Same random numbers in each run in Fortran

Question

I have the below Fortran code:

program Looops
  implicit none 
  
  integer, parameter:: PPPP = 3.1415     
  real*8 , Dimension(:) , allocatable  ::dkk25,dkk26,dkk27,dkk28 
  integer:: n_38
  
  Allocate(dkk25(40))
  Allocate(dkk26(40))
  Allocate(dkk27(40))
  Allocate(dkk28(40))

  call RANDOM_NUMBER(dkk25)
  call RANDOM_NUMBER(dkk26)
  dkk27(:) = sqrt(-2*log((dkk25(:))))*cos(2*pppp*(dkk26(:)))

  do n_38 = 1 , 40
    dkk28(n_38) = ((dkk27(n_38)-(sum(dkk27)/40))**2)/(40-1)
  end do
  
  print*,dkk25
end program Looops

In each run of this code I have same value of dkk25 and dkk26. I want to generate different random number in each run.

Where is `RANDOM_NUMBER` defined? – Scott Hunter Jan 31 '22 at 16:21 — Scott Hunter, Jan 31 '22 at 16:21
it is pre defined by fortran – nima sahakie Jan 31 '22 at 16:23 — nima sahakie, Jan 31 '22 at 16:23

L. Scott Johnson · Answer 1 · 2022-01-31T16:55:34.933

0

Use RANDOM_SEED (with no arguments).

From https://gcc.gnu.org/onlinedocs/gfortran/RANDOM_005fSEED.html :

8.225 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 seeded with random data retrieved from the operating system.

Or use RANDOM_INIT if your compiler supports it.

edited Jan 31 '22 at 16:55

answered Jan 31 '22 at 16:31

L. Scott Johnson

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please edit the code. random_seed is not defined. – nima sahakie Jan 31 '22 at 16:36
3

@nimasahakie I do not understand your comment. `random_seed` is a standard intrinsic subroutine. We have many examples of its use on this site. – Vladimir F Героям слава Jan 31 '22 at 16:37
3

No, don't do that - the behavior is undefined in Fortran. Yes, some implementations have that behavior, but you can't depend on it. Fortran 2018 added `RANDOM_INIT` for just this purpose. `call RANDOM_INIT(REPEATABLE=.false.)` gfortran now supports RANDOM_INIT. – Steve Lionel Jan 31 '22 at 16:38
2

One of the examples I mentioned is at https://stackoverflow.com/questions/65966220/random-number-generating-in-fortran `random_init()` is also shown there. – Vladimir F Героям слава Jan 31 '22 at 16:45

Same random numbers in each run in Fortran

1 Answers1