Programme Units, Etc.

• external procedures (SUBROUTINEs and FUNCTIONs);
• internal procedures;
• MODULE procedures.

External Procedures/Subprogrammes

Seen these already:

• old-fashioned;
• can be within same source file as main PROGRAM or separate;
• lead to bugs and difficult to maintain --- compiler does not check that actual and dummy arguments match;
• procedure/subprogram interfaces are said to be implicit.

Don't!

Internal Procedures/Subprogrammes

• dummy and actual arguments must match --- interfaces are said to be explicit;
  
  
• rarely the best solution:
  • subprograms internal to a PROGRAM are often best moved to a MODULE --- see MODULE procedures (below);
  • subprograms internal to a subprogram are simply rarely good;

PROGRAM my_prog ! -- A PROGRAM may CONTAIN INTEGER :: count ! procedures/subprograms REAL :: total ... CONTAINS SUBROUTINE my_int_sub ! -- internal SUBROUTINE ... END SUBROUTINE my_int_sub FUNCTION my_int_func ! -- internal FUNCTION ... END FUNCTION my_int_func END PROGRAM my_prog

In addition, procedures/subprograms may CONTAIN other (internal) procedures/subprograms :

SUBROUTINE my_sub !...or FUNCTION CHARACTER, INTENT(INOUT) :: C ... CONTAINS FUNCTION my_func !...internal FUNCTION (could ... ! be SUBROUTINE also) END FUNCTION my_func END SUBROUTINE

Module Procedures/Subprogrammes

• ...we consider these when we consider MODULEs...