Assignment #1 Solution

overview

Assignment 1 begins a series of two related assignments on computerized games. In this assignment you will hone your skills at designing and implementing classes, including inheritance. You will end up with a simple game interface that will pit you against a weak computer opponent.

Assignment 2 will continue this theme. You will implement classes that provide a stronger computer opponent, face eciency challenges, and add more complex games.

games background

In these assignments you will be programming computers to play games of a restricted type, namely two-player, sequential-move, zero-sum, perfect-information games. Lots of games have these features: tic-tac-toe, chess, go, checkers, mancala, and nim, for example. Important characteristics are:

two-player: There are two distinct players. We'll call them Player A and Player B.

sequential-move: Players take turns making moves, one after the other, i.e., in sequence. Moves change the

state of the game. (For example, in tic-tac-toe, the state of the game is the conguration of Xs and Os, together with which player is about to take a turn.) The only possible outcomes of the game are a win for Player A (which is a loss for Player B), a win for Player B (which is a loss for Player A), or a tie. In some games, no tie is possible.

zero-sum: The benet of any move for Player A is exactly inverse to that move's benet for Player B. For

example, a move that wins for Player A loses for Player B. A move that takes Player A closer to a win takes Player B closer to a loss. If we can measure and add the benet for Player A to the benet for Player B of any move, the result is zero.

perfect-information: No information is hidden. Both players know everything about the game state, and all

moves made by their opponent. Compare this to a typical card game, where players keep the cards in their hand hidden from the other players.

description of classes

the state of the game

You'll need code to keep track of the game state, or a snapshot of the current situation in the game. The game state will need to represent which player is currently playing, which legal moves (if any) are available to that player, and whether the game is over. For example: In checkers, you would want to keep track of whether White or Black is playing, and which pieces are where. From this information, you could determine


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all available moves for the current player. That is, the "state" determines the game. If a player has no pieces left in checkers, you can determine that the game is over, and the player who still has pieces is the winner. It must be possible to go from one game state to another by making a legal move, if there is one.

Design your code so that it is not specic to a particular game. We want it to be general so that in a later assignment you will be able to add additional games and to let the user choose which one to play. But here's a paradox: a generic game state cannot possibly know things such as what are the legal moves available, whether the game is over and who has won, because it doesn't know which particular game is being played. You will deal with the paradox using inheritance: implement the game state features that are common to all games in a generic game state class, but implement game-specic features in a subclass for each game. As you will see in lecture, Python provides a NotImplementedError exception that you may use in

the generic class for features that you know are necessary, but that can only be implemented in subclasses. The exact design and naming of your classes is left up to you. The constraints are that they must allow

the code we provide you in game interface.py to play subtract square and chopsticks, both described below.


the computer’s strategy

You'll also need to complete code, in strategy.py, that uses some strategy to choose a move for the computer

to make. For Assignment 1, this will be an extremely simple strategy, randomly choosing one of the legal moves available. Your strategies must work for as-yet-unspecied games, so they should have no game-specic detail. In the nal version of your code, for Assignment 2, you will let the user choose which strategy the computer will use.

the overall play of the game

Finally, we provide you with code in game interface.py that manages the player's \view" of the game.  It


allows a user to play a complete game against a computer opponent. The player's view will be text-based. It will include informing them of the aim of the game and relevant rules, prompting for a move, indicating whether a chosen move is legal, showing what move the computer has chosen, and indicating whether a player has won.

By carefully reading our code in game interface.py you will be able to gure out what the classes that you design must provide.


Be sure to modify only the portions we indicate in game interface.py and strategy.py, or else we will not be able to test your code.


subtract square

You'll implement subtract square, a game which is played as follows:

    1. A non-negative whole number is chosen as the starting value by some neutral entity. In our case, a player will choose it (i.e. through the use of input()).

    2. The player whose turn it is chooses some square of a positive whole number (such as 1, 4, 9, 16, . . . ) to subtract from the value, provided the chosen square is not larger. After subtracting, we have a new value and the next player chooses a square to subtract from it.

    3. Play continues to alternate between the two players until no moves are possible. Whoever is about to play at that point loses!




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chopsticks

You'll implement chopstick, a game played as follows:

    1. Each of two players begins with one nger pointed up on each of their hands.

    2. Player A touches one hand to one of Player B's hands, increasing the number of ngers pointing up on Player B's hand by the number on Player A's hand. The number pointing up on Player A's hand remains the same.

    3. If Player B now has ve ngers up, that hand becomes \dead" or unplayable. If the number of ngers should exceed ve, subtract ve from the sum.

    4. Now Player B touches one hand to one of Player A's hands, and the distribution of ngers proceeds as above, including the possibility of a \dead" hand.

    5. Play repeats steps 2{4 until some player has two \dead" hands, thus losing.

your job

We provide you with starter les

    • game interface.py, where you have to complete the "TODOs" and reason about what classes you need to design to make this interface work.

    • a1 pyta.txt, to specify exceptions from python ta's default settings.

    • strategy.py, where you have to complete a "TODO" for a random move-choosing strategy

    • chopsticks unittest subset.py, subtractsquare unittest subset.py, which should increase your condence that basic features of your chopstick and subtract squares game work.


You will design and implement classes to support the simple game-playing interface described above. Class design, names, and other implementation choices are left up to your taste, applying the concepts you have learned in this course. However, we insist on the following:

¤ All your *.py les must contain, in the if    name    == "  main  ": block, the lines:


import python_ta

python_ta.check_all(config="a1_pyta.txt")

... and produce no errors or warnings when your le is run.

¤ You must implement appropriate    init  ,    str  , and    eq    methods for each class that you dene.


    • We must be able to play your games, on a teach.cs machine, by typing $ python3.6 game_interface.py

The command above is appropriate on unix-like platforms. Another way of saying this is we must be able to:

        1. Open game interface.py in Pycharm.

        2. Click the green arrow ("run")


3. ... in order to play your game.


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Submitting your work

Submit all your code on MarkUs by 10 p.m. January 30 | or earlier! You may submit  multiple versions

of your les, and only the most recent version is marked. Click on the \Submissions" tab near the top. Click \Add a New File" and either type a le name or use the \Browse" button to choose one. Then click \Submit". You can submit a new version of a le later (before the deadline, of course); look in the \Replace" column.

Submit:

    • game interface.py

    • strategy.py

    • ...plus all les containing classes that support your implementation of these games.
















































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