关于binary search
关于 search Binary
2023-09-11 14:16:04 时间
编程珠玑Column 4关于binary search的部分相当精彩,1946年就有人发表binary search,但直到1962第一个正确运行的算法才写出来。尽管算法描述看起来简单明了,但是要写的正确却是有许多地方要仔细考虑。作者着重强调的是对程序正确性的分析方法,仔细分析方法的pre-condition, invariant,和post-condition。g9老大翻译过Joshua Bloch在google blog上的文章《号外,号外,几乎所有的binary search和mergsort都有错》,原文在这里。今天看了下原文,有更新,对于求中值索引的c/c++方法原文仍然是有错的,本来是这样:
int mid = ((unsigned) (low + high)) 1。 但是在c99标准中,对于两个signed数值之和,如果溢出结果是未定义的(undefined),也就是说与编译器实现相关。上面的代码应该修改为:
mid = ((unsigned int)low + (unsigned int)high)) 1; 我查了下jdk6的java.util.Arrays的源码,joshua bloch说的这个问题已经解决,现在的binary search如下:
// Like public version, but without range checks.
private static int binarySearch0(int[] a, int fromIndex, int toIndex,
int key) {
int low = fromIndex;
int high = toIndex - 1;
while (low = high) {
int mid = (low + high) 1;
int midVal = a[mid];
if (midVal key)
low = mid + 1;
else if (midVal key)
high = mid - 1;
else
return mid; // key found
}
return -(low + 1); // key not found.
}
Constrained Output Embeddings for End-to-End Code-Switching Speech Recognition with Only Monolingual Data 立即下载
int mid = ((unsigned) (low + high)) 1。 但是在c99标准中,对于两个signed数值之和,如果溢出结果是未定义的(undefined),也就是说与编译器实现相关。上面的代码应该修改为:
mid = ((unsigned int)low + (unsigned int)high)) 1; 我查了下jdk6的java.util.Arrays的源码,joshua bloch说的这个问题已经解决,现在的binary search如下:
// Like public version, but without range checks.
private static int binarySearch0(int[] a, int fromIndex, int toIndex,
int key) {
int low = fromIndex;
int high = toIndex - 1;
while (low = high) {
int mid = (low + high) 1;
int midVal = a[mid];
if (midVal key)
low = mid + 1;
else if (midVal key)
high = mid - 1;
else
return mid; // key found
}
return -(low + 1); // key not found.
}
如原文所讨论的,采用了 操作符替代除以2
文章转自庄周梦蝶 ,原文发布时间2008-04-02
Constrained Output Embeddings for End-to-End Code-Switching Speech Recognition with Only Monolingual Data 立即下载