CSE143 handout #16

handout #16

CSE143—Computer Programming II

Programming Assignment #5

due: Sunday, 7/24/05, 9 pm

(courtesy of Stuart Reges)

This assignment will give you practice with recursion and grammars. You will complete a program that reads an input file with a grammar in Backus-Naur Form (BNF) and will allow the user to randomly generate elements of the grammar.

You will be given a main program that does the file processing and user interaction. It is called GrammarMain.java. You are to write a class called GrammarSolver that manipulates the grammar. A grammar will be specified as a sequence of Strings, each of which represents the rules for a nonterminal symbol. Each String will be of the form:

Notice that this is the standard BNF format of a non-terminal symbol on the left-hand-side and a series of rules separated by vertical bar characters (“|”) on the right-hand side. If there is only one rule for a particular nonterminal, then there will be no vertical bar characters. Normally BNF rules have the characters “::=” separating the symbol from the rules, so this is a slight variation where the separator is a simple colon (“:”).

There will be exactly one colon per String. The text appearing before the colon is a nonterminal symbol. You may assume it does not contain any whitespace. Often we surround nonterminal symbols with the characters “<” and “>”, but this will not always be the case. The text appearing after the colon will be a series of rules separated by vertical bar characters (“|”). Each of these rules will have a series of tokens separated and potentially surrounded by whitespace. There could be any amount of whitespace surrounding tokens. Any token that appears to the left of a colon in the grammar is considered a nonterminal. All other tokens are considered terminals.

The grammars you will be asked to process will be stored in text files with each line of the file being of the form described above. GrammarMain reads this file into an ArrayList of Strings and passes the list to the constructor of your GrammarSolver. Your solver has to be able to perform certain tasks, most notably generating random elements of the grammar.

To generate a random instantiation of a non-terminal, you simply pick at random one of its rules and generate whatever that rule tells you to generate. Notice that this is a recursive process. Generating a non-terminal involves picking one of its rules at random and then generating each part of that rule, which might involve more non-terminal symbols to generate for which you pick rules at random and generate each part of those rules, and so on. Depending upon the grammar, this process could continue indefinitely. Any grammar you will be asked to work with will be guaranteed to converge in a finite period of time. Most often this process doesn’t go on indefinitely because many rules involve terminals rather than non-terminals. When you encounter a terminal, you simply include it. This becomes the base case of the recursive process. Your generating method produces an array of Strings. Each String should be compact in the sense that there should be exactly one space between each terminal and there should be no leading or trailing spaces.

Your class must include the following public methods.

Method	Description
GrammarSolver(ArrayList grammar)	Constructs a grammar solver for the given grammar. Throws a NullPointerException if the grammar is null and throws an IllegalArgumentException if there are two or more entries in the grammar for the same non-terminal.
boolean grammarContains(String symbol)	Returns true if the given symbol is a non-terminal of the grammar; returns false otherwise.
String[] generate(String symbol, int times)	Uses the grammar to randomly generate given number of occurrences of the given symbol and returns the result as an array of Strings. For any given non-terminal symbol, each of its rules should be applied with equal probability. Throws an IllegalArgumentException if the grammar does not contain the given non-terminal symbol or if the number of times is less than 0.
String getSymbols()	Returns a String representation of the various non-terminal symbols from the grammar as a sorted, comma-separated list enclosed in square brackets, as in “[<np>, <s>, <vp>]”

Case matters when comparing symbols. For example, <S> would not be considered the same as <s>.

You are allowed to use whatever constructs you want from the Java class libraries, but there are several highly convenient options described below that you are strongly advised to consider.

The Random class in java.util can be used to generate a random integer by calling its nextInt method. It is generally best to have one Random object stored as a data field that is constructed just once. If you construct multiple Random objects, they are likely not to be as random as you’d like.
You can store the grammar rules any way you want, but you will want to check out the Map interface and the implementation TreeMap, both in java.util. Maps keep track of key/value pairs. Each key is associated with a particular value. In our case, we want to store something for each non-terminal symbol. So the non-terminal symbols become the keys and the rules become the values. If you use this approach, you will find that the getSymbols method can be written quickly because the Map interface includes a method called keySet that returns a set of keys from the map. If you ask for the “toString” of this set, you will get the desired string. It is important to use a TreeMap because it keeps the keys in sorted order (notice that getSymbols requires that the non-terminals be listed in sorted order).
The String class has a method called “trim” that will return a new version of the String minus any leading or trailing whitespace.
One problem you will have to deal with is breaking up strings into various parts. There are many ways to do this. StringTokenizer, for example, is a class that is specifically designed to do this. The new Scanner class can be used to read a sequence of tokens from a String. Another approach is to use the split method added to the String class in jdk1.4. It makes use of what are called “regular expressions” and this can be confusing, but you will find that learning about regular expressions is extremely helpful for computer scientists and computer programmers. Many unix tools, for example, take regular expressions as input.

The regular expressions we want are fairly simple. For example, to split a string on the colon character you simply put the colon inside square brackets. The split method returns an array of strings, which means we can perform this split by saying:

String[] parts = s.split("[:]");

This square bracket notation works for any single character, even a space. In the case of whitespace, we want to include both spaces and tabs and we want to have one or more of them. This can be accomplished in a regular expression by putting both a space and a tab inside the square brackets and putting a plus sign after the brackets which indicates “1 or more of these”:

String[] parts = s.split("[ \t]+");

As mentioned above, the various parts of a rule are guaranteed to be separated by whitespace. Otherwise you would have a difficult time separating the parts of a rule. But this means that once you’ve used the spaces to split the rule up, the spaces are gone. That means that when you generate Strings, you will have to include spaces yourself.

