Utilizing BinarySearch to do an Autocomplete exercise

Question

Hi I'm making a program that takes an array of "Terms" each of which have their own weights (Long value) and queries (String value). It should take many terms from a .txt file and initialize them into a private "Terms" array, and then it uses arrays.sort to rearrange their queries lexicographically. The Term[] allmatches(String prefix) function is meant to utilize a binary search to find within the already lexicographically sorted "Terms" array where the first match begins and the last match ends (I'm confident that my prefixOrder comparator and my binary search methods work fine on their own). Then using a "for" loop, I try to copy all those specific terms into a new array called "newterms." The problem is that I only return "null" values.

public class Autocomplete {
private Term[] terms;

/**
 * Initializes the data structure from the given array of terms.
 * @param terms a collection of terms.
 */
public Autocomplete(Term[] terms) {
   if (terms == null) {
        throw new java.lang.NullPointerException(); }
   this.terms = terms;
   Arrays.sort(terms);
}

/**
 * Returns all terms that start with the given prefix, in descending order 
 * of weight.
 * @param prefix term prefix.
 * @return all terms that start with the given prefix, in descending order 
 * of weight.
 */
public Term[] allMatches(String prefix) {
    if (prefix == null) {
        throw new java.lang.NullPointerException(); }
    Term term = new Term(prefix);
    Comparator<Term> prefixOrder = Term.byPrefixOrder(prefix.length());
    int a = BinarySearchDeluxe.firstIndexOf(this.terms, term, prefixOrder);
    int b = BinarySearchDeluxe.lastIndexOf(this.terms, term, prefixOrder);
    int c = a;
    Term[] newterms = new Term[b - a];
    for (int i = 0; i > b - a; i++) {
        newterms[i] = this.terms[c];
        c++;
    }
    Arrays.sort(newterms, Term.byReverseWeightOrder());
    return newterms;
}

And this is the "test" client that I'm using:

     public static void main(String[] args) {
    String filename = args[0];
    In in = new In(filename);
    int N = in.readInt();
    Term[] terms = new Term[N];
    for (int i = 0; i < N; i++) {
        long weight = in.readLong(); 
        in.readChar(); 
        String query = in.readLine(); 
        terms[i] = new Term(query, weight); 
    }
    int k = Integer.parseInt(args[1]);
    Autocomplete autocomplete = new Autocomplete(terms);
    while (StdIn.hasNextLine()) {
        String prefix = StdIn.readLine();
        Term[] results = autocomplete.allMatches(prefix);
        for (int i = 0; i < Math.min(k, results.length); i++) {
            StdOut.println(results[i]);
        }
    }
}

What I'm I doing wrong here? Help would be greatly appreciated, thanks.

Answer 1

Your for-loop in allMatches(String prefix) has a wrong boolean predicate,instead of

for (int i = 0; i > b - a; i++)

it should be

for (int i = 0; i < b - a; i++)