C++ random number generation

Question

For example, the following expression:

r = (rand() % 10)+1;

Generates a random number from 1-10.

How can we make it generate random numbers from 0-10?

Thanks.

Answer 1

You're almost there! The rand() function returns a random value in a pretty large range (0 to RAND_MAX). The use of the modulus operator wraps this down into a smaller range (from 0 to 9, since you're modding by 10) and then the +1 moves this to be from 1 to 10.

To get values between 0 and 10, you can take rand and mod its value by 11:

r = rand() % 11;

More generally, to get random values in the range [0, n], you can write

r = rand() % (n + 1);

And finally, to get values in the range [k, n + k], you can write

r = rand() % (n + 1) + k;

Of course, as purists will point out, this isn't necessarily going to give you truly uniform values because modding rand() by some value will not distribute all the integers evenly. This usually isn't a problem (you'll be off by a very, very small amount), but if it is you may want to consider looking into a more robust random number generator than rand().