Need Help Getting A Context Free Grammar Generator Working

Question

Hello,
This is my first real programming excercise in a college programming class. I am trying to make a context free grammar generator. I keep getting this error:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String ind
ex out of range: 2
at java.lang.String.substring(String.java:1500)
at Generator.FindCat(Generator.java:48)
at Generator.Generate_Sentence(Generator.java:28)
at Generator.main(Generator.java:16)
I'm not even sure what its trying to tell me. If anyone could take a few minutes to look at it and give me some help I'd appreciate it.
Thanks,
Mark
[email protected]
import java.util.Random;
public class Generator{
//Generate 10 Sentences
public static void main(String[] args) {
for (int a=0; a<10; a++){
System.out.println(Generate_Sentence("S"));
//Expand Categories
public static String Generate_Sentence(String category){
int startleft=0;
while (HasCategory(category)==0) {
startleft=LeftIndex(category);
category=FindCat(category,startleft);
return category;
// Find Complete Category and Separate
public static String FindCat(String category, int start){
String nextLetter="", cat="";
String left="", right="";
int endCat=0;
while ((!(nextLetter.equals(""))) || (!(nextLetter.equals(" ")))){
int z=start;
String orig = category.substring(z,z+1) ;
String folded = orig.toUpperCase() ;
if (orig.equals(folded))
cat=cat+orig;
z++;
nextLetter=category.substring(z,z+1);
endCat=z;
for (int r=0; r<start; r++){
left=left+category.substring(r,r+1);
for (int s=endCat; s<category.length(); s++){
right=right+category.substring(s,s+1);
String tempString = left+Select_Expansion(category)+right;
return tempString;
// Find Left Index
public static int LeftIndex(String category){
int e=-1;
boolean bol=false;
while (!bol){
e++;
String orig = category.substring(e,e+1) ;
String folded = orig.toUpperCase() ;
// If folding up had no effect original must have been uppercase:
if (orig.equals(folded))
bol=true;
return e;
// Check for Category 0=yes - 1=no -- WORKS FINE
public static int HasCategory(String Input) {
String original, folded_up ;
boolean b=false;
// Check each character:
for (int j = 0 ; j < Input.length() ; j++) {
// Extract single character and fold it to uppercase:
original = Input.substring(j,j+1) ;
folded_up = original.toUpperCase() ;
// If folding up had no effect original must have been uppercase:
if (original.equals(folded_up)){
b=true;
if (b)
return 0;
else
return 1;
// Select_Expansion(category) will randomly expand Category according
// to the following grammar.
static String[] Grammar= {
"S -> NAME VP",
"VP -> V NP",
"NP -> ART NN",
"NN -> ADJ NN",
"NN -> N",
"ART -> a",
"V -> is",
"V -> looks like",
"V -> acts like",
"NAME -> dr aronis",
"NAME -> mr aronis",
"NAME -> john",
"ADJ -> nice",
"ADJ -> mean",
"ADJ -> smart",
"ADJ -> stupid",
"ADJ -> hip",
"ADJ -> geeky",
"ADJ -> normal",
"ADJ -> strange",
"ADJ -> great",
"ADJ -> terrible",
"N -> person",
"N -> teacher",
"N -> fool",
"N -> tool",
static Random generator = new Random();
public static String Select_Expansion(String Category) {
// Variables:
String This_Rule = "", Rule_LHS="", Rule_RHS="";
int Number_of_Expansions=0, Expansion_to_Use=0;
// Count expansions for Category:
for (int n=0 ; n<Grammar.length ; n++) {
This_Rule=Grammar[n];
Rule_LHS = This_Rule.substring(0,This_Rule.indexOf(" ->"));
if (Rule_LHS.equals(Category)) Number_of_Expansions++;
// return error if no expansions:
if (Number_of_Expansions == 0) return "error";
//Randomly select which expansion to use:
Expansion_to_Use = Math.abs(generator.nextInt() % Number_of_Expansions);
//Go through rules while counting down through expansions of Category,
// and break with selected expansion:
for (int n=0; n<Grammar.length; n++) {
This_Rule=Grammar[n];
Rule_LHS = This_Rule.substring(0,This_Rule.indexOf(" -> "));
Rule_RHS = This_Rule.substring(This_Rule.indexOf(" -> ") + 4);
if (Rule_LHS.equals(Category) && (Expansion_to_Use ==0)) break;
if (Rule_LHS.equals(Category)) Expansion_to_Use--;
return Rule_RHS;

Answer 1

In general terms, any of your loops over a string should contain a test to make sure you are still within the bounds of the string. I noticed that several methods have the potential to throw exceptions like the one you saw.
You will also need (want?) to deal with the case that the string is not found - getting the substring(-1, 0) will throw an exception, too.
If you are dealing with single characters of strings, you may prefer to use the charAt(pos) method rather than substring(pos, pos+1), since comparing a character is faster (and simpler, IMHO) than comparing strings of length 1.
I also noticed, in particular, in your FindCat method, the following loopwhile ((!(nextLetter.equals(""))) || (!(nextLetter.equals(" ")))){
    int z=start;
    String orig = category.substring(z,z+1) ;
    String folded = orig.toUpperCase() ;
    if (orig.equals(folded))
        cat=cat+orig;
    z++;
    nextLetter=category.substring(z,z+1);
    endCat=z;
}You may want to move the line 'int z = start;' outside the loop, otherwise you may experience infinite looping, since you are constantly incrementing then resetting the value of z.
A common implementation technique is incremental implementation. That is, to code a single method, then test to make sure that method works as expected before moving on to the next method. This reduces the code that you will have to search for any errors when they do occur, although stack traces (as you have shown) can help in determining where an error occurs.
One final comment, a matter of style... it's considered a convention in Java to name methods and variables starting with a lowercase letter... thus 'public static String findCat' rather than 'public static String FindCat', and 'numberOfExpansions' rather than 'Number_of_Expansions'... once you get used to it, you can easily differentiate between return types, method calls, method and variable declarations and other statements. Nothing forcing you though :-)
Otherwise, it looks like you have a grasp on the language, and you understand the problem... it's just a couple of standard 'tricks' that are holding you back... which come with practice.
Good luck with your exercise!
-Troy