generating a random number within range 0 to n where n can be > RAND_MAX

Question

How can I generate a random number within range 0 to n where n can be > RAND_MAX in c,c++?

Thanks.

Answer 1

Do x random numbers (from 0 to RAND_MAX) and add them together, where

x = n % RAND_MAX

Answer 2

Random numbers is a very specialized subject that unless you are a maths junky is very easy to get wrong. So I would advice against building a random number from multiple sources you should use a good library.

I would first look at boost::Random

If that is not suffecient try of this group sci.crypt.random-numbers Ask the question there they should be able to help.

Answer 3

suppose you want to generate a 64-bit random number, you could do this:

uint64_t n = 0;
for(int i = 0; i < 8; ++i) {
    uint64_t x = generate_8bit_random_num();
    n = (n << (8 * i)) | x;
}

Of course you could do it 16/32 bits at a time too, but this illustrates the concept.

How you generate that 8/16/32-bit random numbers is up to you. It could be as simple as rand() & 0xff or something better depending on how much you care about the randomness.