Possible Duplicate:
Generate Random numbers uniformly over entire range
C++ random float
How can I generate a random number between 5 and 25 in c++ ?
#include <iostream> #include <cstdlib> #include <time.h> using namespace std; void main() { int number; int randomNum; srand(time(NULL)); randomNum = rand(); } Do rand() % 20 and increment it by 5.
rand() is a pseudo random number generator, not a true random number generator so there's a bias right there in the first place. Secondly, you're taking a modulo of a number which has a upper bound that is not a factor of 20 ( look up MAX_RAND ). This can't make the output uniformly distributed. I know I should have talked about this in my answer, but looking at the quality of the question, i assumed that he didn't really need the true RNG.In C++11:
#include <random> std::default_random_engine re; re.seed(time(NULL)); // or whatever seed std::uniform_int_distribution<int> uni(5, 25); // 5-25 *inclusive* int randomNum = uni(re); Or it could just as well be:
std::uniform_int_distribution<int> d5(1, 5); // 1-5 inclusive int randomNum = d5(re) + d5(re) + d5(re) + d5(re) + d5(re); which would give a different distribution on the same range.
The C++ way:
#include <random> typedef std::mt19937 rng_type; // pick your favourite (i.e. this one) std::uniform_int_distribution<rng_type::result_type> udist(5, 25); rng_type rng; int main() { // seed rng first! rng_type::result_type random_number = udist(rng); } 
#include <cstdlib> #include <time.h> using namespace std; void main() { int number; int randomNum; srand(time(NULL)); number = rand() % 20; cout << (number) << endl; } 