program write_sin
      open(unit=10,file='data1.dat')
      dx=(20.0-0.0)/1000
      write(10,*) 1000
      do i=1,1000
        x=(i-1)*dx
        y=sin(x)
        write(10,*)x,y
      enddo
      close(10)
      write(*,*)'File written to data1.dat.'
      end