为什么 binarySearch 方法不返回正确的结果？

Question

Recently I am working on arrays in Java and I needed to use the method binarySearch(arr, v) in Arrays class to search the index of value v in the array arr if found.

Then I do some tests with some different arrays. When I try to search value 4 in the array {4, 3, 2, 1} I found -5 .

Would someone can explain that please ?

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The code is below:

int[] arr   = {4, 3, 2, 1};
System.out.println(Arrays.binarySearch(arr, 4));

-5

Answer 1

You should do Arrays.sort(arr); before the binary search. From the documentation for Arrays.binarySearch :

Searches the specified array of ints for the specified value using the binary search algorithm. The array must be sorted (as by the sort(int[]) method) prior to making this call. If it is not sorted, the results are undefined.

Answer 2

Just look at thejavadoc . You need to have a sorted array to use this method. You can do the following:

Arrays.sort(arr);
Arrays.binarySearch(arr, 4)

Answer 3

对于使用标准二分搜索算法，您的输入数组必须按递增值顺序排列，否则您会得到意想不到的结果。