program ex_7_18_v
   use Environment

   implicit none
      character(*), parameter :: input_file = "../data/input.txt", output_file = "output.txt"
      integer                 :: In = 0, Out = 0, N = 0, i = 0, j=0, l, N_max, m, k
      real(R_)                :: max
      real(R_), allocatable   :: A(:, :), B(:)
      integer, allocatable    :: Indexes(:, :), Ind_max(:, :)
      logical, allocatable    :: Mask(:)

      !READ AN NxN ARRAY
      open (file=input_file, newunit=In)
         read (In, *) N
         allocate (A(N, N))
         allocate (B(2*N))
         read (In, *) (A(i, :), i = 1, N)
      close (In)

      allocate (Indexes(2*N, 2)) !AN N*N MATRIX CONTAINS M*N ELEMENTS. THIS ARRAY WILL CONTAIN INDICES COLUMNWISE.
      allocate (Mask(N), source=.false.)

      !Indexes(:, 1) = [((i, i = 1, N), j = 1, N)]
      !Indexes(:, 2) = [((j, i = 1, N), j = 1, N)]

       i=1
       j=1
      do m=1, N
         B(i) = abs(A(i, i))
         Indexes(i, 1) = i
         Indexes(i, 2) = j
         write (Out, *) Indexes(i, :)
         i=i+1
        j=j+1
      end do

      i=N
      l=N+1
      j=1
      do k=1, N
         B(l) = abs(A(i, j))
         Indexes(i, 1) = i
         Indexes(i, 2) = j
         write (Out, *) Indexes (i, :)
         i=i-1
         l=l+1
         j=j+1
      end do

    write (Out, *) Indexes(:, :)

      max=maxval(B)
      mask = [B == max]
      N_max = count(Mask)
      write (Out, *) N_max

      allocate (ind_max(N_max, 2))
      Ind_max(:, 1) = Pack(Indexes(:, 1), Mask)
      Ind_max(:, 2) = Pack(Indexes(:, 2), Mask)

 open (file=output_file, encoding=E_, newunit=Out)
      write (Out, *) (A(i, :), i = 1, N)
      write (Out, *) "------------------------"
      write (Out, *) B
      write (Out, *) "MAX = ", max
      write (Out, *) (Ind_max(i, :), i = 1, UBound(Ind_max, 1))
 close (Out)

end program ex_7_18_v