What's wrong with this Binary Search function?

Question

I am tring to solve a Spoj problems of Binary Search but I keep getting "wrong answer" and I can't see my problem. Here is my bsearch function:

int binarySearch(int numbers[], int size, int key)
{
    int start = 0;
    int end = size - 1;
    int middle;

    while(start <= end)
    {
        middle = start + (end - start)/2;

        if(key < numbers[middle])
            end = middle - 1;
        else if(key > numbers[middle])
            start = middle + 1;
        else
            return middle;
    }

    return -1;
}

And this is my main function

int main()
{
    int *numbers;
    int n_numbers, n_queries, key, i, found;

    scanf("%d %d", &n_numbers, &n_queries);
    numbers = (int*)malloc(n_numbers * sizeof(int));

    for(i = 0; i<n_numbers; i++)
        scanf("%d", &numbers[i]);

    for(i = 0; i<n_queries; i++)
    {
        scanf("%d", &key);
        found = binarySearch(numbers, n_numbers, key);
        printf("%d\n", found);
    }

    return 0;
}

Here is the SPOJ problem: http://www.spoj.com/problems/BSEARCH1/

Answer 1

Your algorithm is correct. The data is not sorted, so you binary search algorithm cannot correctly zero in on the solution.

Answer 2

Not totally clear which C based language you are using but might the expression (end - start)/2 possibly return a floating point value that is then truncated to an integer when you actually would want the value rounded?