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Lab 2: Bacon Number

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Table of Contents

1) Preparation

This lab assumes you have Python 3.5 or later installed on your machine.

The following file contains code and other resources as a starting point for this lab: lab2.zip

Most of your changes should be made to lab.py, which you will submit at the end of this lab. Importantly, you should not add any imports to the file.

This lab is worth a total of 4 points. Your score for the lab is based on:

  • correctly answering the questions throughout this page (1 point)
  • passing the test.py tests and the server-only tests efficiently (2 points, see below), and
  • a brief "checkoff" conversation with a staff member to discuss your code (1 point).

Your points for the test cases are based on how quickly your code runs on the server together with correctness. Your lab code will be expected to run in a certain amount of time (which varies depending on the test); this execution time is based on the amount of time it takes to run that test on the server (not your own machine).

The questions on this page, and the code submission at the end, are due at 4pm on Friday, Feb 22.

2) Introduction

Have you heard of Six Degrees of Separation? This simple theory states that at most 6 people separate you from any other person in the world. (Facebook actually showed that the number among their users is significantly lower, at about 4.6. You can read more about it in a research paper.)

Hollywood has its own version: Kevin Bacon is the center of the universe (not really, but let's let him feel good about himself). Every actor who has acted with Kevin Bacon in a movie is assigned a "Bacon number" of 1, every actor who acted with someone who acted with Kevin Bacon is given a "Bacon number" of 2, and so on. (What Bacon number does Kevin Bacon have? Think about it for a second.)

Note that if George Clooney acts in a movie with Julia Roberts, who has acted with Kevin Bacon in a different film, George has a Bacon number of 2 through this relationship. If George himself has also acted in a movie with Kevin, however, then his Bacon number is 1 and the connection through Julia is irrelevant. We define the notion of a "Bacon number" to be the smallest number of films separating a given actor (or actress) from Kevin Bacon.

In this lab, we will explore the notion of the Bacon number. We have prepared an ambitious database of approximately 37,000 actors and 10,000 films so that you may look up your favorites. Did Julia Roberts and Kevin Bacon act in the same movie? And what does Robert De Niro have to do with Frozen? Let's find out!

2.1) The Film Database

We've mined a large database of actors and films from IMDB via the www.themoviedb.org API. We present this data set to you as a list of records (3-element lists), each of the form [actor_id_1, actor_id_2, film_id], which tells us that actor_id_2 acted with actor_id_1 in a film denoted by film_id.

Keep in mind that "acts with" is a symmetric relationship. If [a1, a2, f] is in the database, it is true both that a1 acted with a2 and that a2 acted with a1, even if [a2, a1, f] is not explicitly represented in the database.

However, these relationships do not necessarily exhibit the transitive property. That is, if [a1, a2, f] and [a2, a3, f] are in the database, it is not necessarily true that a1 and a3 have acted together (unless [a1, a3, f] or [a3, a1, f] is in the database).

We store these data as JSON files. The server tests will use small.json and large.json, but we have also included a tiny.json that you will use to write your own tests.

2.2) The Names Database

The methods in lab.py expect you to use integer actor IDs, but the tests we give you on this page will have actor names as inputs and outputs.

To help with this mapping, we include a file, resources/names.json, which has a JSON representation of the mapping between actor IDs and names. You can use the load method of Python's json module to get the data out of the file and into Python. For an example of this, check out how we load databases in the setUp function of test.py.

Answer the following questions using Python.

Which of the following best describes the Python object that results from loading resources/names.json?
This question is due on Friday February 22, 2019 at 04:00:00 PM.

What is Jerome Savary's ID number?
This question is due on Friday February 22, 2019 at 04:00:00 PM.

Which actor has the ID 1085707?
This question is due on Friday February 22, 2019 at 04:00:00 PM.

2.3) Using the UI

We have also provided a visualization website which loads your code into a small server (server.py) and visualizes your results. To use the visualization, run server.py and use your web browser navigate to localhost:8000. You will need to restart server.py in order to reload your code if you make changes.

You will be able to see actors as circular nodes (hover above the node to see the actor's name) and the movies as edges linking nodes together.

Above the graph we define three different tabs, one for each component of the lab. Each tab sets up the visualization appropriate for its aspect of the lab.

2.4) lab.py and test.py

These files are yours to edit in order to complete this lab. You should implement the main functionality of the lab in lab.py, and you should implement additional test cases (as described throughout the assignment) in test.py.

In lab.py, you will find a skeleton for the functions we expect you to write.

