Statement Ordering

Fortran has some quite strict rules about the order of statements. Basically in any program or procedure the following rules must be used:

1. The program heading statement must come first, (PROGRAM, FUNCTION or SUBROUTINE). A PROGRAM statement is optional but its use is recommended.
2. All the specification statements must precede the first executable statement. Even though DATA statements may be placed with executable text it is far clearer if they lie in the declaration area. It is also a good idea to group FORMAT statements together for clarity's sakes.
3. The executable statements must follow in the order required by the logic of the program.
4. The program or procedure must terminate with an END statement.

Within the set of specification statements there is relatively little ordering required, however, in general if one entity is used in the specification of another, it is normally required that it has been previously defined. In other words, named constants (PARAMETER s) must be declared before they can be used as part of the declaration of other objects.

The following table details the prescribed ordering:

Execution of a program begins at the first executable statement of the MAIN PROGRAM, when a procedure is called execution begins with the first executable statement after the invoked entry point. The non-executable statements are conceptually `executed' (!) simultaneously on program initiation, in other words they are referenced once and once only when execution of the main program begins.

There now follows a explanation of the table,

• There can be only 1 main PROGRAM,
• there may be many uniquely named FUNCTION s and SUBROUTINE s program units (procedures).
• there may be any number of uniquely named MODULE s which are associated with a program through a USE statement. Modules are very flexible program units and are used to package a number of facilities (for example, procedures, type definitions, object declarations, or semantic extensions). Their use is very much encouraged and they replace a number of unsafe features of FORTRAN 77.
• There can be only one BLOCK DATA subprogram -- these will not be described as part of this course -- their purpose is to define global constants or global initialisation and this is best done by a MODULE and USE statement.
• USE statement:

  This attaches a module whose entities become use-associated with the program unit. When a module is used its public contents are accessible as if they had been declared explicitly in the program unit. Modules may be pre-compiled (like a library) or may be written by the programmer.

  Any global entities should be placed in a module and then used whenever access is required.

• The IMPLICIT NONE statement should be placed after a USE statement. Its use is implored.
• FORMAT and ENTRY statements. Format statements should be grouped together somewhere in the code. ENTRY statements provide a mechanism to `drop' into a procedure halfway through the executable code. Their use is outmoded and strongly discouraged due it its dangerous nature.
• PARAMETER statement and IMPLICIT statement:

  IMPLICIT statements should not be used, IMPLICIT NONE should be the only form of implicit typing considered. (IMPLICIT statements allow the user to redefine the implication of an object's first letter in determining what its implicit type will be if it is not declared. Cannot have an IMPLICIT statement if there is an IMPLICIT NONE line.)

  PARAMETER statements, it is suggested that the attributed (Fortran 90) style of PARAMETER declaration be used.

• DATA statements should (but do not have to) be placed in the declaration, common practice puts them after the declarations and before the executables.
• Interface blocks are generally placed at the head of the declarations and are grouped together.

  Statement functions are a form of in-line statement definition, internal procedures should be used instead.

• executable statements are things like DO statements, IF constructs and assignment statements.
• CONTAINS separates the ``main'' program unit from any locally visible internal procedures.
• the internal procedures follow the same layout as a (normal) procedure except that they cannot contain a second level of internal procedure.
• the END statement is essential to delimit the current program unit.

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