CS152

Lab Exercise 8: Dictionaries and Class Inheritance

This project continues our work with classes within the domain of physical simulations. This is the second part of a multi-part project where we will look at increasingly complex physical simulations. This week we incorporate class inheritance, more custom shapes, and sophisticated collisions.

Lab Tasks (L1-L3)

L0. Setup

Make one folder called Project_08 where you keep your CS 152 files. Then copy the Zelle graphics package and your physics_objects.py into this folder. In the project we are going to we are going to use a dictionary as a “router” or “selector” for what type of collision is required. Just in case you are still a little rusty on dictionaries we will start with a quick review and an exercise.

You can watch a video about dictionaries from Prof. Maxwell:Dictionaries

L1. Review Exercise on Dictionaries

Dictionaries are a different method of storing information. A dictionary is a data type like a list, but the information in a dictionary is not stored in sequential order. Instead, each value stored in a dictionary has a unique key. In a dictionary, the key plays the same role as the index in a list.   The difference is that a key can be any Python type that is immutable (can't be modified). These include the types int, str, float, and tuple. Examples of valid keys are:

1, 42, '42 ', 'ice cream ', 'chocolate ', 'cookie ', 1.7, and (5,

'blueberry ', 'muffins ').

Create a new Python file called wordmap.py. Put your name and a date at the top of the file in  a file header (triple quotes), then start a main function (no parameters). The goal of this program is to give the user a set of word prompts and then record their answer. To record the answer, use a dictionary with the word as the key and the response as the value. The pseudocode to do this is below:

1.   Print out a prompt for the user that tells them what to do.

2.   Create a list of about ten words, call it words. Leave space after each word (ex: “yes_”, “no “ , … )

3.   Create an empty dictionary, called mapping ={}.

4.   Loop over the  words list. Each time through the loop, do two things. First, assign a response the return value of input, using the word as the argument to input. Second, using the word as the key for the empty dictionary mapping, assign to your dictionary the response (the output from input).

5.   Finally, loop over the keys in mapping (for key in mapping.keys ()) and print out the key/response pairs using the dictionary   .get (key) method to show the user their results.

L2. Refactoring (rewriting) our Classes with Inheritance

Watch a video from Prof. Maxwell about inheritance: Inheritance

Inheritance is sharing the capabilities of one class with one or more other classes. This week we are going to use inheritance to simplify the code in the physics_objects.py file and make it easier to create new kinds of shapes. The goal is to create a parent class, Thing, that has all of the common fields and methods for physical objects. Then we will create child classes, like Ball, Block, and  Triangle, that make use of the parent Thing class.

a. Define a parent class named Thing

Edit your physics_objects.py file so that it now has a Thing class defined in it. Add a comment indicating that this class is the parent class for simulated objects.

b. Define the Thing init method

The init method for Thing should have two required parameters, win and the_type. The win parameter will be the GraphWin window for drawing and the_type will be the type of thing being created (e.g. a string like 'ball' or 'block').

You can add any number of default parameters for the remaining fields later on.

In the init method, create the following fields and assign them either a reasonable default value or their corresponding parameter.

type                        A string, indicating the type of the object.  (e.g., self.type

= the_type)

mass                       A scalar value indicating the mass of the object.

position                   A 2-element list indicating the current position of the object.

velocity                   A 2-element list indicating the current velocity of the object.

acceleration            A 2-element list indicating acceleration acting on the object.

elasticity                 The amount of energy retained after a collision.

scale                       The scale factor between the simulation space and pixels.

Default to 10.

win                          A reference to the GraphWin object representing the window.

vis                           An empty list that will hold Zelle graphics.

color                        An (r, g, b) tuple. A good default value is black (0, 0, 0).

drawn                      A boolean indicating if the shape has been drawn, initially False.

c. Create the get methods

Create get methods for all of the above fields except vis, win, and drawn. For most of them, you can copy and paste the code from the classes from last week.

d. Create draw and undraw methods

The draw method should loop over the self.vis list of graphics objects and draw them into the window specified by self.win. The draw method should finish by setting self.drawn to True.

The und raw method should also loop over the self.vis list of graphics objects and undraw each one. The undraw method should finish by setting self.drawn to False.

e. Create the set methods

Create set methods for all of the above fields except type, position, scale, win, vis, color, and drawn. These methods will all be simple and should only update the value of the corresponding field.

f. Create the setPosition method

def setPosition (self, px, py):

There is no change from last week's function.

1.   Calculate the difference between the new position (px, py) and the current position (i.e. self.position[0] and self.position[1]). Put these values in dx and dy.

2.   Update the ball's position fields with the new position.

3.   Multiply dx by self.scale and dy by -self.scale.

4.   In a for loop, move all of the items in the vis list by dx and dy.

g. Create the setColor method

def setColor (self, c): # takes in an (r, g, b) tuple

For the setColor method first set the color field to   c. Then, if c is not None, it should   loop over the self.vis list and set the fill color of each object to the specified color. You may want to override this method in the child classes. (Remember to use gr.color_rgb () to convert the three values in c into a Zelle color object. e.g. ((gr.color_rgb (c [0],c [1],c [2]))

h. Create an update method

The update method is identical to the prior week's method from the Ball class. It should update the simulated position and velocity using the standard equations of motion. It should also move the graphics objects the appropriate amount.

L3. Create a new Ball class that inherits from the Thing class

class Ball(Thing):

The Ball class will need four methods, init , refresh, getRadius, and setRadius.

a. Define the init method

The init method should have win as the only required argument, but you may

also want to add radius, x0, y0, and color as default arguments so you can create a Ball in a location (x0, y0), of a particular size and color.

The first step in the init method is to call the Thing (parent) init method. Call it by using the name of the class, Thing, followed by the name of the method,

init , connected by a dot (. The first three arguments should be self, win, and the string "ball".

Thing. init (self, win, "ball")

b.   You may also want to pass in other parameters if you have them as optional arguments  to the Thing init method. Second, create a field self.radius and assign it     the radius of the Ball. Third, call self.refresh (), which is a function that will define the vis list.

Finally, call self.setColor ().

c. Define a refresh method

The refresh method should implement the following algorithm.

Assign to a local variable (e.g. drawn)the value of self.drawn

if drawn:

undraw the object (use self.undraw ())

Define the self.vis list of graphics objects using the current position,

radius, and window

if drawn:

draw the object

To define the vis list you can use the same code as last week.

d. Define getRadius and setRadius def getRadius (self):

The get Radius method should return the current value of the radius field. def setRadius (self, r):

The set Radius method should assign r to the radius field and then call self.refresh ().

Once you have completed this, download the files test_ballclass.py and .py. Put them both in the same folder as your new physics_objects.py file.

Running test_ballclass.py should create two balls and send them into a single collision. When you are done with the lab exercises, begin the project.