strategy, doesn't that open the door for an alternative set of question paths that only covers the now-reduced dictionary? (cue reference to the Unexpected Hanging Paradox) In many ways, the entropy is a heuristic for finding a good question path given the known probabilities for each word in the dictionary. The formula that I quoted above can be trivially extended for nonuniform distributions of the chosen words. It sounds like the Nash equilibrium would be designed such that (after eliminating dominated strategies), the expected information gain at each step would be the same for all question paths. I wouldn't want to have to prove it, though.