Identifies optional arguments
for procedures.
Syntax
The syntax of the type declaration statement with the OPTIONAL attribute is:
type , attrib-list :: dummy-argument-name-list |
- type
is a valid type specification (INTEGER, REAL, LOGICAL, CHARACTER, TYPE ( name), etc.).
- attrib-list
is a comma-separated list of attributes including OPTIONAL and optionally those attributes compatible with
it, namely:
Table 10-41 Title not available (OPTIONAL (statement and attribute))
DIMENSION | INTENT | TARGET |
EXTERNAL | POINTER | VOLATILE |
- dummy-argument-name-list
is a comma-separated list of dummy-argument-names.
The syntax of the OPTIONAL statement is:
OPTIONAL [::] dummy-argument-name-list |
Description
If a dummy argument has
the OPTIONAL attribute, the corresponding actual argument need not
appear in a procedure reference. In cases where there are arguments
that generally do not change from one reference to another, it is
convenient to specify that the arguments are optional and provide
default values for them. They can then be omitted from references
in these general cases. The presence of an optional argument in
a procedure may be determined by using the PRESENT intrinsic function.
Many uses of the ENTRY statement in FORTRAN 77 programs can be replaced
by the use of optional arguments.
The following restrictions
apply to the use of the OPTIONAL attribute:
The OPTIONAL attribute may be specified only for dummy arguments.
It may occur in a subprogram and in any corresponding interface
body.
An optional dummy argument
whose actual argument is not present may not be referenced or defined
(or invoked if it is a dummy procedure), except that it may be passed to
another procedure as an optional argument and will be considered
not present.
When an argument
is omitted in a procedure reference, all arguments that follow it
must use the keyword form.
If a procedure
has an optional argument, the procedure interface must be explicit.
Examples
The following are two examples of the OPTIONAL statement. In the first example, the call to the
subroutine trip can legally omit the path argument because it
has the OPTIONAL attribute:
CALL TRIP ( distance = 17.0 ) ! path is omitted
SUBROUTINE trip ( distance, path )
OPTIONAL distance, path |
In the next example, the subroutine plot uses the PRESENT function to determine whether or not to execute
code that depends on the presence of arguments that have the OPTIONAL attribute:
SUBROUTINE plot (pts, o_xaxis, o_yaxis, smooth)
TYPE (point) pts
REAL, OPTIONAL :: o_xaxis, o_yaxis
! Origin - default (0.,0.)
LOGICAL, OPTIONAL :: smooth
REAL ox, oy
IF (PRESENT (o_xaxis)) THEN
ox = o_xaxis
ELSE
ox = 0.
! Note that the o_xaxis dummy argument cannot be referenced if
! the actual argument is not present. The same applies
! to o_yaxis (below).
END IF
IF (PRESENT (o_yaxis)) THEN
oy = o_yaxis
ELSE
oy = 0.
END IF
IF (PRESENT(smooth)) THEN
IF (smooth) THEN
... ! Smooth algorithm
RETURN
END IF
END IF
... ! Plot points
END SUBROUTINE plot
! Some valid calls to plot.
CALL plot (points)
CALL plot (observed, o_xaxis = 100., o_yaxis = 1000.)
CALL plot (random_pts, smooth = .TRUE.) |
Related statements
SUBROUTINE and FUNCTION
Related concepts
For related information, see the following:
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