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

There are many ways to do this.

If you are OK with less granularity (higher chance of dupes), then something like (in pseudocode) rand() * n / RAND_MAX will work to spread the values across a larger range. The catch is that in your real code you'll need to avoid overflow, either via casting rand() or n to a large-enough type (e.g. 64-bit int if RAND_MAX is 0xFFFFFFFF) to hold the multiplication result without overflow, or use a multiply-then-divide API (like GNU's MulDiv64 or Win32's MulDiv) which is optimized for this scenario.

If you want granuarity down to each integer, you can call rand() multiple times and append the results. Another answer suggests calling rand() for each 8-bit/16-bit/32-bit chunk depending on size of RAND_MAX.

But, IMHO, the above ideas can rapidly get complicated, inaccurate, or both. Generating random numbers is a solved problem in other libraries, and it's probably much easier to borrow existing code (e.g. from Boost) than try to roll your own. Open source random number generation algorithm in C++? has answers with more links if you want something besides Boost.

[ EDIT: revising after having a busy day... meant to get back and clean up my quick answer this morning, but got pulled away and only getting back now. :-) ]