Name

ran16807 − Routine to generate single precision random numbers using a=16807 and m=2∗∗31-1

FORMAT

RAN16807 (s)

Function Value

ran16807real∗4
The uniform[0,1] value returned.

Arguments

sinteger∗4
On entry, the seed s>=1. 
On exit, the updated seed.

Description

The RAN16807 routine implements the "minimal standard" or Lehmer multiplicative generator to compute updated seeds using a=16807, m=2∗∗31-1, as follows:

s = 16807∗s  mod 2∗∗31-1

and returns s∗(1.0/m) as its uniform[0,1] output. 

The following code example simulates a random walk using RAN16807.  A particle starts at (0,0) and proceeds N., E., S., or W. with probability 0.25 each. 

Example

      integer ix,iy,i,j,iseed,nsteps,nwalks
      real∗4 x
c     random walk: go N E S W each with probability 0.25
      nsteps = 1000000
      nwalks = 10
      iseed = 1234
       do j=1,nwalks
      ix=0
      iy=0
      do i = 1, nsteps
      x=ran16807(iseed)
      if(x.le.0.25)then
         ix=ix+1
      else if(x.le.0.5)then
         iy=iy+1
      else if(x.le.0.75)then
        ix=ix-1
      else
         iy=iy-1
      end if
      end do
       print∗,’final position ’,ix,iy
      end do
      end