How to get a random element from a C++ container?

Question

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

The best I can come up with is to do std::random_shuffle(c.begin(), c.end()) an

Answer 1

C++17 std::sample

This is a convenient method to get several random elements without repetition.

main.cpp

#include <algorithm>
#include <iostream>
#include <random>
#include <vector>

int main() {
    const std::vector<int> in{1, 2, 3, 5, 7};
    std::vector<int> out;
    size_t nelems = 3;
    std::sample(
        in.begin(),
        in.end(),
        std::back_inserter(out),
        nelems,
        std::mt19937{std::random_device{}()}
    );
    for (auto i : out)
        std::cout << i << std::endl;
}

Compile and run:

g++-7 -o main -std=c++17 -Wall -Wextra -pedantic main.cpp
./main

Output: 3 random numbers are picked from 1, 2, 3, 5, 7 without repetition.

For efficiency, only O(n) is guaranteed since ForwardIterator is the used API, but I think stdlib implementations will specialize to O(1) where possible (e.g. vector).

Tested in GCC 7.2, Ubuntu 17.10. How to obtain GCC 7 in 16.04.

Answer 2

You can use 0~1 random function to generate a float number for every element in the container as its score. And then select the one with highest score.