CS63 Lab 2: A* Search

Due by 11:59pm Thursday, September 22

For this assignment, we will apply A* search to the eight puzzle problem. The eight puzzle consists of a three by three board with eight numbered tiles and a blank space. A tile adjacent to the blank space can slide into the space. The object is to figure out the steps needed to get from one configuration of the tiles to another.

For this problem, a state should specify the location of each the eight tiles as well as the blank space in the three by three grid. The operators are most efficiently represented as moving the blank left, right, up, or down, rather than creating operators for each of the numbered tiles.

For example, suppose we wanted to get from the initial state to the goal state given below.

 1 2 3         1 2 3
 8   6   -->   8   4
 7 5 4         7 6 5 
initial        goal
We could accomplish this with the following sequence of operators on the blank.
 1 2 3         1 2 3        1 2 3        1 2 3       1 2 3
 8   6  right  8 6    down  8 6 4  left  8 6 4  up   8   4
 7 5 4         7 5 4        7 5          7   5       7 6 5
initial                                              goal

You may work with a partner on this assignment. Be sure to add both names to the top of every file. Only one of the partners needs to turn in the code.

Before you begin do an update63 to get a copy of the files you'll need to complete this homework. They should appear in the directory cs63/labs/2.

Informed Search

Informed search is very similar to the basic search that we used last week. Because they are so similar it makes sense to have the informed classes inherit from the basic classes.

  1. Copy your search.py file from last week's lab directory to this week's lab directory.
  2. Implement the InformedProblemState class.
    Informed problem states will need one additional method called heuristic that takes the goal state as a parameter and returns the estimate of the distance from the current state to the goal state.
  3. Implement the InformedNode class.
    Informed nodes will need one additional instance variable and one additional method. Informed nodes need to store the goal state. They will also need a method called priority to calculate the node's f value for A*. Recall that the f(n) = g(n) + h(n). The g(n) represents the cost of getting from the starting node to the current node, which in this case is the depth of the node. The h(n) represents the estimate of the distance from the current node to the goal, which is this case will be obtained by getting the state from the current node and calling its heuristic method.
  4. Implement the InformedSearch class.
    You'll need to override the __init__ and the execute methods of the basic search. You should replace the queue with a priority queue and you should use informed nodes rather than standard nodes.

Eight Puzzle

Similar to last week, you'll need to come up with a representation of the state of the problem as well as its operators. In addition, you'll need to implement several heuristics.

  1. Implement a representation of the eight puzzle state.
    In the physical world, the eight puzzle is a three by three grid. However, in the virtual world of your program, it may be simpler to implement it as a flat list.
  2. Implement the four operators to move the blank up, down, left, and right.
    Your operators should make a copy of the current state and then move the tiles of the copy to create the next state. To make a copy of a list do the following: newLs = ls[:].
  3. Implement both of the eight puzzle heuristics that we discussed in class: tiles out of place and Manhattan distance. Remember that you DO NOT count the blank in either of these heuristics.
  4. Test your solution on the following starting states A-H:
     1 3     1 3 4     1 3   7 1 2   8 1 2   2 6 3   7 3 4   7 4 5
     8 2 4   8 6 2   4 2 5   8   3   7   4   4   5   6 1 5   6   3
     7 6 5     7 5   8 7 6   6 5 4   6 5 3   1 8 7   8   2   8 1 2
       A       B       C       D       E       F       G       H
    using the same goal each time:
     1 2 3
     8   4
     7 6 5
    State A should take 2 steps, state B should take 6 steps, and state C should take 8 steps. You'll need to determine the length of the other solutions.


In order to demonstrate that the Manhattan distance heuristic is more informed than the tiles out of place heuristic, you will compare the number of nodes that each search expands on the problems given above (A-H). A node is expanded when it is removed from the priority queue. Inside a comment in your 8puzzle.py file, create and fill in the following table:

             Node Expansions
Problem    A*(tiles)  A*(dist) 


Once you are satisfied with your programs, hand them in by typing handin63 at the unix prompt.