J3/06-126

To: J3
From: Michael Ingrassia
Subject: Formatted output of a negative real zero value
Date: 2006 January 30

NUMBER: F03/0080
TITLE: Formatted output of a negative real zero value
KEYWORDS: formatted output, negative zero, IEEE
DEFECT TYPE: CLARIFICATION/ERRATUM
STATUS: J3 consideration in progress

QUESTION:

     Suppose a Fortran processor's representation of
     the real zero value is signed.  When a negative
     real zero value is written using formatted output:

     (a)  Does the Fortran 2003 standard allow the
          representation of the zero value in the
          output field to be prefixed with a minus
          sign?

     (b)  Does the Fortran 2003 standard require the
          representation of the zero value in the
          output field to be prefixed with a minus
          sign?

ANSWER:  (a) Yes.  Although Section 4.4.2 lists some
        contexts as requiring positive and negative zeros to
        be treated as equivalent, formatted output is not
        on that list of contexts.  Negative zero can already
        be produced by formatted output, for example by

                PRINT '(F4.1)', -0.000001

        which in Fortran 95 (but not Fortran 90) is required
        to write "-0.0" or " -.0".

        (b)  Yes, or Not Necessarily (edits provided for both).  The first
        choice promotes portability.

        (b1) The negative sign is required to appear in formatted output
                of a negative zero value.
        (b2) Negative zero values may be treated as negative values
        in formatted output at the option of the implementor.

DISCUSSION:  A negative zero value is negative by name but zero
        by nature.  Therefore a context which refers to negative values
        must always be inspected to see if it is appropriate
        to also include negative zero values, and the instance in 10.6.1
        needs amendment.

EDITS:

        For (b1):
        [227:3-4] Change from

        The representation of a negative internal value in the field
        shall be prefixed with a minus sign.

        to

        The representation of a negative internal value or a negative
        zero value in the field shall be prefixed with a minus sign.


        For (b2):
        [227:4] Add the sentence

        The representation of a negative zero value
        in the field may be prefixed with a minus sign.

SUBMITTED BY: Michael Ingrassia
HISTORY: J3/06-126 submitted to J3 meeting 175