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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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<p>
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Next: <a href="RANGE.html#RANGE" accesskey="n" rel="next">RANGE</a>, Previous: <a href="RANDOM_005fNUMBER.html#RANDOM_005fNUMBER" accesskey="p" rel="prev">RANDOM_NUMBER</a>, Up: <a href="Intrinsic-Procedures.html#Intrinsic-Procedures" accesskey="u" rel="up">Intrinsic Procedures</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="RANDOM_005fSEED-_002d_002d_002d-Initialize-a-pseudo_002drandom-number-sequence"></a>
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<h3 class="section">9.223 <code>RANDOM_SEED</code> — Initialize a pseudo-random number sequence</h3>
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<a name="index-RANDOM_005fSEED"></a>
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<a name="index-random-number-generation_002c-seeding"></a>
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<a name="index-seeding-a-random-number-generator"></a>
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<dl compact="compact">
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<dt><em>Description</em>:</dt>
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<dd><p>Restarts or queries the state of the pseudorandom number generator used by
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<code>RANDOM_NUMBER</code>.
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</p>
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<p>If <code>RANDOM_SEED</code> is called without arguments, it is seeded with
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random data retrieved from the operating system.
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</p>
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<p>As an extension to the Fortran standard, the GFortran
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<code>RANDOM_NUMBER</code> supports multiple threads. Each thread in a
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multi-threaded program has its own seed. When <code>RANDOM_SEED</code> is
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called either without arguments or with the <var>PUT</var> argument, the
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given seed is copied into a master seed as well as the seed of the
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current thread. When a new thread uses <code>RANDOM_NUMBER</code> for the
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first time, the seed is copied from the master seed, and forwarded
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<em>N * 2^{512}</em> steps to guarantee that the random stream does not
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alias any other stream in the system, where <var>N</var> is the number of
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threads that have used <code>RANDOM_NUMBER</code> so far during the program
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execution.
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</p>
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</dd>
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<dt><em>Standard</em>:</dt>
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<dd><p>Fortran 95 and later
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</p>
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</dd>
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<dt><em>Class</em>:</dt>
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<dd><p>Subroutine
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</p>
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</dd>
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<dt><em>Syntax</em>:</dt>
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<dd><p><code>CALL RANDOM_SEED([SIZE, PUT, GET])</code>
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</p>
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</dd>
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<dt><em>Arguments</em>:</dt>
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<dd><table>
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<tr><td width="15%"><var>SIZE</var></td><td width="70%">(Optional) Shall be a scalar and of type default
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<code>INTEGER</code>, with <code>INTENT(OUT)</code>. It specifies the minimum size
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of the arrays used with the <var>PUT</var> and <var>GET</var> arguments.</td></tr>
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<tr><td width="15%"><var>PUT</var></td><td width="70%">(Optional) Shall be an array of type default
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<code>INTEGER</code> and rank one. It is <code>INTENT(IN)</code> and the size of
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the array must be larger than or equal to the number returned by the
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<var>SIZE</var> argument.</td></tr>
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<tr><td width="15%"><var>GET</var></td><td width="70%">(Optional) Shall be an array of type default
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<code>INTEGER</code> and rank one. It is <code>INTENT(OUT)</code> and the size
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of the array must be larger than or equal to the number returned by
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the <var>SIZE</var> argument.</td></tr>
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</table>
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</dd>
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<dt><em>Example</em>:</dt>
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<dd><div class="smallexample">
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<pre class="smallexample">program test_random_seed
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implicit none
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integer, allocatable :: seed(:)
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integer :: n
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call random_seed(size = n)
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allocate(seed(n))
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call random_seed(get=seed)
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write (*, *) seed
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end program test_random_seed
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</pre></div>
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</dd>
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<dt><em>See also</em>:</dt>
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<dd><p><a href="RANDOM_005fNUMBER.html#RANDOM_005fNUMBER">RANDOM_NUMBER</a>
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</p></dd>
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</dl>
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Next: <a href="RANGE.html#RANGE" accesskey="n" rel="next">RANGE</a>, Previous: <a href="RANDOM_005fNUMBER.html#RANDOM_005fNUMBER" accesskey="p" rel="prev">RANDOM_NUMBER</a>, Up: <a href="Intrinsic-Procedures.html#Intrinsic-Procedures" accesskey="u" rel="up">Intrinsic Procedures</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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