I’m looking for an algorithm to pick M random elements from a given array. The prerequisites are:

• the sampled elements must be unique,
• the array to sample from may contain duplicates.

This is what I’ve managed to come up with. Here I’m also making an assumption that the amount of unique elements in the array is greater (or equal) than M.

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


const std::vector<int> sample(const std::vector<int>& input, size_t n) {
    std::random_device rd;
    std::mt19937 engine(rd());
    std::uniform_int_distribution<int> dist(0, input.size() - 1);
    
    std::vector<int> result;
    result.reserve(n);

    size_t id;
    do {
        id = dist(engine);
        if (std::find(result.begin(), result.end(), input[id]) == result.end())
            result.push_back(input[id]);
    } while (result.size() < n);

    return result;
}


int main() {
    std::vector<int> input{0, 0, 1, 1, 2, 2, 3, 3, 4, 4};

    std::vector<int> result = sample(input, 3);

    for (const auto& item : result)
        std::cout << item << ' ';
    std::cout << std::endl;
}

This algorithm does not seem to be the best. Is there a more efficient algorithm to solve this task? It would be good if this algorithm could also assert the amount of unique elements in the input array is not less than M.