8. Keyword arguments and default arguments

The use of keyword and default argument is not just as simple as it appears from Appendix 3, section 6, program units. It is one of the cases where an explicit interface is required. Therefore we give a complete example here. As keywords the formal parameters of the interface are being used, they do not have to be the same names as in the actual subprogram. They shall not be specified in the calling program unit.

       IMPLICIT NONE
       INTERFACE
          SUBROUTINE SOLVE (A, B, N)
             INTEGER, INTENT (IN)          : : N
             REAL, INTENT(OUT)             : : A
             REAL, INTENT(IN), OPTIONAL    : : B
          END SUBROUTINE SOLVE
       END INTERFACE

       REAL X
!      Note that A, B and N are not specified as REAL
!      or INTEGER in this unit.

       CALL SOLVE(B=10.0,N=50,A=X)
       WRITE(*,*) X
       CALL SOLVE(B=10.0,N=100,A=X)
       WRITE(*,*) X
       CALL SOLVE(N=100,A=X)
       WRITE(*,*) X
       END

       SUBROUTINE SOLVE(A, B, N)
       REAL, OPTIONAL, INTENT (IN)  : : B
       IF (PRESENT(B)) THEN
              TEMP_B = B
       ELSE
              TEMP_B = 20.0
       END IF
       A = TEMP_B + N
       RETURN
       END

It is run with the following statements

       % f90 program.f90
       % a.out
          60.0000000
           1.1000000E+02
           1.2000000E+02
       %

It is convenient to place the interface INTERFACE in a module so the user does not have to care so much about it. The interface is a natural complement to the routine library. Fortran 90 looks automatically for modules in the present directory, in the directories given in the I-list and also in /usr/local/lib/f90 (this is the standard library for Fortran 90 using UNIX). The concept I-list is explained in Appendix 6. If you forget INTERFACE or have an incorrect interface, usually compilation or execution gives the error message "Segmentation error", and nothing more.

Note that if an output variable is given as OPTIONAL and INTENT(OUT) then you have to have it included in the argument list, if the program at execution assigns a value to this variable. You can therefore not use OPTIONAL in order to get a choice whether you wish to have a certain variable outputted or not. The solution to this problem will be discussed a little later, the PRESENT statement.

Exercises

(8.1) Write a routine for the calculation of an integral of a function. You will use keyword arguments and default arguments so that

• if there is no left integration limit A use the value zero.
• if there is no right integration limit B use the value one.
• if there is no tolerance keyword TOL use the value 0.001 for the absolute error.

(8.2) Write the interface that is required in the calling routine in order to use the above integration routine.
Solution.