! rw2d.f90
! A simple random walk program  in 2D. 

PROGRAM drunk
  IMPLICIT NONE
  INTEGER :: i, N, stepLength
  REAL :: phi, rnd
!  INTEGER, DIMENSION(2) :: seed

  REAL :: x=0.0, y=0.0        ! Put drunk initially at the origin
  INTEGER, PARAMETER :: out=1, out1=2 ! Set output unit

  REAL, PARAMETER :: step=1.0, twopi=2.0*3.1415926 ! step size and constants
  CHARACTER(LEN=15), SAVE :: FORMAT1 = "(2F14.7)"
  
  PRINT*,"Enter number of steps:"
  READ*, N
  PRINT*,"Enter  stepLenght (integer=>1) :"
  READ*, StepLength
  OPEN(out, FILE='10.dat', STATUS="REPLACE", ACTION="WRITE")
  OPEN(out1, FILE='1.dat', STATUS="REPLACE", ACTION="WRITE")
 ! PRINT*,"Enter seed (1 and 2)"
 ! READ*,seed

 ! CALL RANDOM_SEED(PUT=seed)
  i = 0
  WRITE(UNIT=out,FMT=FORMAT1)x,y
  WRITE(UNIT=out1,FMT=FORMAT1)x,y

  DO  i=1, N*stepLength
     CALL RANDOM_NUMBER(rnd)
     phi=twopi*rnd
     x=x+step*COS(phi)
     y=y+step*SIN(phi)
     if(i<N+1)WRITE(unit=out1,FMT=FORMAT1)x,y
     if(mod(i,stepLength).eq.0)WRITE(UNIT=out,FMT=FORMAT1)x,y
  END DO

  CLOSE(out)
  CLOSE(out1)

  print*,' in 1.dat: dati scritti ogni step'
  print*,' in 10.dat: dati scritti ogni step multiplo di steplength'

END PROGRAM drunk