[Bug-gsl] gsl_ran_flat: bug or improper usage ?

[Brad Bell]

I seem to be getting some strange results when I use gsl_ran_flat on some 
systems (and not on other systems). The attached bash script extracts a c 
program, compiles it and runs it twice: once using gsl_rng_uniform and then 
using gsl_ran_flat. In the case where gsl_ran_flat is used, the value gamma, on 
some systems, comes out as nan, and on other systems comes out as 1 (as it 
should be).

Here are the results I get with Fedora:

> gcc -g gsl_ran_flat.c -L/usr/lib -lgsl -lgslcblas -lm -o gsl_ran_flat
> ./gsl_ran_flat uniform
gamma = 1.000000
> ./gsl_ran_flat flat
gamma = nan
> gsl-config --version
1.7
> uname -sr
Linux 2.6.17-1.2174_FC5
> rpm -q gsl
gsl-1.7-1.2.1
> cat /etc/redhat-release
Fedora Core release 5 (Bordeaux)

Here are the results I get with Cygwin

> gcc -g gsl_ran_flat.c -L/usr/lib -lgsl -lgslcblas -lm -o gsl_ran_flat
Info: resolving _gsl_rng_default by linking to __imp__gsl_rng_default 
(auto-import)
> ./gsl_ran_flat uniform
gamma = 1.000000
> ./gsl_ran_flat flat
gamma = 1.000000
> gsl-config --version
1.7
> uname -sr
CYGWIN_NT-5.0 1.5.19(0.150/4/2)

I am including a copy of the c program below for your inspection:

# include <stdio.h>
# include <gsl/gsl_rng.h>
# include <string.h>

int main(int argc, char *argv[])
{       size_t i;
        int uniform;
        int flat;
        double gamma;
        double w[] = {1., 2.};

        uniform = strcmp(argv[1], "uniform") == 0;
        flat    = strcmp(argv[1], "flat")    == 0;

        // initialize random number generator
        const gsl_rng_type *T;
        gsl_rng_env_setup();
        T = gsl_rng_default;
        gsl_rng *rng = gsl_rng_alloc(T);

        for(i = 1; i < 7; i++)
        {
                if( uniform )
                        gsl_rng_uniform(rng);
                if( flat )
                        gsl_ran_flat(rng, 0., 1.);
        }

        gamma    = w[0] + 0. * w[1];
        printf("gamma = %f\n", gamma);

        gsl_rng_free(rng);
        return 0;
}

gsl_ran_flat.sh
Description: Bourne shell script