Fortran: Choosing the rank of an allocatable array

Question

I am trying to write a program where I want the allocatable array A to be of either rank 1, 2, or 3, depending on my input at run-time. I want to do this since the subsequent operations on A are similar, and I have defined in a module an interface work with module procedures that when acted on A, gives the desired result.

What I am doing currently is this:

program main
implicit none
integer :: rank,n=10
real*8, allocatable :: A1(:)
real*8, allocatable :: A2(:,:)
read (*,*) rank

if (rank.eq.1) then
    allocate (A1(n))
else if (rank.eq.2) then
    allocate (A2(n,n))
end if

! operate on the array
if (rank.eq.1) then
    call work(A1)
else if (rank.eq.2) then
    call work(A2)
end if

end program

Things would be much easier if somehow I could choose the rank of A, as then the if statements are not needed. Maybe this is not possible, but all help are appreciated.

Answer 1

The next Fortran standard (2015) has the select rank construct similar to select case. My example uses the select case construct on the rank intrinsic of an assumed-rank dummy variable.

    module my_type

  use, intrinsic :: iso_fortran_env, &
       ip => INT32, &
       wp => REAL64

  implicit none
  private
  public :: MyType

  type MyType
     real (wp)              :: rank0
     real (wp), allocatable :: rank1(:)
     real (wp), allocatable :: rank2(:,:)
     real (wp), allocatable :: rank3(:,:,:)
   contains
     procedure :: create => create_my_type
     procedure :: destroy => destroy_my_type
  end type MyType

contains

  subroutine create_my_type(this, array)
    ! calling arguments
    class (MyType), intent (in out) :: this
    real (wp),      intent (in)     :: array(..) !! Assumed-rank dummy variable

    ! local variables
    integer (ip), allocatable :: r(:)

    select case(rank(array))
    case (0)
       return
    case (1)
       r = shape(array)
       allocate( this%rank1(r(1)) )
    case (2)
       r = shape(array)
       allocate( this%rank2(r(1), r(2)) )
    case (3)
       r = shape(array)
       allocate( this%rank3(r(1), r(2), r(3)) )
    case default
       error stop 'array must have rank 0,1,2, or 3'
    end select

    ! Release memory
    if (allocated(r)) deallocate( r )

  end subroutine create_my_type


  subroutine destroy_my_type(this)
    ! calling arguments
    class (MyType), intent (in out) :: this

    if (allocated(this%rank1)) deallocate( this%rank1 )
    if (allocated(this%rank2)) deallocate( this%rank2 )
    if (allocated(this%rank3)) deallocate( this%rank3 )

  end subroutine destroy_my_type

end module my_type

program main

  use, intrinsic :: iso_fortran_env, only: &
       ip => INT32, &
       wp => REAL64

  use my_type, only: &
       MyType

  implicit none

  type (MyType) :: foo
  real (wp)     :: a0, a1(42), a2(42,42), a3(42,42,42)

  print *, rank(a0)
  print *, rank(a1)
  print *, rank(a2)
  print *, rank(a3)

  ! Allocate array of rank 3
  call foo%create(a3)

  print *, rank(foo%rank3)
  print *, shape(foo%rank3)
  print *, size(foo%rank3)

  ! Release memory
  call foo%destroy()

end program main

Answer 2

Declare the array to be rank three. If a lower rank array is required, allocate the relevant trailing dimensions to be of size one.

real, allocatable :: array(:,:,:)
...
select case (desired_rank)
case (1) ; allocate(array(n,1,1))
case (2) ; allocate(array(n,n,1))
case (3) ; allocate(array(n,n,n))
case default ; error stop 'bad desired rank'
end select

You can then use an array section to get a contiguous slice of array that is consistent with your desired rank. Alternatively, write the relevant procedures that operate on array to take a rank three argument, and make them aware of the meaning of a size one extent for the higher dimensions.