program basic_sort
      implicit none
      integer i, N_max, N, M, N_total, yes_no
      parameter(N_max=2000)
      real A(N_max)
      character*3 switch
      character*20 filename
      
      write(*,*) 'Which file name has your data ? (No more than 20 chrs.
     ^, please.)'
      read(*,*) filename

      write(*,*) 'How many number are there in the file ?'
      write(*,*) 'Less than',N_max,'numbers, please'
      read(*,*) N_total

      if(N_total.gt.N_max) then
        write(*,*) 'Please change your progeam to make N_total larger.'
        write(*,*) 'The code will stop now, sorry.'
        stop
      end if

      open(unit=10, file=filename, status='OLD')
      do i=1,N_total
        read(10,*) A(i)
      end do

      do M=1, N_total-1
        N = N_total - M + 1
        call pick_maxmin(N, A, 'MAX')
      end do

      write(*,*) 'Sorting completed, the numbers from big to small are:'
      do i=1,N_total
        write(*,*) A(N_total + 1 - i)
      end do

 200  write(*,*) 'Do you want to replace/overwrite the original file ?'
      write(*,*) '1 mean YES, 0 means NO.'
      read(*,*) yes_no
      if (yes_no .eq. 1) then
        close (10)
        open(unit=10, file=filename, status='OLD')
        do i =1, N_total
          write(*,*) A(i)
        end do
        write(*,*) 'Sorted numbers written in file in assending order.'
       else
        goto 200
      end if
      close(10)

      end



      subroutine pick_maxmin(N,A,switch)
C
C This subroutine reads in an array A(N) and find its Max (or Min) number to
C put it in A(N) (or A(1)), the rest of numbers occupie the rest N-1 positions
C in the array and are unchanged in their order. 
C 
      implicit none
      integer i,N
      real A(*), winner, losser
      character*3 switch

      if(N.lt.2) then
        write(*,*) 'Need at least two number to compare, please check.'
        stop
      end if
     
      if (switch.eq.'MAX') then
        do i=1,N-1
          if (A(i).gt.A(N)) then
            winner = A(i)
            losser = A(N)
           else
            winner = A(N)
            losser = A(i)
          end if
          A(N) = winner
          A(i) = losser
        end do 
      end if

      if (switch.eq.'MIN') then
        do i=2,N
          if (A(i).lt.A(1)) then
            winner = A(i)
            losser = A(1)
           else
            winner = A(1)
            losser = A(i)
          end if
          A(1) = winner
          A(i) = losser
        end do
       end if
       
       return

       end