[英]Why is sorting not taking O(n log (n)) in time
在下面的代码片段中, std::sort
的时间消耗。 这应该花费O(nlog(n))时间。 std::chrono
仅用于测量std::sort
。
我用英特尔编译器18.0.3编译了以下代码,优化级别为-O3
。 我使用Redhat6。
#include <vector>
#include <random>
#include <limits>
#include <iostream>
#include <chrono>
#include <algorithm>
int main() {
std::random_device dev;
std::mt19937 rng(dev());
std::uniform_int_distribution<std::mt19937::result_type> dist(std::numeric_limits<int>::min(),
std::numeric_limits<int>::max());
int ret = 0;
const unsigned int max = std::numeric_limits<unsigned int>::max();
for (auto j = 1u; j < max; j *= 10) {
std::vector<int> vec;
vec.reserve(j);
for (int i = 0; i < j; ++i) {
vec.push_back(dist(rng));
}
auto t_start = std::chrono::system_clock::now();
std::sort(vec.begin(), vec.end());
const auto t_end = std::chrono::system_clock::now();
const auto duration = std::chrono::duration_cast<std::chrono::duration<double>>(t_end - t_start).count();
std::cout << "Time measurement: j= " << j << " took " << duration << " seconds.\n";
ret + vec[0];
}
return ret;
}
这个程序的输出是
Time measurement: j= 1 took 1.236e-06 seconds.
Time measurement: j= 10 took 5.583e-06 seconds.
Time measurement: j= 100 took 1.0145e-05 seconds.
Time measurement: j= 1000 took 0.000110649 seconds.
Time measurement: j= 10000 took 0.00142651 seconds.
Time measurement: j= 100000 took 0.00834339 seconds.
Time measurement: j= 1000000 took 0.098939 seconds.
Time measurement: j= 10000000 took 0.938253 seconds.
Time measurement: j= 100000000 took 10.2398 seconds.
Time measurement: j= 1000000000 took 114.214 seconds.
Time measurement: j= 1410065408 took 163.824 seconds.
这似乎非常接近线性行为。
为什么std::sort
需要O(n)
而不是O(nlog(n))
?
您提供的图表非常适合y = x log (x)
。 与x
相比, log(x)
的影响很小。 我推测你的结果将通过x log (x)
的卡方具有重要意义。
这里没有惊喜。
这是一个试金石,供您欣赏,O(n log n)并不比O(n)差。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.