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<title>The GNU Fortran Compiler: RANDOM_SEED</title>

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<a name="RANDOM_005fSEED"></a>
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<a name="RANDOM_005fSEED-_002d_002d_002d-Initialize-a-pseudo_002drandom-number-sequence"></a>
<h3 class="section">9.223 <code>RANDOM_SEED</code> &mdash; Initialize a pseudo-random number sequence</h3>
<a name="index-RANDOM_005fSEED"></a>
<a name="index-random-number-generation_002c-seeding"></a>
<a name="index-seeding-a-random-number-generator"></a>

<dl compact="compact">
<dt><em>Description</em>:</dt>
<dd><p>Restarts or queries the state of the pseudorandom number generator used by 
<code>RANDOM_NUMBER</code>.
</p>
<p>If <code>RANDOM_SEED</code> is called without arguments, it is seeded with
random data retrieved from the operating system.
</p>
<p>As an extension to the Fortran standard, the GFortran
<code>RANDOM_NUMBER</code> supports multiple threads. Each thread in a
multi-threaded program has its own seed.  When <code>RANDOM_SEED</code> is
called either without arguments or with the <var>PUT</var> argument, the
given seed is copied into a master seed as well as the seed of the
current thread. When a new thread uses <code>RANDOM_NUMBER</code> for the
first time, the seed is copied from the master seed, and forwarded
<em>N * 2^{512}</em> steps to guarantee that the random stream does not
alias any other stream in the system, where <var>N</var> is the number of
threads that have used <code>RANDOM_NUMBER</code> so far during the program
execution.
</p>
</dd>
<dt><em>Standard</em>:</dt>
<dd><p>Fortran 95 and later
</p>
</dd>
<dt><em>Class</em>:</dt>
<dd><p>Subroutine
</p>
</dd>
<dt><em>Syntax</em>:</dt>
<dd><p><code>CALL RANDOM_SEED([SIZE, PUT, GET])</code>
</p>
</dd>
<dt><em>Arguments</em>:</dt>
<dd><table>
<tr><td width="15%"><var>SIZE</var></td><td width="70%">(Optional) Shall be a scalar and of type default 
<code>INTEGER</code>, with <code>INTENT(OUT)</code>. It specifies the minimum size 
of the arrays used with the <var>PUT</var> and <var>GET</var> arguments.</td></tr>
<tr><td width="15%"><var>PUT</var></td><td width="70%">(Optional) Shall be an array of type default 
<code>INTEGER</code> and rank one. It is <code>INTENT(IN)</code> and the size of 
the array must be larger than or equal to the number returned by the 
<var>SIZE</var> argument.</td></tr>
<tr><td width="15%"><var>GET</var></td><td width="70%">(Optional) Shall be an array of type default 
<code>INTEGER</code> and rank one. It is <code>INTENT(OUT)</code> and the size 
of the array must be larger than or equal to the number returned by 
the <var>SIZE</var> argument.</td></tr>
</table>

</dd>
<dt><em>Example</em>:</dt>
<dd><div class="smallexample">
<pre class="smallexample">program test_random_seed
  implicit none
  integer, allocatable :: seed(:)
  integer :: n

  call random_seed(size = n)
  allocate(seed(n))
  call random_seed(get=seed)
  write (*, *) seed
end program test_random_seed
</pre></div>

</dd>
<dt><em>See also</em>:</dt>
<dd><p><a href="RANDOM_005fNUMBER.html#RANDOM_005fNUMBER">RANDOM_NUMBER</a>
</p></dd>
</dl>


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