COSC-270: Artificial Intelligence

Project 2
Spring 2022

Due: F 2/25 @ 5 PM ET
9 points

For this project, you will implement a resolution theorem prover for propositional logic.

I have divided the project into three phases:

1. For the first phase, write the classes and methods necessary to read, parse, store, and output propositional well-formed formulas (wffs). Formulas must be stored internally as expression trees.

   The following grammar specifies the syntax for wffs.

   formula ::= '(' proposition ')'
            | '(' 'not' formula-or-proposition ')'
            | '(' 'or' formula-or-proposition formula-or-proposition ')'
            | '(' 'and' formula-or-proposition formula-or-proposition ')'
            | '(' 'cond' formula-or-proposition formula-or-proposition ')'
   
   formula-or-proposition ::= formula | proposition
   
   proposition ::= letter letters-and-digits
   
   letters-and-digits ::= letter-or-digit letters-and-digits | \epsilon
   
   letter-or-digit ::= letter | digit
   
   letter ::= 'a'..'z' | 'A'..'Z'
   
   digit ::= '0'..'9'
   

   Examples of wffs written in this syntax are:

   • Wishes are horses provided that horses cannot fly.
   • \( \neg\mbox{HF} \rightarrow \mbox{W} \)
   • (cond (not hf) w)
   • If it is not the case that both beggars ride and wishes are nonequine, then horses can fly.
   • \( \neg (\mbox{BRD} \; \& \; \neg\mbox{W}) \rightarrow \mbox{HF} \)
   • (cond (not (and brd (not w))) hf)

   To support development, I put some starter code on cs-class. To get started, log on to cs-class and copy over the following zip file:

   cs-class-1% cd
   cs-class-1% cp ~maloofm/cosc270/p2.zip ./
   cs-class-1% unzip p2.zip
   

   In the p2 directory, you will find a class implementation for Tokenizer and a partial implementation for Formula. The Tokenizer class demonstrates how to configure Scanner to tokenize propositional logic formulae. The directory also contains files for the two proofs we discussed in lecture, Example 1 and the proof that beggars do not ride horses. As you can see in the files, comments begin with a forward slash, formulas begin with a left parenthesis, and both are confined to a single physical line. By convention, the last formula in the file is the conclusion.

   In addition to the these proofs, you must find two additional proofs from reputable sources consisting of at least three formulas and three propositions. Include these proof files with your submission to Autolab. Naturally, I will test your program on my own set of proofs.

   For this first phase, you must implement the class Formula with the methods

   • public Formula()
   • public void set( String s ), which constructs an expression tree from the specified string s
   • public String toString(), which returns a string representation of the expression tree

   When you submit to Autolab, the autograder will call these three methods. Naturally, you will need to implement additional methods.

2. For the second phase, implement the routines to convert wffs to clausal form. Clauses must be stored internally as expression trees or more precisely a linked list of literals. The following grammar specifies the syntax for clauses.

   clause ::= '{' literals '}'
   
   literals ::= literal literals-comma-separated | \epsilon
   
   literals-comma-separated ::= ',' literal literals-comma-separated | \epsilon
   
   literal ::= proposition
            | '(' 'not' proposition ')'
   

   You must implement

   • public void Formula.cnf(), which converts a formula to conjunctive normal form
   • public class Clauses, which must extend ArrayList<Clause>
   • public Clauses Formula.getClauses(), which converts a formula to clausal form and returns the resulting clauses
   • public String Clause.toString(), which returns the string representation of the clause
3. Finally, implement the routines required to conduct proofs using resolution. For this final phase, you must implement Main.java so it takes a file name as a command-line argument (e.g., java Main lecture.txt), reads the formulas in the file, negates the conclusion, converts the formulas to clauses, and conducts a proof using resolution. Main.main should print the formulas, the converted clauses, and a message indicating whether the conclusion follows from the premises.

   You must implement

   • public Clause()
   • public void Clause.set( String ), which sets the Clause to the clause that the specified String represents
   • public boolean Clause.empty(), which returns true if the Clause is empty and returns false otherwise
   • public boolean Clause.failure(), which returns true if a failed resolution produced the Clause. Modify Clause.toString() to return the String "failure" if failure() is true
   • public Clause Clause.resolve( Clause ), which resolves this Clause with the specified Clause and returns the resolvent. If the two clauses do not resolve, then the method sets the Clause's internal state so Clause.failure() returns true
   • public static boolean Resolution.prove( String filename ), which returns true if the conclusion follows from the premises and returns false otherwise

Include with your submission the proof files and a transcript of your program's execution for the four proofs. The transcript should be a plain ASCII file named README. In a file named HONOR, include the following statement:

In accordance with the class policies and Georgetown's Honor Code,
I certify that, with the exceptions of the class resources and those
items noted below, I have neither given nor received any assistance
on this project.

Name
NetID

When you are ready to submit your project for grading, put your source files, Makefile, proof files, transcript, and honor statement in a zip file named submit.zip. Upload the zip file to Autolab using the assignment p2. Make sure you remove all debugging output before submitting.

Plan B

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