3) Acting Together

To get used to the structure of the databases, complete the definition of did_x_and_y_act_together in lab.py. This function should take three arguments in order:

  • The database to be used (a list of records of actors who have acted together in a film, as well as a film ID: [actor_id_1, actor_id_2, film_id]),
  • Two IDs representing actors

This function should return True if the two given actors ever acted together in a film and False otherwise. For example, Kevin Bacon (id=4724) and Steve Park (id=4025) did not act in a film together, meaning did_x_and_y_act_together(..., 4724, 4025) should return False.

Inside test.py, we have included a TestTiny class which has a setUp method but no tests. Add at least one test testing did_x_and_y_act_together on the tiny database (you can open tiny.json in a text editor to see the data it contains).

When you are done implementing this method and it passes the associated tests, use your code to answer the following questions according to the data in the resources/small.json database. (Hint: You will need to load names.json to get the mapping from actors to IDs, then find the actor corresponding to the ID; consider writing a helper method that does this automatically.)

According to the small.json database, have Geena Davis and Folke Lind acted together?
This question is due on Friday February 22, 2019 at 04:00:00 PM.

According to the small.json database, have Christopher Showerman and Lew Knopp acted together?
This question is due on Friday February 22, 2019 at 04:00:00 PM.

Please note that did_x_and_y_act_together is a warm-up and was given to help you get familiar with the structure of the databases. You don't have to use the function in subsequent sections. Also note, though, that test.py does test this function.

4) Bacon Number

Complete the definition of get_actors_with_bacon_number in lab.py. This function should take two arguments in order:

  • The database to be used (the same structure as before)
  • The desired Bacon number

This function should return a Python set containing the ID numbers of all the actors with that Bacon number. Note that we'll define the Bacon number to be the smallest number of films separating a given actor from Kevin Bacon, whose actor ID is 4724.

Look at the data in tiny.json (we suggest you do this without using Python i.e. open it up in a text editor) and answer these questions:

What are the ID numbers of the actors who have a Bacon number of 0 in tiny.json? Enter your answer below as a Python set of integers:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

What are the ID numbers of the actors who have a Bacon number of 1 in tiny.json? Enter your answer below as a Python set of integers:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

What are the ID numbers of the actors who have a Bacon number of 2 in tiny.json? Enter your answer below as a Python set of integers:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

What are the ID numbers of the actors who have a Bacon number of 3 in tiny.json? Enter your answer below as a Python set of integers:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

Add the four above questions as tests in the TestTiny class. Use your get_actors_with_bacon_number function to compute the sets of actors with Bacon numbers 0, 1, 2, and 3 and compare them against the results you just found above. So that they are recognized as individual tests, define each as a separate method in the class, starting with test_ (e.g. test_bacon_number_0).

Now you're ready to write your Bacon number code! Here are some things to think about when writing your implementation. Consider the set of actors with a Bacon number of 1. Here is a visual representation of the data from the small.json database. (You can use our provided server to generate pictures like these.)

Given the set of actors with a Bacon number of 1, think of how you can find the set of actors with a Bacon number of 2:

Once you get a sense for how to get the Bacon number 2 actors from the Bacon number 1 actors, try to generalize to getting the Bacon number i+1 actors from the Bacon number i actors.

Note that the test cases in test.py run against small and large databases of actors and films, and that your implementation needs to be efficient enough to handle the large database in a timely manner. Your code should also handle the case of arbitrary Bacon numbers (not just n \leq 6) since some databases may be structured to assign some actors quite large Bacon numbers.

When you're done writing this method and it passes all of your tests, answer the following question that uses the large.json database:

In the large.json database, what is the set of actors with Bacon number 6? Enter your answer below as a Python set of strings representing actor names:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

5) Paths

Now we'll turn our attention to finding the chain of actors that connects Kevin Bacon to someone else.

5.1) Bacon Paths

Complete the definition of get_bacon_path in lab.py. The function should take two arguments in order:

  • The database to be used (the same structure as before),
  • An ID representing an actor

Your function should produce a list of actor IDs (any such shortest list if there are several) detailing a "Bacon path" from Kevin Bacon to the actor denoted by actor_id. If no path exists, return None.

Please note that the paths are not necessarily unique, so any shortest list that connects Bacon to the actor denoted by actor_id is valid. The tester does not hard-code the correct paths and only verifies the length of the path you find (as well as that it is indeed a path that exists in the database).

For example, if we run this method on large.json with Julia Roberts's ID (actor_id=1204), one valid path is [4724, 3087, 1204], showing that Kevin Bacon (4724) has acted with Robert Duvall (3087), who in turn acted with Julia Roberts (1204).

