10. Generic routines

From Fortran 77 (but not from Fortran 66) we are used to have that the elementary functions are generic, which means that a call SIN(1.0) returns a value of type REAL , but SIN (1.0D0) returns a value with a higher precision and of type DOUBLE PRECISION. We now also have the possibility to write our own generic functions or subroutines. Here we first give a complete example of a routine SWAP(A, B), which swaps the values of variables A and B (replaces the value with each other), using different underlying routines depending on the type of the variables REAL, INTEGER or CHARACTER.

       PROGRAM SWAP_MAIN
       IMPLICIT NONE
       INTEGER        :: I, J, K, L
       REAL           :: A, B, X, Y
       CHARACTER      :: C, D, E, F
       INTERFACE SWAP
              SUBROUTINE SWAP_R(A, B)
              REAL, INTENT (INOUT)          :: A, B
              END SUBROUTINE SWAP_R
              SUBROUTINE SWAP_I(A, B)
              INTEGER, INTENT (INOUT)       :: A, B
              END SUBROUTINE SWAP_I
              SUBROUTINE SWAP_C(A, B)
              CHARACTER, INTENT (INOUT)     :: A, B
              END SUBROUTINE SWAP_C
       END INTERFACE

       I = 1   ; J = 2       ;       K = 100 ; L = 200
       A = 7.1 ; B = 10.9    ;       X = 11.1; Y = 17.0
       C = 'a' ; D = 'b'     ;       E = '1' ; F = '"'

       WRITE (*,*) I, J, K, L, A, B, X, Y, C, D, E, F
       CALL SWAP(I, J) ; CALL SWAP(K, L)
       CALL SWAP(A, B) ; CALL SWAP(X, Y)
       CALL SWAP(C, D) ; CALL SWAP(E, F)
       WRITE (*,*) I, J, K, L, A, B, X, Y, C, D, E, F
       END

       SUBROUTINE SWAP_R(A, B)
       IMPLICIT NONE
       REAL, INTENT (INOUT)                 :: A, B
       REAL                                 :: TEMP
               TEMP = A ; A = B ; B = TEMP
       END SUBROUTINE SWAP_R

       SUBROUTINE SWAP_I(A, B)
       IMPLICIT NONE
       INTEGER, INTENT (INOUT)              :: A, B
       INTEGER                              :: TEMP
               TEMP = A ; A = B ; B = TEMP
       END SUBROUTINE SWAP_I

       SUBROUTINE SWAP_C(A, B)
       IMPLICIT NONE
       CHARACTER, INTENT (INOUT)            :: A, B
       CHARACTER                            :: TEMP
               TEMP = A ; A = B ; B = TEMP
       END SUBROUTINE SWAP_C

The above works very well, but the user is not so happy to have to care with all the information about SWAP in these three different variants in the program. The solution to this is to move everything that has to do with the SWAP into a module and then the module can be used from the main program with the statement USE module name. Please note that in the INTERFACE of the module the specific statement MODULE PROCEDURE has to be used in order to avoid that the routines are specified both in the INTERFACE and in the CONTAINS part. At the use you will have to link both the module and the main program together, e.g. with the statement

	f90 part2.f90 part1.f90

Here the modules follow, it could be in the file part2.f90,

MODULE BO
       INTERFACE SWAP
              MODULE PROCEDURE SWAP_R, SWAP_I, SWAP_C
       END INTERFACE
CONTAINS

       SUBROUTINE SWAP_R(A, B)
       IMPLICIT NONE
       REAL, INTENT (INOUT)                 :: A, B
       REAL                                 :: TEMP
               TEMP = A ; A = B ; B = TEMP
       END SUBROUTINE SWAP_R

       SUBROUTINE SWAP_I(A, B)
       IMPLICIT NONE
       INTEGER, INTENT (INOUT)              :: A, B
       INTEGER                              :: TEMP
               TEMP = A ; A = B ; B = TEMP
       END SUBROUTINE SWAP_I

       SUBROUTINE SWAP_C(A, B)
       IMPLICIT NONE
       CHARACTER, INTENT (INOUT)            :: A, B
       CHARACTER                            :: TEMP
                  TEMP = A ; A = B ; B = TEMP
       END SUBROUTINE SWAP_C
END MODULE BO

Here follows the main program, which is now cleaned of all uninteresting information about SWAP. It could be in the file part1.f90.

PROGRAM SWAP_MAIN
USE BO
       IMPLICIT NONE
       INTEGER                    :: I, J, K, L
       REAL                       :: A, B, X, Y
       CHARACTER                  :: C, D, E, F

       I = 1   ;  J = 2         ;     K = 100 ; L = 200
       A = 7.1 ;  B = 10.9      ;     X = 11.1; Y = 17.0
       C = 'a' ;  d = 'b'       ;     E = '1' ; F = '"'

       WRITE (*,*) I, J, K, L, A, B, X, Y, C, D, E, F
       CALL  SWAP (I, J)  ;  CALL SWAP (K, L)
       CALL  SWAP (A, B)  ;  CALL SWAP (X, Y)
       CALL  SWAP (C, D)  ;  CALL SWAP (E, F)
       WRITE (*,*) I, J, K, L, A, B, X, Y, C, D, E, F
END

Last modified: 3 February 1997
boein@nsc.liu.se