HW12: Lunar Lander I

The goal of this assignment is to use object-oriented design techniques to create a lunar lander simulation game.

This is an individual assignment.

Create a directory named lander that will contain your code for this project. All your work will be done in this directory.

Download these two files and put them in your lander directory:

• interfaces.py
• graphics.py (this is the same graphics library we used for the landscape assignments earlier in the term, but please make sure to download it new for this assignment.)

This assignment and the next one will involve building a game that simulates a lunar landing craft — you may have played this game before, if you're into retro gaming. We'll implement a simple version of it. In our version, you are in control of a lunar lander that is descending toward the surface of the moon. Gravity steadily accelerates your craft downward. You have a single control: A button labeled “thrust”. Applying thrust slows your ship down (that is, it accelerates you upward). Your goal is to get your lander to be moving at a slow enough speed that it doesn't crash upon impact. However, you have a limited amount of fuel, and when that runs out, you can no longer apply thrust.

In addition to looking at the code that we've created in class and reading in the book, you may find it relevant to read through my tutorial on functions, classes, and methods a couple times.

Your task is to create a class that represents a lunar lander, containing all relevant information about a spacecraft attempting to safely land on the moon.

Create a file called lander.py. In that file, create a class named LunarLander. This class will represent the ship itself. It should keep track of the following information:

• Altitude: How far the ship is above the surface of the moon.
• Velocity: How fast the ship is moving, in meters/second. A negative velocity indicates that the ship is moving downward, towards the moon's surface. A positive velocity indicates that the ship is moving away from the moon's surface.
• Fuel: How much fuel is left in the tank.

The LunarLander class must have the following methods, with precisely the signatures given:

• __init__(self, alt, vel, fuel) — the constructor that will take parameters for the above variables and initialize them.
• get_altitude(self) — a method that takes no parameters (other than self) and returns the current altitude of the lander.
• get_velocity(self) — a method that takes no parameters (other than self) and returns the current velocity of the lander.
• get_fuel(self) — a method that takes no parameters (other than self) and returns the current amount of fuel in the lander.
• update(self, thrust_amount) — a method that takes a parameter, thrust_amount, that will update the lander's altitude and velocity based on the given amount of thrust. Specifically, this method should perform the following steps:
  1. Expend thrustamount units of fuel if possible. (If not, expend what is left.)
  2. For each unit of fuel burnt, increase the ship's velocity by 4 meters/second for firing the engine. (Note that increasing the velocity makes it a smaller negative number — remember that downward motion has a negative velocity!)
  3. Subtract 2 meters/second from the velocity to account for gravity. (Again, this is making the lander move faster in the direction of the moon's surface.)
  4. Determine and set the new altitude. The new altitude is the old altitude plus velocity. If this value is less than 0, then the lander has impacted the surface (for better or worse) in which case you should set altitude to 0.

A reflection on object-oriented design: note that the LunarLander class is responsible only for its own behavior. It doesn't interact with the user (either by taking input or by printing anything out), and it does all the work to assure that the user can rely on its interface — the user doesn't need to understand the internal details in order to use the methods successfully, just based on their interfaces.

Remember that the docstring is part of the interface. Every function (including class methods) and class that you write should have a docstring.

Testing Your Class

To be able to play our game, we'll need to set up code that will create a lander, then repeatedly ask a user how much fuel to spend on thrust, spending that much thrust, and all the time checking to see if our lander landed.

For this assignment, you'll use a pre-existing class to interact with the user through a text-based interface. Take a look at the TextLanderInterface class in interfaces.py; you'll be using it in this part of the assignment.

Create a class LanderGame in your lander.py module. For now, it will have a constructor and one other method: play(). The constructor should set the following instance variables:

• self.lander — An instance of the LunarLander class. Initalize it with initial velocity 0, altitude 200, fuel 30.
• self.interface — An interface to interact with the user. Initialize this to an instance of the TextLanderInterface class.

The play() method should do the following things:

1. As long as the self.lander has altitude above 0, loop and do the following things:
   • Ask the interface to show the current status of the lander (altitude, velocity, fuel).
   • Ask the interface to get a thrust amount from the user.
   • Update the lander's information with the returned amount.

2. When the lander finally impacts the ground, check the lander's velocity. If its velocity is less than -10, the lander has crashed. If not, it has landed safely.

   Based on the result, ask the interface to either show the crash info or show the landing info.

3. Finally, call the interface's close() method.

In your main function, comment out the call to your testing function, and instead write code that will create a LanderGame instance, then call its play() method to play the game.

Here is a sample run (by a person who's not very good at this game...):

Lander Status: Altitude 200, Velocity 0, Fuel 30
Thrust amount? 0
Lander Status: Altitude 198, Velocity -2, Fuel 30
Thrust amount? 0
Lander Status: Altitude 194, Velocity -4, Fuel 30
Thrust amount? 0
Lander Status: Altitude 188, Velocity -6, Fuel 30
Thrust amount? 0
Lander Status: Altitude 180, Velocity -8, Fuel 30
Thrust amount? 0
Lander Status: Altitude 170, Velocity -10, Fuel 30
Thrust amount? 0
Lander Status: Altitude 158, Velocity -12, Fuel 30
Thrust amount? 0
Lander Status: Altitude 144, Velocity -14, Fuel 30
Thrust amount? 0
Lander Status: Altitude 128, Velocity -16, Fuel 30
Thrust amount? 0
Lander Status: Altitude 110, Velocity -18, Fuel 30
Thrust amount? 0
Lander Status: Altitude 90, Velocity -20, Fuel 30
Thrust amount? 0
Lander Status: Altitude 68, Velocity -22, Fuel 30
Thrust amount? 0
Lander Status: Altitude 44, Velocity -24, Fuel 30
Thrust amount? 0
Lander Status: Altitude 18, Velocity -26, Fuel 30
Thrust amount? 0
Crash! Oh no!

Remember to put your name at the top of your source code!

Submit your program, lander.py, via Moodle.

• Proper object-oriented design — 1 points
• Working LunarLander class — 8 points
• Testing code for LunarLander — 4 points
• Working LanderGame class — 8 points
• Comments, style, and other design considerations — 4 points

Good luck, have fun, start early, and ask lots of questions!