cs1120 Problem Set 6: Adventures in Charlottansville

Collaboration Policy - Read Very Carefully

You may work on this assignment alone or with one partner of your choice so long as it is not someone with whom you have partnered on two or more previous problem sets.

Regardless of whether you work alone or with a partner, you are encouraged to discuss this assignment with other students in the class and ask and provide help in useful ways.

Remember to follow the pledge you read and signed at the beginning of the semester. For this assignment, you may consult any outside resources, including books, papers, web sites and people, you wish except for materials from previous cs1120, cs150, and cs200 courses. You may consult an outside person (e.g., another friend who is a CS major but is not in this class) who is not a member of the course staff, but that person cannot type anything in for you and all work must remain your own. That is, you can ask general questions such as "can you explain recursion to me?" or "how do lists work in Scheme?", but outside sources should never give you specific answers to problem set questions. If you use resources other than the class materials, lectures and course staff, explain what you used in your turn-in.

You are strongly encouraged to take advantage of the scheduled help hours and office hours for this course.

Purpose

• Learn how to program with objects
• Develop and use methods
• Understand and use inheritance
• Make an entertaining adventure game
• Gain experience with a new programming language.

Background

In the 1961, Digital Equipment Corporation (later part of Compaq, which is now part of Hewlett-Packard) donated a PDP-1 mini-computer to MIT, hoping they would use it to solve important scientific problems. Instead, Steve Russell invented the first computer game: Spacewar!. Ten years later, Will Crowther produced the first interactive fiction game, Colossal Cave Adventure (or Adventure for short). Inspired by Adventure, a group of MIT students created Zork and released a version for the Apple II. The game became wildly popular, and was the cause of many students failing out of school.

For this problem set, you will create your own adventure game. Our game takes place in a far-off fictional land known as The University in Charlottansville, East Virginia. Set on bucolic grounds of rolling hills, grassy fields, and red-brick buildings, the characters in our adventure game will take us to imaginary places like the Cabal Hall, Cdrs Hill, the Recursa, and Oldbrushe Hall. All of the places and characters in our game are purely fictional (although most are not purely functional, since they use mutation). Any similarity to real persons or places, of course, is purely coincidental.

Programming an adventure game involves modeling objects in a fictional world. Hence, you will build your adventure game using techniques known as object-oriented programming.

Python Warmup

For this assignment (as well as Problem Set 7 and most of the rest of this course), we will use a programming language called Python. We'll talk more in class about the similarities and differences between Python, but one important difference for this problem set is that Python provides built-in support for defining classes and subclasses that use inheritance. This will make our code simpler than the object system defined in Chapter 10 since the details of managing objects and inheriting methods are taken care of directly by Python.

To get started using Python, follow the directions in the Schemer's Guide to Python.

Objects

For our game, we need to represent three basic kinds of things: people, places and things. Every object we make will be a person, a place or a thing, so we will need classes that implement each kind of object.

All of the objects in our game will be instances of subtypes of the SimObject class, defined in adventure.py. The SimObject class defines this constructor:

class SimObject:
    def __init__(self, name):
        self.name = name

(The SimObject class defines some other methods, not shown.)

All objects in our fictional world have names (rumors about a "Nameless Field are as yet unsubstantiated), so the constructor takes a name parameter.

We also define several subclasses of SimObject (in adventure.py):

• PhysicalObject — a SimObject that can be at a location.
• MobileObject — a PhysicalObject that can change its location.
• OwnableObject — a MobileObject that can be owned.

People

Our world also contains people, defined by the Person class in adventure.py. Person is a subclass of simobject.MobileObject (note we need the simboject. here since MobileObject is defined in a different module).

A Person object has two addition instance variables, defined in the constructor:

class Person (simobject.MobileObject):
    def __init__(self, name):
        simobject.MobileObject.__init__(self, name)
        self.__possessions = [] # what person is carrying (initially empty)
        self.__restlessness = 0.0 # propensity to move randomly

The __init__ method first calls the superclass' __init__ method, and then initializes the new instance variables. We use a __ (two underscores) at the begining of the names to indicate that they cannot be accessed outside the Person class.

