It's hard to write computer programs to play games. When we as humans sit down to play a game, we can draw on past experience, adapt to our opponents' strategies, and learn from our mistakes. Computers, on the other hand, blindly follow a preset algorithm that (hopefully) causes them to act somewhat intelligently. Though computers have bested their human masters in some games, most notably checkers and chess, the programs that do so often draw on hundreds of years of human game experience and use extraordinarily complex algorithms and optimizations to outcalculate their opponents. While there are many viable strategies for building competitive computer game players, there is one approach that has been fairly neglected in modern research - cheating! Why spend all the effort trying to teach a computer the nuances of strategy when you can simply write a program to play dirty and win handily all the time? In this assignment, you will build a mischievous program that bends the rules of Hangman to trounce its human opponent time and time again. In doing so, you'll cement your skills with various data types, and will hone your general programming savvy. Plus, you'll end up with a piece of software which will be highly entertaining. At least, from your perspective.
In case you aren't familiar with the game Hangman, the rules are as follows:
Fundamental to the game is the fact the first player accurately represents the word they have chosen. That way, when the other players guess letters, they can reveal whether that letter is in the word. But what happens if the player doesn't do this? This gives the player who chooses the hidden word an enormous advantage. For example, suppose that you're the player trying to guess the word, and at some point you end up revealing letters until you arrive at this point with only one guess remaining:
DO-BLE
There are only two words in the English language that match this pattern: “doable” and “double.” If the player who chose the hidden word is playing fairly, then you have a fifty-fifty chance of winning this game if you guess 'A' or 'U' as the missing letter. However, if your opponent is cheating and hasn't actually committed to either word, then there is no possible way you can win this game. No matter what letter you guess, your opponent can claim that they had picked the other word, and you will lose the game. That is, if you guess that the word is “doable,” they can pretend that they committed to “double” the whole time, and vice-versa.
Let's illustrate this technique with an example. Suppose that you are playing Hangman and it's your turn to choose a word, which we'll assume is of length four. Rather than committing to a secret word, you instead compile a list of every four-letter word in the English language. For simplicity, let's assume that English only has a few four-letter words, all of which are reprinted here:
ALLY BETA COOL DEAL ELSE FLEW GOOD HOPE IBEX
Now, suppose that your opponent guesses the letter 'E'. You now need to tell your opponent which letters in the word you've "picked" are E's. Of course, you haven't picked a word, and so you have multiple options about where you reveal the E's. Here's the above word list, with E's underlined in each word:
ALLY BETA COOL DEAL ELSE FLEW GOOD HOPE IBEX
If you'll notice, every word in your word list falls into one of five "word families":
Since the letters you reveal have to correspond to some word in your word list, you can choose to reveal any one of the above five families. There are many ways to pick which family to reveal - perhaps you want to steer your opponent toward a smaller family with more obscure words, or toward a larger family in the hopes of keeping your options open. In this assignment, in the interests of simplicity, we'll adopt the latter approach and always choose the largest of the remaining word families. In this case, it means that you should pick the family ----. This reduces your word list down to
ALLY COOL GOOD
and since you didn't reveal any letters, you would tell your opponent that his guess was wrong.
Let's see a few more examples of this strategy. Given this three-word word list, if your opponent guesses the letter 'O', then you would break your word list down into two families:
The first of these families is larger than the second, and so you choose it, revealing two O's in the word and reducing your list down to
COOL GOOD
But what happens if your opponent guesses a letter that doesn't appear anywhere in your word list? For example, what happens if your opponent now guesses 'T'? This isn't a problem. If you try splitting these words apart into word families, you'll find that there's only one family - the family ---- in which T appears nowhere and which contains both COOL and GOOD. Since there is only one word family here, it's trivially the largest family, and by picking it you'd maintain the word list you already had.
There are two possible outcomes of this game. First, your opponent might be smart enough to pare the word list down to one word and then guess what that word is. In this case, you should congratulate them - that's an impressive feat considering the scheming you were up to! Second, and by far the most common case, your opponent will be completely stumped and will run out of guesses. When this happens, you can pick any word you'd like from your list and say it's the word that you had chosen all along. The beauty of this setup is that your opponent will have no way of knowing that you were dodging guesses the whole time - it looks like you simply picked an unusual word and stuck with it the whole way.
Your assignment is to write a computer program which plays a game of Hangman using this "Evil Hangman" algorithm. In particular, your program should do the following:
It's up to you to think about how you want to partition words into word families. Think about what data structures would be best for tracking word families and the master word list. Would an associative array (map) work? How about a stack or queue? Thinking through the design before you start coding will save you a lot of time and headache.
Some starter code is available for this project; you can choose to use all or some of it in your solution, as you wish. You are free to start from scratch if you prefer, as long as the program does all of the things detailed in the Outline section above.
The provided starter code largely provides a user interface and a suggested design for a main.cpp to add in that bit of user interface at the very least. The hangman class code provided is all "stubs" - the methods return the required type, but don't do anything.
You'll need to do a bit of planning to figure out what the best data structures are for the program. There is no "right way" to go about writing this program, but some design decisions are much better than others. Here are some general tips and tricks that might be useful:
The algorithm outlined in this handout is by no means optimal, and there are several cases in which it will make bad decisions. For example, suppose that the human has exactly one guess remaining and that computer has the following word list:
DEAL TEAR MONK
If the human guesses the letter 'E' here, the computer will notice that the word family -E-- has two elements and the word family ---- has just one. Consequently, it will pick the family containing DEAL and TEAR, revealing an E and giving the human another chance to guess. However, since the human has only one guess left, a much better decision would be to pick the family ---- containing MONK, causing the human to lose the game.
There are several other places in which the algorithm does not function ideally. For example, suppose that after the player guesses a letter, you find that there are two word families, the family --E- containing 10,000 words and the family ---- containing 9,000 words. Which family should the computer pick? If the computer picks the first family, it will end up with more words, but because it revealed a letter the user will have more chances to guess the words that are left. On the other hand, if the computer picks the family ----, the computer will have fewer words left but the human will have fewer guesses as well.
More generally, picking the largest word family is not necessarily the best way to cause the human to lose. Often, picking a smaller family will be better. After you implement this assignment, take some time to think over possible improvements to the algorithm. You might weight the word families using some metric other than size. You might consider having the computer "look ahead" a step or two by considering what actions it might take in the future.
If you implement something interesting, feel free to include it with your solution for extra credit!
You must submit with your source code a README file. This file is just a plain text file (usually named README or README.txt). Your README must contain the following:
| Code compiles and runs | 5 points |
| README | 5 points |
| Code style | 5 points |
| Correctly functioning user interface (initial prompts) | 10 points |
| Clear and usable user interface (game play) | 10 points |
|
Correct implementation of "evil" algorithm (We'll do our best to detect sub-optimal solutions and give partial credit.) |
30 points |
| Total: | 65 points |
| Extra credit: extensions/innovations of your own design | Up to 3 points |
Submit a zip file on Canvas containing: