如何从 C++ 容器中获取随机元素?

2022-01-07 00:00:00 algorithm c++ stl

从 STL 范围中获取 [伪] 随机元素的好方法是什么?

What is a good way to get a [pseudo-]random element from an STL range?

我能想到的最好方法是做 std::random_shuffle(c.begin(), c.end()) 然后从 c.begin 中取出我的随机元素().

The best I can come up with is to do std::random_shuffle(c.begin(), c.end()) and then take my random element from c.begin().

但是，我可能想要一个来自 const 容器的随机元素，或者我可能不想要完全 shuffle 的成本.

However, I might want a random element from a const container, or I might not want the cost of a full shuffle.

有更好的方法吗?

推荐答案

我在 Google+ 的一篇文章中发布了这个解决方案，其他人引用了这个.把它贴在这里，因为这个比其他的稍微好一点，因为它通过使用 std::uniform_int_distribution 避免了偏见:

I posted this solution on a Google+ article where someone else referenced this. Posting it here, as this one is slightly better than others because it avoids bias by using std::uniform_int_distribution:

#include  <random>
#include  <iterator>

template<typename Iter, typename RandomGenerator>
Iter select_randomly(Iter start, Iter end, RandomGenerator& g) {
    std::uniform_int_distribution<> dis(0, std::distance(start, end) - 1);
    std::advance(start, dis(g));
    return start;
}

template<typename Iter>
Iter select_randomly(Iter start, Iter end) {
    static std::random_device rd;
    static std::mt19937 gen(rd());
    return select_randomly(start, end, gen);
}

示例用途是:

#include <vector>
using namespace std;

vector<int> foo;
/* .... */
int r = *select_randomly(foo.begin(), foo.end());

我最终创建了一个遵循类似方法并具有更好设计的要点.

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