ran16807 

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

FORMAT

  RAN16807 (s)

Function Value

  ran16807            real*4
                      The uniform[0,1] value returned.

Arguments

  s                   integer*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