The Person class defineds several methods for manipulating people including:

say(self, utterance): Person × String → Void

Say the words passed in as the parameter (as a string).

look(self): Person → Void

Report on the immediate surroundings (only works for installed people).

have_fit(self): Person → Void

Throw a temper tantrum.

go(self, direct): Person × Direction → Boolean

Move in the indicated direction. The Direction is a string representing the direction, such as "north". The result is a Boolean indicating if the move was successful (only works for installed people.)

Note that many of the provided Person methods only work for objects that have been installed in a place (using the install method provided by its subclass PhysicalObject).

Places

The Place subclass is defined adventure.py. Its constructor is:

class Place (simobject.SimObject):
    def __init__(self, name):
        simobject.SimObject.__init__(self, name) 
        self.__neighbors = {} # dictionary of direction -> place entries
        self.__things = [] # list of SimObject

A Place object maintains a dictionary of the neighbors of the place. The dictionary type provides a convenient lookup-table. In this case, we use the direction (a string) as the dictionary key. Hence, we can look up the neighboring place in a given direction by looking up the direction in the __neighbors dictionary:

    def neighbor_towards(self, direction):
        if direction in self.__neighbors:
            return self.__neighbors[direction]
        else:
            return False # no neighbor in this direction

A Place object also maintains a list of the things in a place (this can include any SimObject object, such as a Person). It provides methods for keeping track of the things in a place:

get_things(self): Place → List

Returns a list of the things in this place. The list is a new copy of the list of things (so mutating it does not effect the things in the place).

add_thing(self, thing): Place × SimObject → Void

Modifes the place by adding the thing to the place.

has_thing(self, thing): Place × SimObject → Boolean

Returns True if the place has the thing; otherwise, returns False.

remove_thing(self, thing): Place × SimObject → Void

If the place does not contain this thing, produces an error. Otherwise, removes this thing from the place.

Visiting Charlottansville

In charlottansville.py we define some places in our imaginary world. The procedure setup_Cville() creates some places and sets up connections between them.

We can experiment with our world by evaluating

>>> cville = setup_Cville()

and then asking objects in our world to do things.

For example:

>>> cville = setup_Cville()

>>> cabal = cville.get_place('Cabal Hall')

>>> cabal.get_exits()

['north', 'south']

>>> cabal.neighbor_towards('north').name

'Bart Statue'

Our world needs some people in it, so let's create one, and install him in our world:

>>> JT = Person('Jeffus Thomerson')

>>> cville.install_thing(JT, 'Cdrs Hill')

<simobject: Jeffus Thomerson>: Installing at <place: Cdrs Hill>

We can also make things and add them to our world. For example, let's create a sandwich and install it where JT is now. Then, Mr. Thomerson looks around, sees the sandwich and takes it:

>>> sandwich = OwnableObject('sandwich')

>>> cville.install_thing(sandwich, 'Cabal Hall')

<simobject: sandwich>: Installing at <place: Cabal Hall>

>> JT.look()

At Cdrs Hill:  Jeffus Thomerson says -- I see sandwich
At Cdrs Hill:  Jeffus Thomerson says -- I can go north, south

>> JT.take(sandwich)

At Cdrs Hill: Jeffus Thomerson says -- I take sandwich

True

Try playing the adventure game. Load ps6.py in a Python Shell. Then, in the interactions window, start making people and moving them around. Get a feel for how objects work before moving on.

Inheritance

Generic people are okay, but for an interesting game we need to have people who can do special things.

Our Person object provides a say(self, utterance) method:

>>> bill = Person('Bill')

>>> bill.say('To appy or to eval, that is the question.')

Bill says -- To appy or to eval, that is the question.

What if we have lots of different kinds of people and we want to make them speak different ways. For example, a Lecturer is a kind of Person, except that she can lecture as well as say. When lecturing, the lecturer follows every comment with "you should be taking notes".

We can make a Lecturer a subclass of Person that defines a new method (this is defined in adventure.py):

class Lecturer (Person):
    def lecture(self, stuff):
        self.say (stuff + " - you should be taking notes")

When a method is invoked on a Lecturer object, first it will apply the subclass method if there is one. If the Lecturer subclass does not define the method, evaluation will the continue to look for the method in the superclass.

