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Manual Reference Pages  - present (3fortran)

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NAME

PRESENT(3) - [STATE:INQUIRY] Determine whether an optional dummy argument is specified

SYNOPSIS

result = present(a)

         logical function present (a)

          type(TYPE(kind=KIND)) :: a(..)

CHARACTERISTICS

o	A May be of any type and may be a pointer, scalar or array value, or a dummy procedure.

DESCRIPTION

PRESENT(3) can be used in a procedure to determine if an optional dummy argument was present on the current call to the procedure.

A shall be the name of an optional dummy argument that is accessible in the subprogram in which the PRESENT(3) function reference appears. There are no other requirements on A.

Note when an argument is not present when the current procedure is invoked, you may only pass it as an optional argument to another procedure or pass it as an argument to PRESENT.

OPTIONS

o	A : the name of an optional dummy argument accessible within the current subroutine or function.

RESULT

Returns .true. if the optional argument A is present (was passed on the call to the procedure) , or .false. otherwise.

EXAMPLES

Sample program:

    program demo_present
    implicit none
    integer :: answer
       ! argument to func() is not present
       answer=func()
       write(*,*) answer
       ! argument to func() is present
       answer=func(1492)
       write(*,*) answer
    contains
    !
    integer function func(x)
    ! the optional characteristic on this definition allows this variable
    ! to not be specified on a call; and also allows it to subsequently
    ! be passed to PRESENT(3):
    integer, intent(in), optional :: x
    integer :: x_local
       !
       ! basic
       if(present(x))then
         ! if present, you can use x like any other variable.
         x_local=x
       else
         ! if not, you cannot define or reference x except to
         ! pass it as an optional parameter to another procedure
         ! or in a call to present(3f)
         x_local=0
       endif
       !
       func=x_local**2
       !
       ! passing the argument on to other procedures
       ! so something like this is a bad idea because x is used
       ! as the first argument to merge(3f) when it might not be
       ! present
       ! xlocal=merge(x,0,present(x)) ! NO!!
       !
       ! We can pass it to another procedure if another
       ! procedure declares the argument as optional as well,
       ! or we have tested that X is present
       call tattle(’optional argument x’,x)
       if(present(x))call not_optional(x)
    end function
    !
    subroutine tattle(label,arg)
    character(len=*),intent(in) :: label
    integer,intent(in),optional :: arg
       if(present(arg))then
          write(*,*)label,’ is present’
       else
          write(*,*)label,’ is not present’
       endif
    end subroutine tattle
    !
    subroutine not_optional(arg)
    integer,intent(in) :: arg
       write(*,*)’already tested X is defined’,arg
    end subroutine not_optional
    !
    end program demo_present

Results:

        optional argument x is not present
                  0
        optional argument x is present
        already tested X is defined 1492
            2226064

STANDARD

Fortran 95

fortran-lang intrinsic descriptions (license: MIT) @urbanjost

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Nemo Release 3.1

present (3fortran)

April 28, 2024

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