10. Generic routines

With Fortran 77 (but not Fortran 66) we are used to the elementary functions being generic. This means that a call SIN(1.0) returns a value of type REAL, but SIN(1.0D0) returns a value with the higher precision 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 it is a pain keeping track of all the information involving these three different variants of SWAP. The solution is to move everything that has to do with the SWAP into a module. The module can then 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 parts. You will have to link both the module and the main program together, e.g. with the statement

	f90 part2.f90 part1.f90

Here is the module, it is 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 is the main program, now free of all uninteresting information about SWAP. It is 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

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