I need to generate a random number between 1 and n where n is unsigned int. If n were int I would simply write 1 + rand()% n. But unfortunately n is unsigned int. What do you suggest?
1 Answer
rand() should be avoided whenever possible*. Use http://en.cppreference.com/w/cpp/numeric/random
#include <random> #include <iostream> int main() { std::random_device rd; std::mt19937 engine(rd()); std::uniform_int_distribution<unsigned> dist(1, 77); for (int i = 0; i != 5; ++i) std::cout << dist(engine) << '\n'; } * Because it shares a global state, gets often implemented as a linear congruential engine which has a few drawbacks and it's range is often only 0-2^16. Also, % n where n is not an exact multiple of the range does not produce an uniform distribution.
Edit: This might seem like overkill, but technically one would want something like this, since mt19937 needs a bit of "warm up":
std::mt19937 create_seeded_rng() { std::random_device rd; std::array<std::mt19937::result_type, std::mt19937::state_size> seed_data; std::generate(seed_data.begin(), seed_data.end(), std::ref(rd)); std::seed_seq seq(seed_data.begin(), seed_data.end()); return std::mt19937(seq); } int main() { std::mt19937 rng = create_seeded_rng(); std::uniform_int_distribution<int> dist(0, 100); for (unsigned i = 0; i != 100; ++i) std::cout << dist(rng) << '\n'; } 
4 Comments
Igor
Thank you for your answer. May I ask you once again? My initial problem was to write a function which filled in an array with random numbers in random order. So what I did is
Igor
#include <iostream> #include <random> using namespace std; void GenRandArray(int* _result, unsigned int _maxValue) { int i; random_device rd; mt19937 gen(rd()); uniform_int_distribution<> dis1(0, _maxValue - 1); uniform_int_distribution<> dis2(1, _maxValue); for(i = 0; i < _maxValue; i++) _result[ dis1(gen) ] = dis2(gen); } void main() { unsigned int n = 5; int* ar = new int [n]; GenRandArray(ar, n); }
cooky451
@Igor Why would you want to fill it in random order? Is that really a requirement, because it makes not sense to me.
Sebastian Redl
nis an unsigned int ? In C++11, there are much better ways to generate random numbers.1 + rand() % n.