Your Professor should work like this:

>>> ev = Professor('Evan Davis')

>>> ev.profess("(lambda (n) ((lambda (f) (f f n)) (lambda (f k) (if (= k 1) 1 (* k (f f (- k 1))))))) is a familiar function")

Evan Davis says -- It is intuitively obvious that (lambda (n) ((lambda (f) (f f n)) (lambda (f k) (if (= k 1) 1 (* k (f f (- k 1))))))) is a familiar function - you should be taking notes

>>> ev.lecture("Python is cool")

Evan Davis says -- Python is cool - you should be taking notes

Your Dean should work like this:

>>> dean = Dean("Deany Duck")

>>> dean.say("Hello")

Deany Duck says -- Hello - We need your help, please send money now.

>>> dean.profess("You should study all the liberal arts.")

Deany Duck says -- It is intuitively obvious that You should study all the liberal arts. - you should be taking notes - We need your help, please send money now.

Note that when we invoke the profess method, theProfessor class profess method is used. When that method invokes self.lecture, the Lecturer class lecture method is used. When that method invokes self.say, the Dean class say method is used. In each case, searching for a method always begins with the object's most specific subclass (that is, the class used to create the object), and searches up the superclasses until the appropriate method is found.

State

More interesting objects in our game will need to use state to keep track of things that might change during an execution. Consider the Person class defined in adventure.py (and described above). It is pretty long because a Person has many methods. Here we show some of the code, but leave out some methods:

class Person (MobileObject):
    def __init__(self, name):
        MobileObject.__init__(self, name)
        self.__possessions = [] # what person is crrying (initially empty)
        self.__restlessness = 0.0 # propensity to move randomly

    def get_possessions(self):
        return list(self.__possessions)
    
    def display_possessions(self):
        self.say ("I have " + list_to_string(self.__possessions))

    ...

A person has an instance variable __possessions that is a list of objects the person is carrying (we'll get to the __restlessness instance variable later). The method get_possessions can be used to see what a person is holding. Note that instead of returning self.__possessions directly, get_possessions returns a new copy of self.__possessions. This means if the caller of Person.get_possessions mutates the resulting list, it does not change the actual state of the Person object. This preserves encapsulation, since it means that the only way a Person object's possessions can be modified is by using the Person methods.

Your student should work like this:

>>> aph = Student("Alyssa P. Hacker")

>>> aph.install(Place("The Green"))

<simobject: Alyssa P. Hacker>: Installing at <place: The Green>

>>> aph.is_dressed()

True

>>> aph.get_undressed()

At The Green: Alyssa P. Hacker says -- Brr! It's cold!

>>> aph.get_undressed()

At The Green: Alyssa P. Hacker says -- I have no more clothes left to remove.

>>> aph.is_dressed()

False

>>> aph.get_dressed()

At The Green: Alyssa P. Hacker says -- I feel much better now.

>>> aph.is_dressed()

True

The University administration does not condone streaking, and has decided to strategically place police officers on The Green to apprehend streakers.

We have provided (in ps6.py) the beginnings of a PoliceOfficer class. A PoliceOfficer is a special type of Person, with the ability to arrest other people.

The provided __init__ constructor takes an additional parameter jail, which is a Place that arrestees are sent to, and stores the value passed in as jail in an instance variable:

class PoliceOfficer (Person):
    def __init__(self, name, jail):
        Person.__init__(self, name)
        self.__jail = jail
        self.set_restlessness(0.5)

For example, here is how a PoliceOfficer object is constructed in play_game_as_APH (defined in charlottansville.py):

   krumpke = ps6.PoliceOfficer('Officer Krumpke', cville.get_place('Jail'))

This works since cville is initalized as setup_Cville() which produces a world that includes a Jail place. Here are some sample interactions that should work with your PoliceOfficer class:

>>> cville = setup_Cville()

>>> krumpke = PoliceOfficer('Officer Krumpke', cville.get_place('Jail'))

>>> aph = Student('Alyssa P. Hacker')

>>> cville.install_thing(aph, 'The Green')

<simobject: Alyssa P. Hacker>: Installing at <place: The Green>

>>> cville.install_thing(krumpke, 'The Green')

<simobject: Officer Krumpke>: Installing at <place: The Green>

>>> krumpke.arrest(aph)

At The Green: Officer Krumpke says -- Alyssa P. Hacker, you are under arrest!
At The Green: Officer Krumpke says -- You have the right to remain silent, call methods, and mutate instance variables.
Alyssa P. Hacker moves from The Green to Jail
At Jail:  Alyssa P. Hacker says -- Its lonely here...

