How to implement a recursive binary search algorithm? [duplicate]

Question

I need to implement a recursive binary search algorithm for an integer array sorted in ascending order (i.e 1,2,3,4...).

The array I have contains the following numbers:

0 0 0 0 0 0 0 1 2 2 3 3 3 3 5 6 7 7 7 9

However, my current implementation of binary search only finds the numbers to the right of 3. For some reason, it doesn't find 9, 7, 6, and 5.

below is my code:

private int srchHelper(int[] array, int first, int last, int x) {
    if (first > last) return - 1;
    int mid = (first + last) / 2;
    if (array[mid] == x) {
        return mid;
    }
    if (array[mid] < x) {
        return srchHelper(array, (mid + 1), last, x);
    }
    else return srchHelper(array, (mid - 1), last, x);
}

I saw this question yesterday, and about 3 people commented with the correct answer. — Paul Boddington
– Paul Boddington, Commented Mar 21, 2016 at 22:11
If you want to go left, wouldn't your int first be the left most option (first), and not mid, and wouldn't last be mid and not last on the very first go? — Ceelos
– Ceelos, Commented Mar 21, 2016 at 22:11
@PaulBoddington do you have the link to that question, to mark this as a duplicate? — Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans, Commented Mar 21, 2016 at 22:22

Edward Peters · Accepted Answer · 2016-03-21 22:37:21Z

0

Make sure you're clear about what the algorithm does, and then take a good long look at your recursive calls.

answered Mar 21, 2016 at 22:37

Edward Peters

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