DS/CS 442 Spring 2021
Project 1: Search
Due: Sunday 2/21 at 11:59 pm
All those colored walls,
Mazes give Pacman the blues,
So teach him to search.
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 Project 0, this project includes an autograder for you to grade your answers on your machine. This can be
un with the command:
See the autograder tutorial in Project 0 for more information about using the autograder.
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 can download all the code and
supporting files as a zip archive.
Files you'll edit:
search.py Where all of your search algorithms will reside.
searchAgents.py Where all of your search-based agents will reside.
Files you might want to look at:
pacman.py The main file that runs Pacman games. This file describes a Pacman
GameState type, which you use in this project.
game.py The logic behind how the Pacman world works. This file describes several
supporting types like AgentState, Agent, Direction, and Grid.
util.py Useful data structures for implementing search algorithms.
Supporting files you can ignore:
graphicsDisplay.py Graphics for Pacman
graphicsUtils.py Support for Pacman graphics
textDisplay.py ASCII graphics for Pacman
ghostAgents.py Agents to control ghosts
keyboardAgents.py Keyboard interfaces to control Pacman
Feb 13, 2022
layout.py Code for reading layout files and storing their contents
autograder.py Project autograde
testParser.py Parses autograder test and solution files
testClasses.py General autograding test classes
test_cases/ Directory containing the test cases for each question
searchTestClasses.py Project 1 specific autograding test classes
Files to Edit and Submit: You will fill in portions of search.py and searchAgents.py during the
assignment. Once you have completed the assignment, you will submit a token generated by
submission_autograder.py . Please do not change the other files in this distribution or submit any of ou
original files other than these files.
Evaluation: Your code will be autograded for technical co
ectness. Please do not change the names of any
provided functions or classes within the code, or you will wreak havoc on the autograder. However, the
ectness of your implementation – not the autograder’s judgements – will be the final judge of your score. If
necessary, we will review and grade assignments individually to ensure that you receive due credit for your work.
Academic Dishonesty: We will be checking your code against other submissions in the class for logical
edundancy. 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. If you do, we will pursue the strongest consequences available to us.
Getting Help: You are not alone! If you find yourself stuck on something, contact the course staff for help.
Office hours, section, and the discussion forum are there for your support; please use them. If you can’t make ou
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.
Discussion: Please be careful not to post spoilers.
Welcome to Pacman
After downloading the code (search.zip), unzipping it, and changing to the 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 co
idors 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
eflex agent). This agent can occasionally win:
But, things get ugly for this agent when turning is required:
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.
Note that 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:
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 bash commands.txt .
Question 1 (3 points): Finding a Fixed Food Dot using Depth First
python pacman.py --layout testMaze --pacman GoWestAgent
python pacman.py --layout tinyMaze --pacman GoWestAgent
python pacman.py -h
In 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.
First, test that the SearchAgent is working co
ectly by running:
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 Stack , Queue and PriorityQueue data structures provided to
you in util.py ! These data structure implementations have particular properties which are required fo
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).
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
Your code should quickly find a solution for:
The Pacman board will show an overlay of the states explored, and the order in which they were explored
ighter 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 (3 points): Breadth First Search
eadth-first search (BFS) algorithm in the
eadthFirstSearch 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.
Does BFS find a least cost solution? If not, check your implementation.
Hint: If Pacman moves too slowly for you, try the option --frameTime 0 .
Note: If you’ve written your search code generically, your code should work equally well for the eight-puzzle
search problem without any changes.
Question 3 (3 points): Varying the Cost Function
python pacman.py -l tinyMaze -p SearchAgent -a fn=tinyMazeSearch
python pacman.py -l tinyMaze -p SearchAgent
python pacman.py -l mediumMaze -p SearchAgent
python pacman.py -l bigMaze -z .5 -p SearchAgent
python pacman.py -l mediumMaze -p SearchAgent -a fn=bfs
python pacman.py -l bigMaze -p SearchAgent -a fn=bfs -z .5
While BFS will find a fewest-actions path to the goal, we might want to find paths that are “best” in other senses.
Consider mediumDottedMaze and mediumScaryMaze .
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.
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 you
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):
Note: You should get very low and very high path costs for the StayEastSearchAgent and
StayWestSearchAgent respectively, due to their exponential cost functions (see searchAgents.py fo
Question 4 (3 points): A* search
Implement A* graph search in the empty function aStarSearch in 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
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 manhattanHeuristic in
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?
Question 5 (3 points): 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