>>> krumpke.arrest(aph)

At The Green: Officer Krumpke says -- Alyssa P. Hacker is not here

Automating Objects

This kind of world doesn't make for a very challenging or interesting game. All the people and things only do what the player tells them to. For a more interesting game, we need to have people and things that act autonomously.

We do this by creating a list of all the characters to be moved by the computer and by simulating the passage of time in our World class. The World includes a __things instance variable that maintains a list of all the things in our world. The tick method invokes the tick method on each object that provides a tick method:

class World:
    def __init__(self, n):
        self.name = n
        self.__places = {}
        self.__things = [] # a list of things in the world

    ...
    def install_thing(self, t, p):
        assert self.has_place(p)    
        self.__things.append(t)
        t.install(self.get_place(p))
    ...
    def tick(self):
        for t in self.__things:
            if has_method(t, "tick"):
                t.tick() 

(The has_method procedure is defined in adventure.py, but you don't need to worry about how it is defined. It takes an object and a string as inputs, and outputs True if the object has a method with the given name.)

People hang about idly until they get bored enough to do something. To account for this, we give people a __restlessness instance variable that indicates how likely they are to get bored enough to move randomly. A person will move in a random direction with __restlessness probability with each clock tick. For example, if __restlessness is 1.0, the person will move randomly every clock tick. If restlessness is 0.5, the person will move half the time (but not necessarily every other tick, since the decision whether to move or not is random). If restlessness is 0.0, the person will never move randomly. The Person class defines a method set_restlessness(self, value) that sets the __restlessness instance variable to the input value.

Here are some sample interactions, but note that your results will be different since the Person.tick method makes the person move randomly (for more controlled testing, you might want to invoke krumpke.set_restlessness(0).

>>> cville = setup_Cville()

>>> krumpke = PoliceOfficer('Officer Krumpke', cville.get_place('Jail'))

>>> aph = Student('Alyssa P. Hacker')

>>> cville.install_thing(aph, 'The Green')

<simobject: Alyssa P. Hacker>: Installing at <place: The Green>

>>> cville.install_thing(krumpke, 'The Green')

<simobject: Officer Krumpke>: Installing at <place: The Green>

>>> cville.tick()

At The Green: Officer Krumpke says -- No one to arrest. Must find donuts.

Officer Krumpke moves from The Green to The Recursa

At The Recursa: Officer Krumpke says -- Its lonely here...

>>> aph.get_undressed()

At The Green: Alyssa P. Hacker says -- Brr! It's cold!

>>> cville.tick()

At The Recursa: Officer Krumpke says -- No one to arrest. Must find donuts.

Officer Krumpke moves from The Recursa to The Green

At The Green: Officer Krumpke says -- Hi Alyssa P. Hacker

>>> cville.tick()

At The Green: Officer Krumpke says -- Alyssa P. Hacker, you are under arrest!

At The Green: Officer Krumpke says -- You have the right to remain silent, call methods, and mutate instance variables.

Alyssa P. Hacker moves from The Green to Jail

At Jail: Alyssa P. Hacker says -- Its lonely here...

Note that Alyssa is not arrested on the tick() where Officer Krumpke moves to The Green and greets Alyssa. The Officer only makes an arrest if there is a streaking student at its location at the beginning of the tick. (Our PoliceOfficer is friendly, but a bit slow to observe her surroundings.)

The World class defines a method play_interactively(self, character) that provides a better interface to playing the game. The play_game_as_APH() procedure (defined in charlottansville.py) sets up Cville, installs two students and one restless police officer in our world, and starts playing interactively as Alyssa P. Hacker.

