二进制搜索最近值c＃

Question

i am using this binary search algorithm to find items in my array. I am trying to make it so when i search for a value which doesn't appear in the array my program tells me, and it also reveals where the closest items are

Example Array: (4, 6, 7, 8, 9) Search for 5 Result 6, 7.

Every time i search for something which isn't an element within my array it returns 0, or -1. Can someone help me out, thanks?

  public static double BinarySearch(double[] a, double item)
    {
        int first = 0;
        int last = a.Length - 1;
        do
        {
            int mid = first + (last - first) / 2;
            if (item > a[mid])
                first = mid + 1;
            else
                last = mid - 1;
            if (a[mid] == item)
                return mid;
        } while (first <= last);       
        return -1;
    }

Answer 1

Suppose you would declare "mid" outside the loop and return it ? Instead of -1 ?

When your array is sorted and the searched value is not found, you will get a mid result pointing to the lower value in the array closest to your search value. The element above your search value is contained in a[mid+1]. Small change:

public static double BinarySearch(double[] a, double item)
{
    int first = 0;
    int last = a.Length - 1;
    int mid = 0;
    do
    {
        mid = first + (last - first) / 2;
        if (item > a[mid])
            first = mid + 1;
        else
            last = mid - 1;
        if (a[mid] == item)
            return mid;
    } while (first <= last);
    return mid;
}