Fortran Wiki

Fortran initialization on declaration and the implied SAVE attribute

In Fortran a declaration statement cannot be an assignment statement; only a variable initialization. The declaration of CALL_COUNT in the following program can be used to illustrate the difference:

   program called ! @(#) example contrasting initialization vs assignment
   implicit none
   integer :: i
   do i=1,10
      call subinit() ! this has an initialization 
   do i=1,10
      call subdeclare() ! this has an assignment instead


   subroutine subinit()
   integer :: call_count=0  ! <= This is an initialization,
                            !    not as assignment
      write(*,*) 'CALL_COUNT=',call_count
   end subroutine subinit

   subroutine subdeclare()
   integer :: assigned ! <= the type is declared, 
                       !    but the value is undefined at this point
      assigned=0       ! <= This really is an assignment used on each 
                       !    call to this procedure, not just an 
                       !    initialization
      write(*,*) 'ASSIGNED=',assigned
   end subroutine subdeclare

   program called

Execution of SUBINIT() does not print “1” ten times; it prints from “1” to “10”. On the other hand the calls to the almost identical SUBDECLARE() prints “1” ten times.

This is because initialization takes place only once during program execution and from that point on the variable value and any changes to it are subsequently saved between calls to the procedure.

Of course on the other hand a regular assignment statement executes each time it is encountered and clearly takes care of the reinitialization.

The CALL_COUNT variable therefore only has the value zero at the beginning of the first call and does not get reinitialized when called again; and any subsequent changes to its value such as the increment in SUBINIT() are preserved throughout program execution even though it is not a global variable.

This is particularly important to note as such syntax is different than in many other widely used languages which use similar-looking statements for assignment, not initialization.

So to reiterate if you initialize a variable at declaration

integer :: a = 5  ! initialization with implied SAVE

it is the same as:

integer, save :: a = 5  ! type declaration and initialization

or the old FORTRAN 77 equivalent

integer a
save a
data /a/ 5

and not to:

integer :: a   ! type declaration
a = 5          ! assignment

as the functions of an initialization expression are a modern version of a saved value declared in a DATA statement, not an alternative to an assignment.

The above behavior is now all clearly specified by the Fortran standard. It was not in early Fortran versions. There are therefore a few related issues to watch for when porting legacy FORTRAN (ie. pre-Fortran 90) code to a modern Fortran compiler.

Many pre-Fortran90 compilers statically allocated all variables, to the point programmers often relied upon this behavior (ie. that all variables were implicitly saved).

Depending on this behavior is technically a bug, as it is now specifically standard that an unsaved value is assumed undefined upon reentry to a procedure.

This history is why most compilers provide compile-time switches to provide this old behavior, essentially acting as if a plain SAVE statement occurs in all procedures.

The -fno-automatic option of gfortran, and commonly the -static or -save option of other Fortran compilers (check the manual!) provide this option.

It is strongly recommended to not depend on the switch, but to explicitly use the SAVE attribute or SAVE declaration to identify variables that require being saved between calls. Even though a DATA statement or initialization on a declaration imply the SAVE attribute it is much clearer to specify the attribute anyway.

Initializing Floating Point Numbers with constants

The most common mistake when initializing floating-point values is to not make sure the RHS is generating a result with the same precision as the LHS. This often is because the kind of a constant is not declared. Without a declared kind a constant assumes the default kind of the type, not the kind on the LHS (Left Hand Side) of the initialization or assignment, as is sometimes assumed.

For example, assuming the definitions:

integer,parameter :: dp=kind(0.d0) ! double precision
integer,parameter :: sp=kind(0.0 ) ! single precision

Then the following code:

real(dp) :: a
a = 1.0  ! constant is missing a kind

is equivalent to:

real(dp) :: a
a = 1.0_sp  ! Oops! a single-precision value for a doubleprecision

and not to:

real(dp) :: a
a = 1.0_dp  ! correctly matches the constants' precision to the variable

However, the following code:

real(dp) :: a
a = 1

is equivalent to:

real(dp) :: a
a = 1.0_dp

Somewhat surprisingly because of the built-in type conversion rules specified for an assignment assigning an integer to a double does not loose any accuracy!

Another common case is assuming that because a lot of digits were declared the compiler will treat a value with more precision. Try this:

program trunc
integer,parameter :: dp=kind(0.d0) ! double precision
real(dp),parameter :: wrong=3.14159265358979323846264338327950
real(dp),parameter :: better=3.14159265358979323846264338327950_dp !notice the suffix
   write(*,*)'  3.14159265358979323846264338327950'
end program trunc

You will likely get something like this, without any warning about an excessive number of digits or truncation of the value given.

>   3.14159265358979323846264338327950
>   3.14159274101257     
>   3.14159265358979     

A general principle of Fortran is that the type of the RHS (Right Hand Side) of an assignment does not depend on the LHS (Left Hand Side). Once you understand this rule, a lot of things fall into place.

Applying a floating point kind to the wrong type

Another common error is to forget the decimal when creating what is intended to be a floating point constant. In this example the declaration of integer variables with a _dp suffix does not promote them automatically to double precision variables. The literal “111_dp” is an INTEGER!:

integer,parameter :: dp=kind(0  .d0) ! double precision
   a = 111_dp  ! a subtle error because no decimal appears in the number
   write(*,*)dp,'is just an integer'

Kinds are just constant names for an integer and not their own type so the compiler will likely not catch this error if the integer DP happens to also represent a valid integer kind, which is commonly true. (Some) compilers can give unique values to each default kind and type pair and so will catch this, but many will not. As noted above, because of the conversion rules for expressions and assignments you might get lucky and get the right value anyway, but obviously do not depend on it. The value of KINDs is up to the compiler, so this mistake might work in one environment and give different results in another.