How do I scale down numbers from rand()?

Question

The following code outputs a random number each second:

int main ()
{
    srand(time(NULL)); // Seeds number generator with execution time.

    while (true)         


        

Answer 1

All the examples posted so far actually give badly distributed results. Execute the code often and create a statistic to see how the values become skewed.

A better way to generate a real uniform random number distribution in any range [0, N] is the following (assuming that rand actually follows a uniform distribution, which is far from obvious):

unsigned result;
do {
    result = rand();
} while (result > N);

Of course, that method is slow but it does produce a good distribution. A slightly smarter way of doing this is to find the largest multiple of N that is smaller than RAND_MAX and using that as the upper bound. After that, one can safely take the result % (N + 1).

For an explanation why the naive modulus method is bad and why the above is better, refer to Julienne’s excellent article on using rand.