Name

ranl_skip2 − Routine to skip forward 2∗∗d seeds for the RANL and RANL_NORMAL random number generators

FORMAT

RANL_SKIP2 (d, s1, s2, s1_new, s2_new)

Arguments

dinteger∗4
On entry, an integer d>0 specifying the number 2∗∗d of seeds to skip starting at (s1,s2) in the L’Ecuyer algorithm. 

s1integer∗4
On entry, a starting seed s1>=1. 

s2integer∗4
On entry, a starting seed s2>=1. 

s1_newinteger∗4
On exit, a new starting seed 2∗∗d iterations away from s1. 

s2_newinteger∗4
On exit, a new starting seed 2∗∗d iterations away from s2. 
 

Description

The RANL_SKIP2 routine is used to get starting seeds for parallel, independent streams of random numbers. The new starting seeds are computed using the following algorithms:

s1_new = a1∗∗(2∗∗d)∗s1  mod m1
s2_new = a2∗∗(2∗∗d)∗s2  mod m2

where a1,a2,m1,m2 are the constants defining the L’Ecuyer method. 

The following code example calls RANL_SKIP2 to set up separate seeds for 4 streams, (nprocs=4), from RANL. It computes the averages per stream. 

Example

      integer nprocs,n,hop
      parameter (nprocs=4)
      parameter (n=16384,hop=14)
      integer∗4 j,k
      real∗4 v(n,nprocs)
      integer∗4 s1val(nprocs),s2val(nprocs)
      real∗4 sum1(nprocs)
 c     get seeds (2∗∗hop apart) for separate streams
      s1val(1)=1
      s2val(1)=1
      do j=2,nprocs
      call ranl_skip2(hop,s1val(j-1),s2val(j-1),s1val(j),s2val(j))
      end do
 c     parallel calls to ranl can be done with KAP directives:
C∗$∗  ASSERT CONCURRENT CALL
C∗$∗  ASSERT DO (CONCURRENT)
      do j=1,nprocs
         call ranl(s1val(j),s2val(j),v(1,j),n)
         sum1(j)=0.0
         do k=1,n
            sum1(j)=sum1(j)+v(k,j)
         end do
      end do
       print∗,’ per-stream averages’
      do j=1,nprocs
         print∗,sum1(j)/n
      end do
       end