CS662 Assignment 2: Search

Assigned: September 4
Due: September 13.
30 points

To turn in: typed or handwritten answers to question 1 and the questions at the end of the assignment. Also, please check your code into your Subversion repository under a directory named assignment2.

Question 1: (10 points) Environments. For each of the following problems, characterize its environment according to the six dimensions described by Russell and Norvig (fully/partially observable, static/dynamic, deterministic/stochastic, discrete/continuous, episodic/sequential, single/multi-agent). If you feel that it's not clear from the given description, please give any details or assumptions you feel are needed.

Question 2 (20 points total) Search.
In this problem, you'll be helping the members of the rock band U2 get to a concert on time.

Consider the following problem:
U2 has a concert that starts in 17 minutes and they must all cross a bridge to get there. All four men begin on the same side of the bridge. You must help them across to the other side. It is night. There is one flashlight. A maximum of two people can cross at one time. Any party who crosses, either 1 or 2 people, must have the flashlight with them. The flashlight must be walked back and forth, it cannot be thrown, etc. Each band member walks at a different speed. A pair must walk together at the rate of the slower man's pace:

Bono: 1 minute to cross
Edge: 2 minutes to cross
Adam: 5 minutes to cross
Larry: 10 minutes to cross

For example: if Bono and Larry walk across first, 10 minutes have elapsed when they get to the other side of the bridge. If Larry then returns with the flashlight, a total of 20 minutes have passed and you have failed the mission.

(note: this is a fairly well-known puzzle, and solutions to it undoubtedly can be found on the Web. I would suggest that you try to solve it yourself first. This will give you a much better understanding of the problem.)
A skeleton of the code needed to solve the problem can be found here. This will help provide some structure and guidance for solving the problem.
  1. (10 points) To begin, complete the U2State class. To represent a problem state, we'll use a list containing the band members on the left bank, a list containing the band members on the right bank, a variable indicating whether the flashlight is on the left or the right, integers for time elapsed, depth and fvalue, and a pointer back to the previous or parent state (this will be useful for printing the solution)

    I've provided __init__, __lt__, and __hash__. You should implement __repr__, __eq__, __le__, __gt__, __ge__, isGoal, isFailure, and successors. Most of these are only a couple of lines.

    The exception to this is successors, which is the most complex method in this assignment. For a given state, successors should consider all possible legal actions and, for each action, create the state that would result of that action was applied. It should return a list of these states.
    For example, one state might be: (['Edge', 'Bono'],['Larry','Adam'],flashlight='Left',time elapsed=10). The possible moves here are: Edge crosses alone, Bono crosses alone, or they go together. So the successor method for this state would return:

    [ (['Edge'], ['Bono','Larry','Adam'], Right, 11), (['Bono'], ['Edge','Larry','Adam'], Right, 12) , ([], ['Edge', 'Bono','Larry','Adam'], Right, 12)]

    Note: you may be tempted to try to put some "smarts" in successors to only pick "good" moves. Resist this temptation - we'll use the search algorithms to select among states.

  2. Search. Once we have our state representation built, we can think about how to search - recall that search is just a mechanism for considering different states in a particular order.

    (5 points). To begin, we'll need a function called search. It should take as input an initial state and a search queue. (I've provided the prototype for you.)

    Search should repeatedly:
    1. dequeue the front state in the search queue
    2. check to see if it is a goal. If so, we are done. Print out the solution.
    3. If not, place the state in the closed list, and generate its successors.
    4. For each successor, check to see if it is in your closed list (i.e. they've already been visited). If so, discard it. Also check to see if it returns true for isFailure, and if so, discard it.
    5. Enqueue remaining state.
    You should implement the closed list as a dictionary with states as keys and 'True' as a value.

  3. Search queues. (5 points) By using different sorts of queues, we can implement different algorithms. In this assignment, we'll implement three search algorithms: BFS, DFS, and A*.

    I've provided you with an abstract base class called SearchQueue. You'll want to subclass this to implement BFSQueue, DFSQueue, and AStarQueue. Recall that the queues should work as follows:

    Run each of the algorithms on the problem and prepare a table that lists for each algorithm:
    Turn this table in along with your source code. Also, please provide both a "main" that allows your code to be run from the command line and a README describing how to run it.