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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">9.216 <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 initialized
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to a default state. The example below shows how to initialize the
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random seed with a varying seed in order to ensure a different random
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number sequence for each invocation of the program. Note that setting
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any of the seed values to zero should be avoided as it can result in
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poor quality random numbers being generated.
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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">subroutine init_random_seed()
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use iso_fortran_env, only: int64
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implicit none
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integer, allocatable :: seed(:)
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integer :: i, n, un, istat, dt(8), pid
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integer(int64) :: t
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call random_seed(size = n)
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allocate(seed(n))
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! First try if the OS provides a random number generator
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open(newunit=un, file="/dev/urandom", access="stream", &
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form="unformatted", action="read", status="old", iostat=istat)
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if (istat == 0) then
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read(un) seed
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close(un)
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else
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! Fallback to XOR:ing the current time and pid. The PID is
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! useful in case one launches multiple instances of the same
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! program in parallel.
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call system_clock(t)
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if (t == 0) then
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call date_and_time(values=dt)
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t = (dt(1) - 1970) * 365_int64 * 24 * 60 * 60 * 1000 &
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+ dt(2) * 31_int64 * 24 * 60 * 60 * 1000 &
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+ dt(3) * 24_int64 * 60 * 60 * 1000 &
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+ dt(5) * 60 * 60 * 1000 &
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+ dt(6) * 60 * 1000 + dt(7) * 1000 &
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+ dt(8)
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end if
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pid = getpid()
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t = ieor(t, int(pid, kind(t)))
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do i = 1, n
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seed(i) = lcg(t)
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end do
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end if
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call random_seed(put=seed)
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contains
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! This simple PRNG might not be good enough for real work, but is
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! sufficient for seeding a better PRNG.
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function lcg(s)
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integer :: lcg
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integer(int64) :: s
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if (s == 0) then
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s = 104729
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else
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s = mod(s, 4294967296_int64)
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end if
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s = mod(s * 279470273_int64, 4294967291_int64)
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lcg = int(mod(s, int(huge(0), int64)), kind(0))
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end function lcg
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end subroutine init_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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