Take look at the data in tiny.json. What's the shortest path that connects actor 4724 to actor 1640? Enter your answer below as a Python list of ID numbers:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

Add a test for the case above to the TestTiny class in test.py. You can use this test to help make sure your function is implemented correctly.

5.1.1) Speed

When implementing the path-finding algorithm, you should optimize your code to handle the large database, which our testing infrastructure will use when testing your code.

In particular, here are a few ideas about speed:

  • Membership tests (the in operator) on long lists can be very slow. By contrast, the in operator is very fast on sets and dictionaries (regardless of the lengths of these objects). However, sets and dictionaries do not retain information about the order of their elements. Consider whether there are cases in your code where a set or dictionary can be used in place of a list.

  • Running L.pop(0) on a long list is also slow. If you find yourself doing this, ask: do you really need to pop? Or can you just use an index to keep track of which list element you’re working on?

  • Searching through data using a for loop can be slow. Can you reorganize the data so that your search can be implemented with a single dictionary lookup or set-containment check?

You will also need to be careful about your overall algorithm. In particular, avoid repeatedly iterating through all of data. For example, consider the following graph, with a path highlighted:

Here we've started from Kevin Bacon and successfully expanded out our search until we got to the actor we were looking for. What do we need to keep track of during our search if we want to get the path without looking for the actor again?

When you have implemented your function and it passes your tests, use it to answer the question below:

According to the large.json database, what is the path of actors from Kevin Bacon to Rube Miller? Enter your answer as a Python list of actor names below:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

5.2) Arbitrary Paths

What we've done so far is pretty good, but it raises an important question: what makes Kevin Bacon so special? So far, everything we've done has centered around him. Let's expand things a bit and find the path that connects two arbitrary actors to each other.

Complete the definition of get_path in lab.py. The function should take three arguments in order:

  • The database to be used (the same structure as before),
  • Two IDs representing actors

Your function should produce a list of actor IDs (any such shortest list if there are several) detailing a path from one actor to the other.

Add at least one test case for get_path to TestTiny based on the contents of the tiny.json database. It should find the minimal path between two non-Bacon actors.

When you have implemented this function and it passes your tests, use it to answer the question below:

According to the large.json database, what is the minimal path of actors from Venice Hayes to Ellen Page? Enter your answer as a Python list of actor names below:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

6) Movie Paths

After completing the work above, you might be interested to know what sequence of movies you could watch in order to traverse the path from one actor to another. For example, to move from Kevin Bacon to Julia Roberts, one could watch movie ID 94671 ("Jayne Mansfield's Car," which connects Kevin Bacon to Robert Duvall) and 18402 ("Something to Talk About," which connects Robert Duvall to Julia Roberts).

Add some code to your lab.py to determine the list of movie names that connect two arbitrary actors. To this end, we have included the movies.json database, which maps movie names to ID numbers.

When you have finished this code, use it to answer the following question:

According to the large.json database, what is the minimal path of movie titles connecting Meryl Streep to Iva Ilakovac? Enter your answer as a Python list of movie names below:
This question is due on Friday February 22, 2019 at 04:00:00 PM.

7) Code Submission

When you have tested your code sufficiently on your own machine, submit your modified lab.py below.

When submitting lab.py, the server will run the tests and report back the results (including timing) below.

On the server, we will run several additional tests, comprised of the following:

  • we will test that get_actors_with_bacon_number can handle arbitrarily large Bacon numbers (not just n \leq 6)
  • we will test get_actors_with_bacon_number with large numbers as input to make sure your code stops looking for actors as soon as we know there will not be any actors with the given Bacon number (instead of looping all the way to those large numbers)
  • we will test get_path on a few pairs from the large.json database
  • we will test get_path with a large Bacon number
  • we will test get_path on a pair for which a path does not exist (but for which both actors are in the database)

Submit your lab.py in the box below:

 No file selected
This question is due on Friday February 22, 2019 at 04:00:00 PM.

8) Checkoff

Once you are finished with the code, please come to a tutorial, lab session, or office-hours session and add yourself to the queue asking for a checkoff. You must be ready to discuss your code and test cases in detail before asking for a checkoff.

You should be prepared to demonstrate your code (which should be well-commented, avoid repetition, and make good use of helper functions). In particular, be prepared to discuss:

  • Your additional test cases in the TestTiny class.
  • Your implementation of get_actors_with_bacon_number.
  • Your implementation of get_path and get_bacon_path.
  • How you computed paths of movies that connect two actors.

8.1) Grade

You have not yet received this checkoff. When you have completed this checkoff, you will see a grade here.