Function:     presentn

Purpose:      Logical function checking the 'presence' of a numeric
              argument in a subroutine/function call.

Category:     UTILITY

File:         present.c

Author:       K.G. Begeman

Use:          LOGICAL PRESENTN( NUMARG )   Input   any type except character

              PRESENTN  Returns true if argument is present, false if not.
              NUMARG    Numeric argument in argument list which is
                        checked for its presence.

Description:  PRESENTN is intended to be used in routines which perform
              a different action depending on the 'presence' or
              'absence' of certain numeric arguments.
              In C procedures a numeric argument is considered not
              present if the calling routine/program puts NULL instead
              of a pointer to a variable in the argument list, in
              FORTRAN or SHELTRAN one has to put the first element of
              the common block NOARGN in the argument list.

Example:      C     SHELTRAN example to demonstrate use of presentn

                    PROGRAM MAINF
                    INTEGER NULLN
                    COMMON /NOARGN/ NULLN
                    ...
                    CALL SUB( NULLN )
                    ...
                    STOP
                    END
                    SUBROUTINE SUB( ARG )
                    REAL ARG
                    LOGICAL PRESENTN
                    IF (PRESENTN( ARG ))
                    THEN
                       ...
                    CIF
                    RETURN
                    END

              C example to demonstrate use of presentn
              #include "gipsyc.h"
              #include "presentn.h"

              void sub( float *arg )
              {
                 if (presentn_c( arg )) {
                    ...
                 }
              }

              main()
              {
                 ...
                 sub( NULL );
                 ...
              }

Warnings:     System dependent!

Notes:        For character arguments use PRESENTC.

Updates:      Jul 21, 1988: JPT, Original document.
              Jul 12, 1989: KGB, Converted to ANSI C.
              Apr 23, 1991: KGB, implemented for HP.