In terms of correctness, your class must provide all of the functionality described above. In terms of style, we will be grading on your use of comments, good variable names, consistent indentation and good coding style to implement these operations. Remember that you will lose points if you declare variables as data fields that can instead be declared as local variables. You should also avoid extraneous cases (e.g., don’t make something into a special case if it doesn’t have to be).

You MUST name your file GrammarSolver.java and turn it in electronically from the “assignments” link on the class web page. A collection of files needed for the assignment is included on the web page as ass5.zip. You will need to have GrammarMain.java and Scanner.java in the same directory as your GrammarSolver.java in order to run GrammarMain. The second input file contains extraneous whitespace, including tabs.

Sample input file (sentence.txt)

<dp>:the|a

<tv>:hit|honored|kissed|helped

<iv>:died|collapsed|laughed|wept

Sample input file (sentence2.txt)

E: T | E OP T

T: x | y | 42 | 0 | 1 | 92 | ( E ) | F1 ( E ) | - T | F2 ( E , E )

OP: + | - | * | % | /

F1: sin | cos| tan |sqrt | abs

F2:max |min | pow

Main program GrammarMain.java

// Stuart Reges

// 3/10/04

// GrammarMain contains a main program that prompts a user for the name of a

// grammar file and then gives the user the opportunity to generate random

// versions of various elements of the grammar.

import java.io.*;

import java.util.*;

public class GrammarMain {

public static void main(String[] args) throws FileNotFoundException {

Scanner console = new Scanner(System.in);

System.out.println("Welcome to the cse143 random sentence generator.");

System.out.println();

// open grammar file

System.out.print("What is the name of the grammar file? ");

String fileName = console.nextLine();

Scanner input = new Scanner(new File(fileName));

// read the grammar file and construct the grammar solver

ArrayList grammar = new ArrayList();

while (input.hasNextLine())

grammar.add(input.nextLine());

GrammarSolver solver = new GrammarSolver(grammar);

showResults(console, solver);

}

// pre : console open for console reading, solver initialized

// post: allows the user to repeatedly pick a grammar element to generate

public static void showResults(Scanner console, GrammarSolver solver) {

for(;;) {

System.out.println();

System.out.println("Available symbols to generate are:");

System.out.println(solver.getSymbols());

System.out.print("What do you want generated (return to quit)? ");

String target = console.nextLine();

if (target.length() == 0)

break;

if (!solver.grammarContains(target))

System.out.println("Illegal symbol");

else {

System.out.print("How many do you want me to generate? ");

if (!console.hasNextInt())

System.out.println("that's not an integer");

else {

int number = console.nextInt();

if (number < 0)

System.out.println("no negatives allowed");

else {

String[] answers = solver.generate(target, number);

for (int i = 0; i < number; i++)

System.out.println(answers[i]);

}

console.nextLine(); // to position to next line

}

Sample execution (user input underlined)

Welcome to the cse143 random sentence generator.

What is the name of the grammar file? sentence.txt

Available symbols to generate are:

[<adj>, <adjp>, <dp>, <iv>, <n>, <np>, <pn>, <s>, <tv>, <vp>]

What do you want generated (return to quit)? <dp>

How many do you want me to generate? 5

the

Available symbols to generate are:

[<adj>, <adjp>, <dp>, <iv>, <n>, <np>, <pn>, <s>, <tv>, <vp>]

What do you want generated (return to quit)? <np>

How many do you want me to generate? 5

Jane

the wonderful pretentious fat big father

Fred

the wonderful cat

a subliminal pretentious dog

Available symbols to generate are:

[<adj>, <adjp>, <dp>, <iv>, <n>, <np>, <pn>, <s>, <tv>, <vp>]

What do you want generated (return to quit)? <s>

How many do you want me to generate? 20

Jane kissed Spot

the wonderful wonderful pretentious television collapsed

Jane hit Fred

Jane helped a wonderful dog

Elmo helped the fat fat man

a pretentious university helped Elmo

a wonderful green subliminal father hit Fred

Fred kissed Spot

Spot laughed

the green wonderful father collapsed

the big man helped John

a pretentious green faulty dog collapsed

Jane honored a green subliminal green child

Elmo hit Elmo

a green university died

the pretentious child honored a faulty wonderful subliminal television

Jane died

the faulty dog hit John

Elmo helped Fred

Elmo honored the pretentious big green father

Available symbols to generate are:

[<adj>, <adjp>, <dp>, <iv>, <n>, <np>, <pn>, <s>, <tv>, <vp>]

What do you want generated (return to quit)?

Sample execution (user input underlined)

Welcome to the cse143 random sentence generator.

What is the name of the grammar file? sentence2.txt

Available symbols to generate are:

[E, F1, F2, OP, T]

What do you want generated (return to quit)? T

How many do you want me to generate? 5

- y

( ( 1 ) )

Available symbols to generate are:

[E, F1, F2, OP, T]

What do you want generated (return to quit)? E

How many do you want me to generate? 10

x - 1

sin ( 1 + 92 + - 1 / 42 )

max ( y , 92 )

42 % 1

- 42

42 - sin ( 1 )

Available symbols to generate are:

[E, F1, F2, OP, T]

What do you want generated (return to quit)?