Java 递归 Function 不会返回值

Question

I am creating this binary search function that finds the correct point to return but never actually returns a value.

'''

class Test {

static int binarySearch(int[] arr, int start, int end, int key) {
    int mid = (end - start) / 2 + start;
    while (end - start >= 1) {
        System.out.println(start + " " + mid + " " + end);
        if (arr[mid] == key) {
            System.out.println("returning " + mid);
            return mid;
        } else if (arr[mid] > key) {
            binarySearch(arr, start, mid, key);
        }else if (arr[mid] < key) {
            binarySearch(arr, mid, end, key);
        }
    }
    return -1;
}

public static void main(String[] args) {
    int[] arr = {1, 2, 3, 4, 5, 6};
    int x = binarySearch(arr, 0, arr.length, 6);
    System.out.println(x);

}

}

'''

Mid becomes 5 and it prints returning 5 but then skips the return statement and continues to loop. Why doesn't this just return 5?

Answer 1

If you want to return a value from a nested call, you have to write code to do that.

You wrote:

 binarySearch(arr, start, mid, key);

which means (1) the return value from this is ignored, and (2) you will likely eventually fall through to the final 'return -1'. The.net effect is that the nested call is completely ineffective.

You presumably (I have not analyzed the code in detail) wanted:

return binarySearch(arr, start, mid, key);

It occurs to me that perhaps you don't realize that 'return' only terminates the current execution of the method in which it is executed. It does not somehow unwind all of the nested calls and return from the outermost invocation.

So, 'return 5' returns 5 from the innermost level, and you're popped back into the loop at the next level out, and will carry on looping (and ignoring the 5).

But as a commenter observed, there's no need for the recursion with the loop, or else there's no need for the loop with the recursion.