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

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.