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Unique random numbers in O(1)?
Unique random numbers in an integer array in the C programming language

I have a std::vector of unique elements of some undetermined size. I want to fetch 20 unique and random elements from this vector. By ‘unique’ I mean that I do not want to fetch the same index more than once. Currently the way I do this is to call std::random_shuffle. But this requires me to shuffle the entire vector (which may contain over 1000 elements). I don’t mind mutating the vector (I prefer not to though, as I won’t need to use thread locks), but most important is that I want this to be efficient. I shouldn’t be shuffling more than I need to.

Note that I’ve looked into passing in a partial range to std::random_shuffle but it will only ever shuffle that subset of elements, which would mean that the elements outside of that range never get used!

Help is appreciated. Thank you!

Note: I’m using Visual Studio 2005, so I do not have access to C++11 features and libraries.

Answer 1

Use a loop to put random index numbers into a std::set and stop when the size() reaches 20.

std::set<int> indexes;
std::vector<my_vector::value_type> choices;
int max_index = my_vector.size();
while (indexes.size() < min(20, max_index))
{
    int random_index = rand() % max_index;
    if (indexes.find(random_index) == indexes.end())
    {
        choices.push_back(my_vector[random_index]);
        indexes.insert(random_index);
    }
}

The random number generation is the first thing that popped into my head, feel free to use something better.