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 <a name="RANDOM_005fSEED"></a>
 <div class="header">
 <p>
 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> &nbsp; [<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>
 </div>
 <hr>
 <a name="RANDOM_005fSEED-_002d_002d_002d-Initialize-a-pseudo_002drandom-number-sequence"></a>
 <h3 class="section">8.201 <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 initialized
 to a default state. The example below shows how to initialize the
 random seed with a varying seed in order to ensure a different random
 number sequence for each invocation of the program. Note that setting
 any of the seed values to zero should be avoided as it can result in
 poor quality random numbers being generated.
 </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">subroutine init_random_seed()
   use iso_fortran_env, only: int64
   implicit none
   integer, allocatable :: seed(:)
   integer :: i, n, un, istat, dt(8), pid
   integer(int64) :: t

   call random_seed(size = n)
   allocate(seed(n))
   ! First try if the OS provides a random number generator
   open(newunit=un, file=&quot;/dev/urandom&quot;, access=&quot;stream&quot;, &amp;
        form=&quot;unformatted&quot;, action=&quot;read&quot;, status=&quot;old&quot;, iostat=istat)
   if (istat == 0) then
      read(un) seed
      close(un)
   else
      ! Fallback to XOR:ing the current time and pid. The PID is
      ! useful in case one launches multiple instances of the same
      ! program in parallel.
      call system_clock(t)
      if (t == 0) then
         call date_and_time(values=dt)
         t = (dt(1) - 1970) * 365_int64 * 24 * 60 * 60 * 1000 &amp;
              + dt(2) * 31_int64 * 24 * 60 * 60 * 1000 &amp;
              + dt(3) * 24_int64 * 60 * 60 * 1000 &amp;
              + dt(5) * 60 * 60 * 1000 &amp;
              + dt(6) * 60 * 1000 + dt(7) * 1000 &amp;
              + dt(8)
      end if
      pid = getpid()
      t = ieor(t, int(pid, kind(t)))
      do i = 1, n
         seed(i) = lcg(t)
      end do
   end if
   call random_seed(put=seed)
 contains
   ! This simple PRNG might not be good enough for real work, but is
   ! sufficient for seeding a better PRNG.
   function lcg(s)
     integer :: lcg
     integer(int64) :: s
     if (s == 0) then
        s = 104729
     else
        s = mod(s, 4294967296_int64)
     end if
     s = mod(s * 279470273_int64, 4294967291_int64)
     lcg = int(mod(s, int(huge(0), int64)), kind(0))
   end function lcg
 end subroutine init_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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	<a name="RANDOM_005fSEED"></a>
	<div class="header">
	<p>
	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>
	</div>
	<hr>
	<a name="RANDOM_005fSEED-_002d_002d_002d-Initialize-a-pseudo_002drandom-number-sequence"></a>
	<h3 class="section">8.201 <code>RANDOM_SEED</code> — 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 initialized
	to a default state. The example below shows how to initialize the
	random seed with a varying seed in order to ensure a different random
	number sequence for each invocation of the program. Note that setting
	any of the seed values to zero should be avoided as it can result in
	poor quality random numbers being generated.
	</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">subroutine init_random_seed()
	use iso_fortran_env, only: int64
	implicit none
	integer, allocatable :: seed(:)
	integer :: i, n, un, istat, dt(8), pid
	integer(int64) :: t

	call random_seed(size = n)
	allocate(seed(n))
	! First try if the OS provides a random number generator
	open(newunit=un, file="/dev/urandom", access="stream", &
	form="unformatted", action="read", status="old", iostat=istat)
	if (istat == 0) then
	read(un) seed
	close(un)
	else
	! Fallback to XOR:ing the current time and pid. The PID is
	! useful in case one launches multiple instances of the same
	! program in parallel.
	call system_clock(t)
	if (t == 0) then
	call date_and_time(values=dt)
	t = (dt(1) - 1970) * 365_int64 * 24 * 60 * 60 * 1000 &
	+ dt(2) * 31_int64 * 24 * 60 * 60 * 1000 &
	+ dt(3) * 24_int64 * 60 * 60 * 1000 &
	+ dt(5) * 60 * 60 * 1000 &
	+ dt(6) * 60 * 1000 + dt(7) * 1000 &
	+ dt(8)
	end if
	pid = getpid()
	t = ieor(t, int(pid, kind(t)))
	do i = 1, n
	seed(i) = lcg(t)
	end do
	end if
	call random_seed(put=seed)
	contains
	! This simple PRNG might not be good enough for real work, but is
	! sufficient for seeding a better PRNG.
	function lcg(s)
	integer :: lcg
	integer(int64) :: s
	if (s == 0) then
	s = 104729
	else
	s = mod(s, 4294967296_int64)
	end if
	s = mod(s * 279470273_int64, 4294967291_int64)
	lcg = int(mod(s, int(huge(0), int64)), kind(0))
	end function lcg
	end subroutine init_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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