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<title>GNU Scientific Library – Reference Manual: Random number environment variables</title>
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<a name="Random-number-environment-variables"></a>
<div class="header">
<p>
Next: <a href="Copying-random-number-generator-state.html#Copying-random-number-generator-state" accesskey="n" rel="next">Copying random number generator state</a>, Previous: <a href="Auxiliary-random-number-generator-functions.html#Auxiliary-random-number-generator-functions" accesskey="p" rel="previous">Auxiliary random number generator functions</a>, Up: <a href="Random-Number-Generation.html#Random-Number-Generation" accesskey="u" rel="up">Random Number Generation</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Random-number-environment-variables-1"></a>
<h3 class="section">18.6 Random number environment variables</h3>
<p>The library allows you to choose a default generator and seed from the
environment variables <code>GSL_RNG_TYPE</code> and <code>GSL_RNG_SEED</code> and
the function <code>gsl_rng_env_setup</code>. This makes it easy try out
different generators and seeds without having to recompile your program.
</p>
<dl>
<dt><a name="index-gsl_005frng_005fenv_005fsetup"></a>Function: <em>const gsl_rng_type *</em> <strong>gsl_rng_env_setup</strong> <em>(void)</em></dt>
<dd><a name="index-GSL_005fRNG_005fTYPE"></a>
<a name="index-GSL_005fRNG_005fSEED-2"></a>
<a name="index-gsl_005frng_005fdefault"></a>
<a name="index-gsl_005frng_005fdefault_005fseed-1"></a>
<p>This function reads the environment variables <code>GSL_RNG_TYPE</code> and
<code>GSL_RNG_SEED</code> and uses their values to set the corresponding
library variables <code>gsl_rng_default</code> and
<code>gsl_rng_default_seed</code>. These global variables are defined as
follows,
</p>
<div class="example">
<pre class="example">extern const gsl_rng_type *gsl_rng_default
extern unsigned long int gsl_rng_default_seed
</pre></div>
<p>The environment variable <code>GSL_RNG_TYPE</code> should be the name of a
generator, such as <code>taus</code> or <code>mt19937</code>. The environment
variable <code>GSL_RNG_SEED</code> should contain the desired seed value. It
is converted to an <code>unsigned long int</code> using the C library function
<code>strtoul</code>.
</p>
<p>If you don’t specify a generator for <code>GSL_RNG_TYPE</code> then
<code>gsl_rng_mt19937</code> is used as the default. The initial value of
<code>gsl_rng_default_seed</code> is zero.
</p>
</dd></dl>
<p>Here is a short program which shows how to create a global
generator using the environment variables <code>GSL_RNG_TYPE</code> and
<code>GSL_RNG_SEED</code>,
</p>
<div class="example">
<pre class="verbatim">#include <stdio.h>
#include <gsl/gsl_rng.h>
gsl_rng * r; /* global generator */
int
main (void)
{
const gsl_rng_type * T;
gsl_rng_env_setup();
T = gsl_rng_default;
r = gsl_rng_alloc (T);
printf ("generator type: %s\n", gsl_rng_name (r));
printf ("seed = %lu\n", gsl_rng_default_seed);
printf ("first value = %lu\n", gsl_rng_get (r));
gsl_rng_free (r);
return 0;
}
</pre></div>
<p>Running the program without any environment variables uses the initial
defaults, an <code>mt19937</code> generator with a seed of 0,
</p>
<div class="example">
<pre class="example">$ ./a.out
</pre><pre class="verbatim">generator type: mt19937
seed = 0
first value = 4293858116
</pre></div>
<p>By setting the two variables on the command line we can
change the default generator and the seed,
</p>
<div class="example">
<pre class="example">$ GSL_RNG_TYPE="taus" GSL_RNG_SEED=123 ./a.out
GSL_RNG_TYPE=taus
GSL_RNG_SEED=123
generator type: taus
seed = 123
first value = 2720986350
</pre></div>
<hr>
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<p>
Next: <a href="Copying-random-number-generator-state.html#Copying-random-number-generator-state" accesskey="n" rel="next">Copying random number generator state</a>, Previous: <a href="Auxiliary-random-number-generator-functions.html#Auxiliary-random-number-generator-functions" accesskey="p" rel="previous">Auxiliary random number generator functions</a>, Up: <a href="Random-Number-Generation.html#Random-Number-Generation" accesskey="u" rel="up">Random Number Generation</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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