[Help-gsl] Uniformly distributed int in [0,n-1]

[Joseph Wakeling]

Hello all,

As I mentioned in earlier emails I've been playing around a bit with the 
random number generators in the GSL.

I'm wondering if someone can advise on appropriate ways to generate 
uniformly-distributed ints in the range [0,n-1].  My traditional way has 
always been to take a function ranf() which returns a floating-point 
number in [0,1), and do,

x = (int) (n*ranf());

But, if we suppose I have a function like the rand() function in C that 
returns an int between 0 and RAND_MAX - 1, would it be appropriate to do,

x = rand()%n;

Or would this unduly affect the uniform distribution?

I note that the gsl_rng_uniform_int function uses a method like this 
(simplifying slightly...;-):

do { x = rand()/(RAND_MAX/n); } while( x >= n);

Could someone advise?  If they are equivalent, what about the speed of 
these alternatives?

Many thanks,

      -- Joe