Fortran PARAMETER variable initialized from input

Question

In Fortran, is the PARAMETER attribute set at runtime or compilation time? I was wondering if I can pass in the size the of the array at run time and have this set as PARAMETER.

Can this be done? If so, how? If not, why?

Answer 1

Yes, as repeatedly answered, a named constant (an object with the parameter attribute) must have its initial value "known at compile time". However, as you talk about the size of arrays I'll note something else.

When declaring the shape of an array there are many times when the bounds needn't be given by constant expressions (of which a simple named constant is one example). So, in the main program or a module

implicit none
...
integer, dimension(n) :: array  ! Not allowed unless n is a named constant.
end program

the n must be a constant. In many other contexts, though, n need only be a specification expression.

implicit none
integer n

read(*,*) n
block
  ! n is valid specification expression in the block construct
  integer, dimension(n) :: array1
  call hello(n)
end block

contains

  subroutine hello(n)
    integer, intent(in) :: n  ! Again, n is a specification expression.
    integer, dimension(2*n) :: array2
  end subroutine

end program

That is, array1 and array2 are explicit shape automatic objects. For many purposes one doesn't really need a named constant.

Beyond the array size, the following is certainly not allowed, though.

implicit none
integer n, m

read(*,*) n, m
block
  ! n and m are specifications expression in the block construct
  integer(kind=m), dimension(n) :: array  ! But the kind needs a constant expression.
  ...
end block

Answer 2

You need dynamic allocation if the size of your array is to be defined as runtime. All parameter (constants) must be defined as compiling time.