Dummy arguments can be made optional if they are declared with the OPTIONAL attribute. In this case, an actual argument does not have to be supplied for it in a procedure reference.
Nonoptional arguments must appear in an actual argument list before any optional arguments, and positional arguments must appear before argument keywords.
Optional arguments must have explicit procedure interfaces.
The PRESENT intrinsic function can be used to determine if an actual argument is associated with an optional dummy argument in a particular reference.
The following example shows optional arguments:
PROGRAM RESULT
TEST_RESULT = LGFUNC(A, B=D)
...
CONTAINS
FUNCTION LGFUNC(G, H, B)
OPTIONAL H, B
...
END FUNCTION
END
In the function reference, A is a positional argument associated with required dummy argument G. The second actual argument D is associated with optional dummy argument B by its keyword name (B). No actual argument is associated with optional argument H.