Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!rex!ames!skipper!elxsi!maine
From: maine@elxsi.dfrf.nasa.gov (Richard Maine)
Newsgroups: comp.lang.fortran
Subject: Keyword and optional argments (was Fortran vs C)
Message-ID: <MAINE.90Nov29200637@altair.dfrf.nasa.gov>
Date: 30 Nov 90 04:06:37 GMT
References: <1990Nov24.002836.19739@ariel.unm.edu> <1270009@hpcllmv.HP.COM>
	<771@sarnoff.sarnoff.com> <11977@milton.u.washington.edu>
Sender: news@skipper.dfrf.nasa.gov
Organization: NASA Dryden, Edwards, Cal.
Lines: 64
In-reply-to: whit@milton.u.washington.edu's message of 30 Nov 90 02:21:55 GMT

On 30 Nov 90 02:21:55 GMT, whit@milton.u.washington.edu (John Whitmore) said:

John> 	Finally, a topic I can really get behind!  Most languages have
John> some constructs that allow optional arguments: 

John> 	OPEN(UNIT= the_unit, FILE='FileName',FORM='UNFORMATTED)

John> is a Fortran example.  Yet, this sort of 'keyword=value' calling
John> sequence is not supported for user subroutines.  The language implementors
John> needed it; SO DO THE USERS.  

John> 	CALL READ_THE_FILE( file='FileName', 
John> 	&	format='Bruce''s third generation data set', 
John> 	&	into= data_array(5)   )

I 100% agree.  And I note that this is incorporated in Fortran 90
modules, one of my favorite Fortran 90 features.  The statement above
is a perfectly legal Fortran 90 statement (well, except that you'd
probably be using free source form, which does continuation lines
differently).  The subroutine interface might be something like.

   module file_stuff

   use precision

   contains
      subroutine read_the_file(file,format,some_other_argument,into)

        character, intent(in) :: file*(*)
        character, intent(in), optional :: format*(*)
        integer, intent(in), optional :: some_other_argument
        real(r_kind), intent(out) :: into
       ........
      end subroutine read_the_file
   end module file_stuff

where for no relevant reason, except that I like it, I've also choosen
to illustrate the usage of the KIND parameter.  The r_kind parameter
above might be defined in

  module precision

     integer, parameter :: r_kind = selected_real_kind(12,30)
     ......

  end module precision

Selected_real_kind is a Fortran 90 intrinsic that gives the
"kind" of the smallest (loosely speaking) real type with
precision of at least 12 decimal digits and range of at least
10**-30 to 10**+30.  Thus the into variable is double precision
(to use old style Fortran terms) on my Sun or other 32-bit
systems, but single precision on the Cybers or Crays; no source
code changes required.

I think I've got all the above correct, though I'm still a bit
short on compilers to test my Fortran 90 code on.  (Waiting
anxiously for you Keith.  Either a Sun3 or Sparc version will
do fine. :-))

--

Richard Maine
maine@elxsi.dfrf.nasa.gov [130.134.64.6]