Question

我刚刚开始学习Java（这是我第一次编程）。如果print语句（纯粹用于测试目的），代码会反复输出mid而不会更改它。我已经考虑了好几个小时了，无法弄明白。非常感谢帮助。

/*class containing binary search algorithm*/
public class BinarySearch {
    /*conducts a binary search as specified by user*/
    public static int binarySearch(int queryValue, int[] list) {
    int length = list.length; 
    /*last point of list*/
    int top = length-1;       
    /*first point of list*/
    int bottom = 0;    
    /*starting midpoint of list*/
    int mid = (int)Math.round((top + bottom)/2);
    /*binary search*/
    while(bottom < top) {
        if((int)queryValue == (int)list[mid]) {
        return mid;
        }
        else if(queryValue > list[mid]) {
         bottom = mid;
         mid = (int)Math.round((top + bottom) / 2);
         StdOut.print(mid);
        }
        else {
         top = mid;
         mid = (top + bottom) / 2;
         }
    }
    /*returns -1 if user value not found*/
    return -1;
    }
}

Answer 1

如果您的值大于中点，则消除中点。将bottom提前一个mid：

bottom = mid + 1;

同样，对于少于中点的情况，请在当前top之前提前mid：

top = mid - 1;

否则，您可能会遇到bottom和top从未相互交叉的情况。

此外，二进制搜索仅在输入已排序时才有效。请确认/确保您的阵列已经排序。

Answer 2

如果您提供用于测试的数据会更好至于现在，我看到以下输入它会挂起：
binarySearch(9, new int[] {1,2,3,4,5,6,7,8,9})
中期将是7
这是因为（7 + 8）/ 2 = 7，因为你使用int
尝试更换：
mid = (int)Math.round((top + bottom) / 2);
与
mid = (int)Math.round((top + bottom) / 2.0);
它将解决问题祝好运！
更新了代码以考虑边缘情况：

public static int binarySearch(int queryValue, int[] list) {
    int length = list.length;
    /*last point of list*/
    int top = length-1;
    /*first point of list*/
    int bottom = 0;
    /*starting midpoint of list*/
    int mid = (int)Math.round((top + bottom)/2.0);
    /*binary search*/
    do {
        if (queryValue > list[mid]) {
            bottom = mid;
            mid = (int)Math.ceil((top + bottom) / 2.0);
            System.out.println(mid);
        } else {
            top = mid;
            mid = (int)Math.floor((top + bottom) / 2.0);
            System.out.println(mid);
        }
        if(queryValue == list[mid]) {
            return mid;
        }
    } while (mid < top && mid > bottom);
    /*returns -1 if user value not found*/
    return -1;
}

Answer 3

public static int binarySearch(int queryValue, int[] list) {
    int length = list.length;
    /*last point of list*/
    int top = length-1;
    /*first point of list*/
    int bottom = 0;
    /*starting midpoint of list*/
    int mid = (int)Math.round((top + bottom)/2.0);
    /*binary search*/
    do {
        if (queryValue > list[mid]) {
            bottom = mid;
            mid = (int)Math.ceil((top + bottom) / 2.0);
            System.out.println(mid);
        } else {
            top = mid;
            mid = (int)Math.floor((top + bottom) / 2.0);
            System.out.println(mid);
        }
        if(queryValue == list[mid]) {
            return mid;
        }
    } while (mid < top || mid > bottom);
    /*returns -1 if user value not found*/
    return -1;
}

我的二分查找有什么问题？

3 个答案: