Conventions for fortran90 or gfortran - Declaration of integer, real, double and array

I would like to get clarifications about the declaration in Fortran90 or gfortran.

Firstly, for declaring integer and double, it seems that convention is:

! i is an integer

whereas I have also seen a simple declaration like (in my code, I use this one):

! i is an integer
integer i 

i.e, without the '::' symbol.

What is the norm about this declaration (for integer, real, double)?

Is it the same for declaring arrays ? i.e, is symbol '::' necessary like in this following declaration:

! declaration of array "list" with 10 elements
double precision,dimension(10)::list

I looked for a simpler declaration for array, without success.

Finally, I also use the following syntax for allocatable arrays:

! Arrays 
double precision, allocatable :: x(:,:)
! Allocation of 2D Array

Is it a recent way (I mean a recent convention or norm in Fortran) to declare allocatable array?

PS: I don't know very well the evolution of different versions for Fortran (for example the differences between fortran90 and gfortran.


  • In a variable declaration, :: is required for one of two reasons:

    • there is an attribute list in the declaration statement;
    • there is explicit initialization in the statement.

    For the first we see such as

    integer, dimension(5) :: i
    integer, target :: j

    For the second

    integer :: k=1

    It isn't harmful to use :: when optional/not required. Although the token isn't valid in Fortran standards before F90.

    Finally, note that integer i(5) doesn't require the ::. I've seen advice given to use :: always for consistency and saving effort learning/applying the rules.