I've heard it mentioned on occasion that using the modulo operator on a random number (e.g. rand() % 17) is "not good" because it somehow reduces the randomness. I think it was something about dropping the upper bits, but I don't really know anything. Can anybody clarify and elaborate?

x % (2^y) = x & (2^y) (binary and)

The reson why Rnd % number is considered bad, is because:
IIRC It has a different probablility of being zero, or number then of being something else.

Also, if number and rnd aren't coprime, then gcd((rnd % number), number) will not be 1. IE. they weill hhare at least one factor.

Thats about as much as i know about this.
From,
Nice coder

[Edited by - Nice Coder on December 4, 2004 1:52:06 AM]

Here is an extreme case which demonstrates the problem (which in practice is generally a very small problem).

Suppose MyRand() returns the values 0-9 (inclusive).

Now, you use MyRand to generate random numbers between 0 and 3 (inclusive) and you do it like this:

    v = MyRand() % 4; 

Here are the values generated by MyRand() (and their probabilities), and how they map to the final value v:

MyRand():     0   1   2   3   4   5   6   7   8   9probability: 10% 10% 10% 10% 10% 10% 10% 10% 10% 10%MyRand()%3:   0   1   2   3   0   1   2   3   0   1 

You can see that v has the values 0 and 1 more often than 2 or 3 (50% more often!).

v:            0   1   2   3probability: 30% 30% 20% 20%