Question

I don’t know why this method is throwing an ArrayIndexOutOfBounds exception.

When I change the initial "high" value to "int high = array.length - 1;", the program will return any integer value that I search for.

What am I doing wrong?

Thanks in advance!

---

public class BinarySearch {

public static void main(String[] args) {

    int searchValue = 12;
    int[] givenNums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    binarySearch(givenNums, searchValue);
    System.out.println("\nResult: " + searchValue);

}

public static int binarySearch(int[] array, int key) {
    int low = 0;
    int high = array.length;
    int mid = (low + high) / 2;
    int i = 0;
    System.out.println();

    while (low <= high) {
        System.out.print(i + " ");
        if (array[mid] < key) {
            low = mid + 1;
            mid = (low + high) / 2;
        } else if (array[mid] > key) {
            high = mid - 1;
            mid = (low + high) / 2;
        }
        else
            return mid;

        i++;
    }
    return -1;
}
}

Answer 1

You need to be consistent about whether high means the maximum value it can be inclusively or exclusively. You start off with it being an exclusive upper bound:

int high = array.length;

But then your while loop condition is only appropriate if it’s an inclusive upper bound:

while (low <= high)

You should probably just change the while condition to:

while (low < high)

… and change the assignment of high later, too.

Alternatively, you could keep it inclusive, and change the initial value to array.length - 1.

That will stop the situation where low == high == mid == array.length, which is where it would blow up.

I’d also suggest moving the mid = (low + high) / 2 computation to be the first statement within the while loop – then you can get rid of the duplicate code.

while (low < high) {        
    mid = (low + high) / 2;
    System.out.print(i + " ");
    if (array[mid] < key) {
        low = mid + 1;
    } else if (array[mid] > key) {
        high = mid;
    }
    else {
        return mid;
    }
    i++;
}