FORTRAN 95, array must have constant shape error

Question

I'm an absolute beginner to Fortran95 so I apologise for the simplicity of this question. I want to be able define the dimensions of an array in terms of integers but I get the error

'The module or main program array 'arrayx' at (1) must have constant shape'

and 'Variable 'y' cannot appear in the expression at (1)' (same error for x). Is it simply not possible to define the dimensions in terms of variables or am I just missing something obvious.

program arrayForm
    implicit none
    integer, save :: x=40, y=2
    real, dimension(x,y) :: arrayX
end program arrayForm

Answer 1

Variables with the save attribute are not constants and the compiler complains with justification. In your snippet replace that attribute with the parameter attribute which makes them constant and, at compile-time, the array can be fixed in size and the compiler shouldn't complain.

As you have employed it the save attribute is redundant. When applied to variables which may go out of scope, such as variables inside the scope of a subroutine or function, it requires that the value of the variable is saved from one invocation of the sub-scope to the next. But since your variables are declared in your program scope they won't ever go out of scope during execution.

Note also that one of Fortran's gotchas for new (and old) programmers is that variables which are initialised in their declaration acquire the save attribute automatically. So, in a subroutine your line

integer, save :: x=40, y=2

could simply be written

integer :: x=40, y=2

Personally I tend to use the save attribute so that my intentions when I write a subroutine are clear to me later.