Here's what a typical game might look like:

>>> play_game_as_APH()

<simobject: Alyssa P. Hacker>: Installing at <place: The Green>
<simobject: Ben Bitdiddle>: Installing at <place: Cdrs Hill>
<simobject: Officer Krumpke>: Installing at <place: Bart Statue>
what next? look
At The Green: Alyssa P. Hacker says -- I see nothing
At The Green: Alyssa P. Hacker says -- I can go west, north, south
At Bart Statue: Officer Krumpke says -- No one to arrest.  Must find donuts.
Officer Krumpke moves from Bart Statue to Cabal Hall
At Cabal Hall: Officer Krumpke says -- Its lonely here...
what next? go north
Alyssa P. Hacker moves from The Green to The Recursa
At The Recursa: Alyssa P. Hacker says -- Its lonely here...
At Cabal Hall: Officer Krumpke says -- No one to arrest.  Must find donuts.
what next? get undressed
At The Recursa: Alyssa P. Hacker says -- Brr! It's cold!
At Cabal Hall: Officer Krumpke says -- No one to arrest.  Must find donuts.
Officer Krumpke moves from Cabal Hall to South Green
At South Green: Officer Krumpke says -- Its lonely here...
what next? go south
Alyssa P. Hacker moves from The Recursa to The Green
At The Green: Alyssa P. Hacker says -- Its lonely here...
At South Green: Officer Krumpke says -- No one to arrest.  Must find donuts.
what next? go south
Alyssa P. Hacker moves from The Green to Bart Statue
At Bart Statue: Alyssa P. Hacker says -- Its lonely here...
At South Green: Officer Krumpke says -- No one to arrest.  Must find donuts.
what next? look
At Bart Statue: Alyssa P. Hacker says -- I see nothing
At Bart Statue: Alyssa P. Hacker says -- I can go north, south
At South Green: Officer Krumpke says -- No one to arrest.  Must find donuts.
what next? look
At Bart Statue: Alyssa P. Hacker says -- I see nothing
At Bart Statue: Alyssa P. Hacker says -- I can go north, south
At South Green: Officer Krumpke says -- No one to arrest.  Must find donuts.
Officer Krumpke moves from South Green to Cabal Hall
At Cabal Hall: Officer Krumpke says -- Its lonely here...
what next? look
At Bart Statue: Alyssa P. Hacker says -- I see nothing
At Bart Statue: Alyssa P. Hacker says -- I can go north, south
At Cabal Hall: Officer Krumpke says -- No one to arrest.  Must find donuts.
Officer Krumpke moves from Cabal Hall to Bart Statue
At Bart Statue: Officer Krumpke says -- Hi Alyssa P. Hacker
what next? look
At Bart Statue: Alyssa P. Hacker says -- I see Officer Krumpke
At Bart Statue: Alyssa P. Hacker says -- I can go north, south
At Bart Statue: Officer Krumpke says -- Alyssa P. Hacker, you are under arrest!
At Bart Statue: Officer Krumpke says -- You have the right to remain silent, call methods, and mutate instance variables.
Alyssa P. Hacker moves from Bart Statue to Jail
At Jail: Alyssa P. Hacker says -- Its lonely here...
what next? go north
You cannot go north from <place: Jail>
At Bart Statue: Officer Krumpke says -- No one to arrest.  Must find donuts.
what next? go south
You cannot go south from <place: Jail>
Ben Bitdiddle moves from Cdrs Hill to University Ave
At University Ave: Ben Bitdiddle says -- Its lonely here...
At Bart Statue: Officer Krumpke says -- No one to arrest.  Must find donuts.
what next? look
At Jail: Alyssa P. Hacker says -- I see nothing
At Jail: Alyssa P. Hacker says -- I can go nothing
Ben Bitdiddle moves from University Ave to Cdrs Hill
At Cdrs Hill: Ben Bitdiddle says -- Its lonely here...
At Bart Statue: Officer Krumpke says -- No one to arrest.  Must find donuts.
Officer Krumpke moves from Bart Statue to Cabal Hall
At Cabal Hall: Officer Krumpke says -- Its lonely here...
what next? quit
Better luck next time.  Play again soon!

Extensions

With a full command of Python procedures, object-oriented programming, and inheritance, you are now ready to start making an addictive game. Keep in mind that, historically, computer games have been a colossal waste of time for humankind. As simple as this game is, it's easy to get carried away. Spend your time on the problems, not the game.

Now invite a friend over and teach them how to play. But, remember that this is a fictional world. Don't try anything from the game at home (or on the green). If you do get a chance to visit Charlottansville, make sure to see Monty's Viola, the favorite instrument of the founder of the University and author of the influential 1976 treatise, The Ultimate Declarative, which led many Schemers to revolt.