using Binary search with a comparator to find first occurance

Question

im trying to do the autocomplete assignment for the algorithm and data structure class, in the assignment its ask you to create a class that finds the first occurance of a key, and last occurance of a key.

the problem that im running into is that i dont understand how to implement the comparator in this problem,im having problems setting up a binary search since when i try to compare key < a[mid] its says bad operand for binary operator, since im using objects, where i understand the comparator comes into effect, but how?

// Return the index of the first key in a[] that equals the search key, or -1 if no such key. uses binary search

    public static <Key>int firstIndexOf(Key[] a, Key key, Comparator<Key> comparator) {

        int low = 0;
        int high = a.length - 1;
        int result = -1;

        while (low <= high) {
            int mid = (low + high) / 2;
            if (key == a[mid]) {
                result = mid;
                high = mid - 1;
            }else if (key < a[mid]) { //**<--- throws bad operand type for binary operator**         
            high = mid - 1;   // key is probable to lie before mid element
            }else {
           low = mid +1;  // key is probable to lie after mid 
           }
        }
            return result;
}

the comparator in question that im supposed to be passing goes likes this, it find the rValue using substring method in a string to see if the prefix order between two objects match. again i dont know if im even doing this right to begin with, but thats not the issue, the issue is how will i implement this in the other class

// Compare the terms in lexicographic order but using only the first r characters of each query.

 public static class prefixOrder implements Comparator<Term> 
    { public prefixOrder(int r){
      rValue = r;
    }

    @Override
    public int compare(Term v, Term w){
    return  v.queryItem.substring(rValue).compareTo(w.queryItem.substring(rValue));
    }

    }

link to the assignment https://www.cs.princeton.edu/courses/archive/fall14/cos226/assignments/autocomplete.html

possible duplicate of [Implement binary search in objects](http://stackoverflow.com/questions/901944/implement-binary-search-in-objects) — Joe, Mar 15 '15 at 05:47
possible duplicate of [How can i use Comparator to implement Bubble sort?](http://stackoverflow.com/questions/5642192/how-can-i-use-comparator-to-implement-bubble-sort) — Philipp Wendler, Mar 15 '15 at 12:35

score 0 · Accepted Answer · answered Mar 15 '15 at 05:36

You can't write key < a[mid] like that in Java, instead, you call the comparator's compare method:

if (comparator.compare(key, a[mid]) < 0) ...

It works like this: the compare method return positive/zero/negative value depending on how the comparison ended. I remember it like this:

a OP b --> comparator.compare(a, b) OP 0

where OP is any of >, >=, <, <=, ==, !=

Also, you might want to change the method declaration to:

public static <Key>int firstIndexOf(Key[] a, Key key, Comparator<? super Key> comparator)

So that for example you could choose minimum from array of Dogs using an Animal comparator, where Dog extends Animal. (Read more about wildcards.)

using Binary search with a comparator to find first occurance

1 Answers1