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### 8.201 `RANDOM_SEED` — Initialize a pseudo-random number sequence

Description:
  Restarts or queries the state of the pseudorandom number generator used by `RANDOM_NUMBER`.

  If `RANDOM_SEED` 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.

Standard:
  Fortran 95 and later
Class:
  Subroutine
Syntax:
  `CALL RANDOM_SEED([SIZE, PUT, GET])`
Arguments:

  SIZE | (Optional) Shall be a scalar and of type default `INTEGER`, with `INTENT(OUT)`. It specifies the minimum size of the arrays used with the PUT and GET arguments.
  PUT | (Optional) Shall be an array of type default `INTEGER` and rank one. It is `INTENT(IN)` and the size of the array must be larger than or equal to the number returned by the SIZE argument.
  GET | (Optional) Shall be an array of type default `INTEGER` and rank one. It is `INTENT(OUT)` and the size of the array must be larger than or equal to the number returned by the SIZE argument.

Example:

                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

See also:
  RANDOM_NUMBER