# 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
```