In this project, your Pacman agent will find paths through his maze world, both to reach a particular location and to collect food efficiently. You will build general search algorithms and apply them to Pacman scenarios.
As in all future projects, this project includes an autograder for you to grade your answers on your machine. This can be run with the command:
See more on using the autograder, run python autograder.py -h
The code for this project consists of several Python files, some of which you will need to read and understand in order to complete the assignment, and some of which you can ignore. You are provided all the code and supporting files in the zip of this assignment.
|Files you’ll edit:|
||Where all of your search algorithms will reside.|
||Where all of your search-based agents will reside.|
|Files you might want to look at:|
||The main file that runs Pacman games. This file describes a Pacman GameState type, which you use in this project.|
||The logic behind how the Pacman world works. This file describes several supporting types like AgentState, Agent, Direction, and Grid.|
||Useful data structures for implementing search algorithms.|
|Supporting files you can ignore:|
||Graphics for Pacman|
||Support for Pacman graphics|
||ASCII graphics for Pacman|
||Agents to control ghosts|
||Keyboard interfaces to control Pacman|
||Code for reading layout files and storing their contents|
||Parses autograder test and solution files|
||General autograding test classes|
||Directory containing the test cases for each question|
||Project 1 specific autograding test classes|
Files to Edit and Submit: You will fill in portions of
searchAgents.py during the assignment. You should submit these files with your code and comments in .zip files. Please do not change the other files in this distribution or submit any of our original files other than these files.
Evaluation: Your code will be autograded for technical correctness, using the same autograder and test cases you are provided with. Please do not change the names of any provided functions or classes within the code, or you will wreak havoc on the autograder. You should ensure your code passes all the test cases before submitting the solution, as we will not give any points for any questions if not all the test cases for it pass. However, the correctness of your implementation — not the autograder’s judgements — will be the final judge of your score. Even if your code passes the autograder, we reserve the right to check it for mistakes in implementation, though this should only be a problem if your code takes too long or you disregarded announcements regarding the project.
Academic Dishonesty: We will be checking your code against other submissions in the class for logical redundancy. If you copy someone else’s code and submit it with minor changes, we will know. These cheat detectors are quite hard to fool, so please don’t try. We trust you all to submit your own work only; please don’t let us down. Likewise, do not attempt to write your code specifically to pass the autograder’s tests. Either copying or trying to cheat the autograder will be considered violations of the student honor code.
Getting Help: You are not alone! If you find yourself stuck on something, contact the course staff for help. Office hours and Piazza are there for your support; please use them. If you can’t make our office hours, let us know and we will schedule more. We want these projects to be rewarding and instructional, not frustrating and demoralizing. But, we don’t know when or how to help unless you ask.
Welcome to Pacman
After unzipping the code and changing to its directory, you should be able to play a game of Pacman by typing the following at the command line:
Pacman lives in a shiny blue world of twisting corridors and tasty round treats. Navigating this world efficiently will be Pacman’s first step in mastering his domain.
The simplest agent in searchAgents.py is called the
GoWestAgent, which always goes West (a trivial reflex agent). This agent can occasionally win:
python pacman.py --layout testMaze --pacman GoWestAgent
But, things get ugly for this agent when turning is required:
python pacman.py --layout tinyMaze --pacman GoWestAgent
If Pacman gets stuck, you can exit the game by typing CTRL-c into your terminal.
Soon, your agent will solve not only
tinyMaze, but any maze you want.
pacman.py supports a number of options that can each be expressed in a long way (e.g.,
--layout) or a short way (e.g.,
-l). You can see the list of all options and their default values via:
python pacman.py -h
Also, all of the commands that appear in this project also appear in commands.txt, for easy copying and pasting. In UNIX/Mac OS X, you can even run all these commands in order with
Note: if you get error messages regarding Tkinter, see this page
Here’s a glossary of the key objects in the code base related to search problems, for your reference:
- A SearchProblem is an abstract object that represents the state space, successor function, costs, and goal state of a problem. You will interact with any SearchProblem only through the methods defined at the top of
- A specific type of SearchProblem that you will be working with — it corresponds to searching for a single pellet in a maze.
- A specific type of SearchProblem that you will define — it corresponds to searching for a path through all four corners of a maze.
- A specific type of SearchProblem that you will be working with — it corresponds to searching for a way to eat all the pellets in a maze.
- Search Function
- A search function is a function which takes an instance of SearchProblem as a parameter, runs some algorithm, and returns a sequence of actions that lead to a goal. Example of search functions are
breadthFirstSearch, which you have to write. You are provided
tinyMazeSearchwhich is a very bad search function that only works correctly on
SearchAgentis a class which implements an Agent (an object that interacts with the world) and does its planning through a search function. The
SearchAgentfirst uses the search function provided to make a plan of actions to take to reach the goal state, and then executes the actions one at a time.
Finding a Fixed Food Dot using Search Algorithms
searchAgents.py, you’ll find a fully implemented
SearchAgent, which plans out a path through Pacman’s world and then executes that path step-by-step. The search algorithms for formulating a plan are not implemented — that’s your job. As you work through the following questions, you might find it useful to refer to the object glossary (the second to last tab in the navigation bar above).
First, test that the
SearchAgent is working correctly by running:
python pacman.py -l tinyMaze -p SearchAgent -a fn=tinyMazeSearch
The command above tells the
SearchAgent to use
tinyMazeSearch as its search algorithm, which is implemented in
search.py. Pacman should navigate the maze successfully.
Now it’s time to write full-fledged generic search functions to help Pacman plan routes! Pseudocode for the search algorithms you’ll write can be found in the lecture slides. Remember that a search node must contain not only a state but also the information necessary to reconstruct the path (plan) which gets to that state.
