[Help-gsl] accessing random number generator from functions

[Nathan Moore]

This is a general question about the scope of random number generators. 
 If I declare and initialize a random number generator (named "dice") 
within the main function, how can I get numbers from "dice" if I'm in a 
sub-function?  Do I need to pass "dice" somehow?

Specific example follows.  Would appreciate any constructive comments.

regards,

Nathan Moore
University of Minnesota


eg, if I have the main function,

- - - - -
#define RANDOM_SEED 53

int main (void)
{
        /* allocate an instance of the random number generator */
        gsl_rng * dice = gsl_rng_alloc (gsl_rng_mt19937);

        /* seed the generator */
        gsl_rng_set( dice, (long)RANDOM_SEED );

        /* generate a few doubles with the generator */
        int i,i_limit=1000000;
        double ave_x=0.0;
        for(i=0;i<i_limit;i++) {
                x = gsl_rng_uniform(dice);
                ave_x+=x;
        }
        printf("average over %ld numbers was 
%lf\n",i_limit,ave_x/(double)i_limit);

        /* free up memory that the generator uses */
        gsl_rng_free (dice);

        return 0;
}
 - - - - -

How should the following code be phrased to use dice, but calculate the 
average within a function?  (the code below DOESN'T work)

 - - - -


#define RANDOM_SEED 53

double calculate_average( long num_terms);
double calculate_average( long num_terms)
{
        int i;
                double ave_x=0.0;
                for( i=0; i<num_terms; i++) {
                x = gsl_rng_uniform(dice);
                ave_x += x;
                }
        return ave_x/(double)num_terms;
}

int main (void)
{
                /* allocate an instance of the random number generator */
                gsl_rng * dice = gsl_rng_alloc (gsl_rng_mt19937);

                /* seed the generator */
                gsl_rng_set( dice, (long)RANDOM_SEED );

                /* generate a few doubles with the generator */
        double ave_x;
        long num_terms=1000000;
        ave_x = calculate_average(num_terms);
        printf("average over %ld numbers was %lf\n",num_terms,ave_x);

                /* free up memory that the generator uses */
                gsl_rng_free (dice);

                return 0;
}