二进制搜索已排序的数组

Question

我正在尝试使用此二进制搜索代码搜索降序排序数组.然而,在我对它进行排序并尝试搜索之后,它不会带来任何结果,只是一个永远不会消失的加载图标就好像它有一个无限循环.我不确定问题是什么,因为代码看起来合乎逻辑.

这是带有4.0框架的aspx,c#.提前致谢!

    protected void Button2_Click(object sender, EventArgs e)
    {
        String item = TextBox1.Text;
        int target = Convert.ToInt16(item);
        int mid, first = 0, last = mynumbers.Length - 1;

        //for a sorted array with descending values
        while (first<=last)
        {
            mid = (first + last) / 2;
            if (target < mynumbers[mid])
                first = mid + 1;
            if (target > mynumbers[mid])
                last = mid - 1;
            else
                Label11.Text = "Target " + item + " was found at index " + mynumbers[mid];

        }

Answer 1

在Array课堂上有二进制搜索:

int index = Array.BinarySearch(mynumbers, target);

对于降序,这可以很容易地完成,ReverseComparer如下所示:

    public class ReverseComparer<T> : IComparer<T>
    {
        public int Compare(T x, T y)
        {
            return Comparer<T>.Default.Compare(y, x);
        }
    }

然后:

int index = Array.BinarySearch(numbers, 7, new ReverseComparer<int>());

如果这是学术练习,您必须使用自定义搜索,当然,这不适用.如果它必须是一个类的自定义算法,那么问题是你必须在找到时突破循环,并且索引位于mid,而不是mynumbers[mid]:

    //for a sorted array with descending values
    while (first<=last)
    {
        mid = (first + last) / 2;

        if (target < mynumbers[mid])
        {
            first = mid + 1;
        }

        if (target > mynumbers[mid])
        {
            last = mid - 1;
        }

        else
        {
            // the index is mid, not mynumbers[mid], and you need to break here
            // once found or it's an infinite loop once it finds it.
            Label11.Text = "Target " + item + " was found at index " + mid;
            break;
        }
    }

实际上,我可能会设置一个bool标志来保持算法的纯净,而不是将find与输出问题混合在一起,这也可以让你更容易分辨出如果你没有找到它就退出循环:

    bool found = false;

    //for a sorted array with descending values
    while (!found && first<=last)
    {
        mid = (first + last) / 2;

        if (target < mynumbers[mid])
        {
            first = mid + 1;
        }

        if (target > mynumbers[mid])
        {
            last = mid - 1;
        }

        else
        {
            // You need to stop here once found or it's an infinite loop once it finds it.
            found = true;
        }
    }

    Label11.Text = found 
        ? "Item " + item + " was found at position " + mid
        : "Item " + item + " was not found";