Important note: All of your search functions need to return a list of actions that will lead the agent from the start to the goal. These actions all have to be legal moves (valid directions, no moving through walls).
Important note: Make sure to use the
PriorityQueue data structures provided to you in
util.py! These data structure implementations have particular properties which are required for compatibility with the autograder.
Hint: Each algorithm is very similar. Algorithms for DFS, BFS, UCS, and A* differ only in the details of how the fringe is managed. So, concentrate on getting DFS right and the rest should be relatively straightforward. Indeed, one possible implementation requires only a single generic search method which is configured with an algorithm-specific queuing strategy. (Your implementation need not be of this form to receive full credit).
Question 1 (2 points)
Implement the depth-first search (DFS) algorithm in the
depthFirstSearch function in
search.py. To make your algorithm complete, write the graph search version of DFS, which avoids expanding any already visited states.
Your code should quickly find a solution for:
python pacman.py -l tinyMaze -p SearchAgent
python pacman.py -l mediumMaze -p SearchAgent
python pacman.py -l bigMaze -z .5 -p SearchAgent
The Pacman board will show an overlay of the states explored, and the order in which they were explored (brighter red means earlier exploration). Is the exploration order what you would have expected? Does Pacman actually go to all the explored squares on his way to the goal?
Hint: If you use a
Stack as your data structure, the solution found by your DFS algorithm for
mediumMaze should have a length of 130 (provided you push successors onto the fringe in the order provided by getSuccessors; you might get 246 if you push them in the reverse order). Is this a least cost solution? If not, think about what depth-first search is doing wrong.
Question 2 (2 points)
Implement the breadth-first search (BFS) algorithm in the
breadthFirstSearch function in
search.py. Again, write a graph search algorithm that avoids expanding any already visited states. Test your code the same way you did for depth-first search.
python pacman.py -l mediumMaze -p SearchAgent -a fn=bfs
python pacman.py -l bigMaze -p SearchAgent -a fn=bfs -z .5
Does BFS find a least cost solution? If not, check your implementation.
Hint: If Pacman moves too slowly for you, try the option
Note: If you’ve written your search code generically, your code should work equally well for the eight-puzzle search problem without any changes.
Varying the Cost Function
While BFS will find a fewest-actions path to the goal, we might want to find paths that are “best” in other senses. Consider
By changing the cost function, we can encourage Pacman to find different paths. For example, we can charge more for dangerous steps in ghost-ridden areas or less for steps in food-rich areas, and a rational Pacman agent should adjust its behavior in response.
Question 3 (2 points) Implement the uniform-cost graph search algorithm in the
uniformCostSearch function in
search.py. We encourage you to look through
util.py for some data structures that may be useful in your implementation. You should now observe successful behavior in all three of the following layouts, where the agents below are all UCS agents that differ only in the cost function they use (the agents and cost functions are written for you):
python pacman.py -l mediumMaze -p SearchAgent -a fn=ucs
python pacman.py -l mediumDottedMaze -p StayEastSearchAgent
python pacman.py -l mediumScaryMaze -p StayWestSearchAgent
Note: You should get very low and very high path costs for the
StayWestSearchAgent respectively, due to their exponential cost functions (see
searchAgents.py for details).
Question 4 (3 points)
Implement A* graph search in the empty function
search.py. A* takes a heuristic function as an argument. Heuristics take two arguments: a state in the search problem (the main argument), and the problem itself (for reference information). The
nullHeuristic heuristic function in
search.py is a trivial example.
You can test your A* implementation on the original problem of finding a path through a maze to a fixed position using the Manhattan distance heuristic (implemented already as
python pacman.py -l bigMaze -z .5 -p SearchAgent -a fn=astar,heuristic=manhattanHeuristic
You should see that A* finds the optimal solution slightly faster than uniform cost search (about 549 vs. 620 search nodes expanded in our implementation, but ties in priority may make your numbers differ slightly). What happens on
openMaze for the various search strategies?
Finding All the Corners
The real power of A* will only be apparent with a more challenging search problem. Now, it’s time to formulate a new problem and design a heuristic for it.
In corner mazes, there are four dots, one in each corner. Our new search problem is to find the shortest path through the maze that touches all four corners (whether the maze actually has food there or not). Note that for some mazes like tinyCorners, the shortest path does not always go to the closest food first! Hint: the shortest path through
tinyCorners takes 28 steps.
Question 5 (2 points)
Note: Make sure to complete Question 2 before working on Question 5, because Question 5 builds upon your answer for Question 2.
CornersProblem search problem in
searchAgents.py. You will need to choose a state representation that encodes all the information necessary to detect whether all four corners have been reached. Now, your search agent should solve:
python pacman.py -l tinyCorners -p SearchAgent -a fn=bfs,prob=CornersProblem
python pacman.py -l mediumCorners -p SearchAgent -a fn=bfs,prob=CornersProblem
To receive full credit, you need to define an abstract state representation that does not encode irrelevant information (like the position of ghosts, where extra food is, etc.). In particular, do not use a Pacman
GameState as a search state. Your code will be very, very slow if you do (and also wrong).
Hint: The only parts of the game state you need to reference in your implementation are the starting Pacman position and the location of the four corners.
Our implementation of
breadthFirstSearch expands just under 2000 search nodes on mediumCorners. However, heuristics (used with A* search) can reduce the amount of searching required.
Question 6 (3 points)
Note: Make sure to complete Question 4 before working on Question 6, because Question 6 builds upon your answer for Question 4.
Implement a non-trivial, consistent heuristic for the
python pacman.py -l mediumCorners -p AStarCornersAgent -z 0.5
AStarCornersAgent is a shortcut for
-p SearchAgent -a fn=aStarSearch,prob=CornersProblem,heuristic=cornersHeuristic.
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