Random numbers keep coming out the same, despite random seed being used

Question

I have the following small piece of code:

  REAL(8)       :: x
  INTEGER       :: i

  call system_clock(i)
  WRITE(*,*) 'cpu time', i
  CALL random_seed(i)

  CALL random_number(x)
  WRITE(*,*) 'uniform RandVar', x

CPU time is working fine, but every time I run this I get the same uniform RandVar number = 0.99755959009261719, almost like random_number is using the same default seed over and over again and ignoring random seed.

What am I doing wrong?

Answer 1

The same seed may well be being used: that is processor-dependent. The reason for this is that your call to random_seed is not setting the seed.

With the reference

CALL random_seed(i)

the argument i is not the (intent(in)) seed, but is the (intent(out)) size of the seed used by the processor. This call is like

CALL random_seed(SIZE=i)  ! SIZE is the first dummy argument

To set the seed you need to explicitly associate with the PUT dummy argument: call random_seed(put=seed). Here the seed is a rank 1 array of size at least n where n - again processor-dependent - is the size given by call random_seed(size=n). From your call i holds this value.

Full details are given in 13.7.136 of F2008.

A common way to seed the generator is:

integer, allocatable :: seed(:)
integer size

call random_seed(size=size)
allocate(seed(size))
! set seed(:) somehow
call random_seed(put=seed)

Setting seed appropriately is not a simple process. I don't address how to do that here, but detail can be found in answers to this other question.

Use of srand(), which is mentioned in the comments, is non-standard.