random - Why is (rand() % anything) always 0 in C++?

Question

I am having trouble getting rand() to work in C++. rand() normally gives me a very large number. When I try to use the modulo operator (%) to give it a range, it always returns 0, no matter what.

Seeding the random number generator at the beginning of the program doesn't help either.

Answer 1

I think this is common if the random generator algorithm leaves a certain pattern of bits as zero. (For example, if the low-order bits are zero, the number mod some low constant will always be zero.)

Maybe you should try something like:

const int desired_maximum = /* ... */;

int r = (((double)rand()) / RAND_MAX) * desired_maximum;

For example in this manpage link, it says:

In Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press, 1992 (2nd ed., p. 277)), the following comments are made:

"If you want to generate a random integer between 1 and 10, you should always do it by using high-order bits, as in

j = 1 + (int) (10.0 * (rand() / (RAND_MAX + 1.0)));

and never by anything resembling

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

(which uses lower-order bits)."