Random number generator, C++

Question

I know there is a bit of limitations for a random number generation in C++ (can be non-uniform). How can I generate a number from 1 to 14620?

Thank you.

Answer 1

A common approach is to use std::rand() with a modulo:

#include
#include

// ...
std::srand(std::time(0));  // needed once per program run
int r = std::rand() % 14620 + 1;

However, as @tenfour mentions in his answer, the modulo operator can disrupt the uniformity of values std::rand() returns. This is because the modulo translates the values it discards into valid values, and this translation might not be uniform. For instance, for n in [0, 10) the value n % 9 translates 9 to 0, so you can get zero by either a true zero or a 9 translated to zero. The other values have each only one chance to yield.

An alternative approach is to translate the random number from std::rand() to a floating-point value in the range [0, 1) and then translate and shift the value to within the range you desire.

int r = static_cast(std::rand()) / RAND_MAX * 14620) + 1;