module md_module
  implicit none

contains
  subroutine matrix_diagonal(a,diag,n)
    implicit none
    real, intent (in), dimension (:,:) :: a
    real, intent (out), dimension (:) :: diag
    integer, intent (in) :: n
    real, dimension (1:size(a,1)*size(a,1)) :: temp

! subroutine to extract the diagonal elements of an n * n matrix A

    temp = pack(a,.true.)
    diag = temp(1:n*n:n+1)
  end subroutine matrix_diagonal
end module md_module

program ch2503
! program reads the n * n matrix from a file
  use md_module
  implicit none
  integer :: i, n
  real, allocatable, dimension (:,:) :: a
  real, allocatable, dimension (:) :: adiag
  character (len=20) :: filename

  print *, 'input name of data file'
  read '(a)', filename
  open (unit=1,file=filename)
  read (1,*) n
  allocate (a(1:n,1:n),adiag(1:n))
  do i = 1, n
    read (1,*) a(i,1:n)
  end do
  call matrix_diagonal(a,adiag,n)
  print *, ' diagonal elements of a are:'
  print *, adiag
end program ch2503